Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
Am Sat, 13 Nov 2021 00:27:12 +0100
schrieb Frederic Weisbecker :
> On Thu, Nov 04, 2021 at 10:05:02PM +0100, Henning Schild wrote:
> > Am Sat, 30 Oct 2021 16:04:35 +0200
> > schrieb Salvatore Bonaccorso :
> >
> > > Control: tags -1 + moreinfo
> > >
> > > On Wed, Oct 27, 2021 at 10:16:56AM +0200, Georg Müller wrote:
> > > > > But for other configurations it is worse:
> > > > >
> > > > > config NO_HZ_FULL
> > > > > bool "Full dynticks system (tickless)"
> > > > > ...
> > > > > This is implemented at the expense of some overhead
> > > > > in user <-> kernel transitions: syscalls, exceptions and
> > > > > interrupts. Even when it's dynamically off.
> > > > >
> > > > > Say N.
> > > > >
> > > >
> > > >
> > > > Upstream commit 176b8906 changed the description regarding
> > > > NO_HZ_FULL:
> > > >
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=176b8906c399a170886ea4bad5b24763c6713d61
> > > >
> > > >
> > > > > By default, without passing the nohz_full parameter, this
> > > > > behaves just like NO_HZ_IDLE.
> > > > >
> > > > > If you're a distro say Y.
> > >
> > > While this is changed, and distros encouraged to select it,
> > > selecting this would enable both CONFIG_VIRT_CPU_ACCOUNTING_GEN
> > > and CONFIG_RCU_NOCB_CPU.
> > >
> > > For CONFIG_VIRT_CPU_ACCOUNTING_GEN
> > >
> > > Select this option to enable task and CPU time
> > > accounting on full dynticks systems. This accounting is
> > > implemented by watching every kernel-user boundaries using the
> > > context tracking subsystem. The accounting is thus performed at
> > > the expense of some significant overhead.
> > >
> > > For now this is only useful if you are working on the
> > > full dynticks subsystem development.
> > >
> > > If unsure, say N.
> > >
> > > which indicates some significant overhead.
> >
> > I can not answer that from the back of my head. Would have to dig as
> > well. Might get back in about two weeks if nobody else finds an
> > answer.
> >
> > But i took the liberty to include Frederic into Cc, the author of
> > the "distro reassure" patch.
> >
> > Not sure such a change would be allowed for bullseye (5.10) and if
> > the answer for 5.10 would be another than for i.e. 5.15
> >
> > Reading what it is, maybe it can in fact be decoupled from
> > NO_HZ_FULL. Which would mean an upstream patch and backporting (if
> > preempt would do that, but could be considered a "performance bug"
> > i guess)
> > > And for CONFIG_RCU_NOCB_CPU
> > >
> > > Use this option to reduce OS jitter for aggressive HPC
> > > or real-time workloads. It can also be used to offload RCU
> > > callback invocation to energy-efficient CPUs in
> > > battery-powered asymmetric multiprocessors. The price of this
> > > reduced jitter is that the overhead of call_rcu() increases and
> > > that some workloads will incur significant increases in
> > > context-switch rates.
> > >
> > > This option offloads callback invocation from the set of
> > > CPUs specified at boot time by the rcu_nocbs parameter. For each
> > > such CPU, a kthread ("rcuox/N") will be created to
> > > invoke callbacks, where the "N" is the CPU being offloaded, and
> > > where the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s"
> > > for RCU-sched (!PREEMPTION kernels). Nothing prevents this
> > > kthread from running on the specified CPUs, but (1) the kthreads
> > > may be preempted between each callback, and (2) affinity or
> > > cgroups can be used to force the kthreads to run on whatever set
> > > of CPUs is desired.
> > >
> > > Say Y here if you need reduced OS jitter, despite added
> > > overhead. Say N here if you are unsure.
> > >
> > > Adding as well overhead.
> > >
> > > Is this still to be considered true?
> >
> > probably but only for people that actively choose to use it and only
> > for the CPUs they choose. "rcu_nocbs" cmdline param, if not set
> > everything will be as it was.
> > I already indicated that in the commit message of my MR:
> > https://salsa.debian.org/kernel-team/linux/-/merge_requests/385
>
> Ok so the past traditional combo for a distro is:
>
>CONFIG_NO_HZ_IDLE=y
># CONFIG_RCU_NOCB_CPU is not set
>CONFIG_TICK_CPU_ACCOUNTING=y
>
> Then nohz_full support has been introduced which allows userspace
> tasks to run without being annoyed by tick interrupts. But ths feature
> is for extreme workloads. So we arranged for this support not to add
> additional overhead when it is not used.
>
> This means that
>
>CONFIG_NO_HZ_FULL=y
>
> which also automatically selects:
>
>CONFIG_RCU_NOCB_CPU=y
>CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
>
> are not expected to bring more overhead than their traditional
> counterparts, unless kernel boot parameters such as "nohz_full="
> or "rcu_nocbs=" are passed.
>
> So you can safely enable CONFIG_NO_HZ_FULL=y. I guess the
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
Hi Frederic,
On Sat, Nov 13, 2021 at 12:27:12AM +0100, Frederic Weisbecker wrote:
> On Thu, Nov 04, 2021 at 10:05:02PM +0100, Henning Schild wrote:
> > Am Sat, 30 Oct 2021 16:04:35 +0200
> > schrieb Salvatore Bonaccorso :
> >
> > > Control: tags -1 + moreinfo
> > >
> > > On Wed, Oct 27, 2021 at 10:16:56AM +0200, Georg Müller wrote:
> > > > > But for other configurations it is worse:
> > > > >
> > > > > config NO_HZ_FULL
> > > > > bool "Full dynticks system (tickless)"
> > > > > ...
> > > > > This is implemented at the expense of some overhead in
> > > > > user <-> kernel transitions: syscalls, exceptions and interrupts.
> > > > > Even when it's dynamically off.
> > > > >
> > > > > Say N.
> > > > >
> > > >
> > > >
> > > > Upstream commit 176b8906 changed the description regarding
> > > > NO_HZ_FULL:
> > > >
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=176b8906c399a170886ea4bad5b24763c6713d61
> > > >
> > > >
> > > > > By default, without passing the nohz_full parameter, this behaves
> > > > > just like NO_HZ_IDLE.
> > > > >
> > > > > If you're a distro say Y.
> > >
> > > While this is changed, and distros encouraged to select it, selecting
> > > this would enable both CONFIG_VIRT_CPU_ACCOUNTING_GEN and
> > > CONFIG_RCU_NOCB_CPU.
> > >
> > > For CONFIG_VIRT_CPU_ACCOUNTING_GEN
> > >
> > > Select this option to enable task and CPU time accounting
> > > on full dynticks systems. This accounting is implemented by watching
> > > every kernel-user boundaries using the context tracking subsystem.
> > > The accounting is thus performed at the expense of some
> > > significant overhead.
> > >
> > > For now this is only useful if you are working on the full
> > > dynticks subsystem development.
> > >
> > > If unsure, say N.
> > >
> > > which indicates some significant overhead.
> >
> > I can not answer that from the back of my head. Would have to dig as
> > well. Might get back in about two weeks if nobody else finds an answer.
> >
> > But i took the liberty to include Frederic into Cc, the author of the
> > "distro reassure" patch.
> >
> > Not sure such a change would be allowed for bullseye (5.10) and if the
> > answer for 5.10 would be another than for i.e. 5.15
> >
> > Reading what it is, maybe it can in fact be decoupled from NO_HZ_FULL.
> > Which would mean an upstream patch and backporting (if preempt would do
> > that, but could be considered a "performance bug" i guess)
> >
> > > And for CONFIG_RCU_NOCB_CPU
> > >
> > > Use this option to reduce OS jitter for aggressive HPC or
> > > real-time workloads. It can also be used to offload RCU
> > > callback invocation to energy-efficient CPUs in
> > > battery-powered asymmetric multiprocessors. The price of this
> > > reduced jitter is that the overhead of call_rcu() increases and that
> > > some workloads will incur significant increases in context-switch
> > > rates.
> > >
> > > This option offloads callback invocation from the set of
> > > CPUs specified at boot time by the rcu_nocbs parameter. For each
> > > such CPU, a kthread ("rcuox/N") will be created to invoke
> > > callbacks, where the "N" is the CPU being offloaded, and
> > > where the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
> > > RCU-sched (!PREEMPTION kernels). Nothing prevents this
> > > kthread from running on the specified CPUs, but (1) the kthreads may
> > > be preempted between each callback, and (2) affinity or cgroups can
> > > be used to force the kthreads to run on whatever set of
> > > CPUs is desired.
> > >
> > > Say Y here if you need reduced OS jitter, despite added
> > > overhead. Say N here if you are unsure.
> > >
> > > Adding as well overhead.
> > >
> > > Is this still to be considered true?
> >
> > probably but only for people that actively choose to use it and only
> > for the CPUs they choose. "rcu_nocbs" cmdline param, if not set
> > everything will be as it was.
> > I already indicated that in the commit message of my MR:
> > https://salsa.debian.org/kernel-team/linux/-/merge_requests/385
>
> Ok so the past traditional combo for a distro is:
>
>CONFIG_NO_HZ_IDLE=y
># CONFIG_RCU_NOCB_CPU is not set
>CONFIG_TICK_CPU_ACCOUNTING=y
>
> Then nohz_full support has been introduced which allows userspace
> tasks to run without being annoyed by tick interrupts. But ths feature
> is for extreme workloads. So we arranged for this support not to add
> additional overhead when it is not used.
>
> This means that
>
>CONFIG_NO_HZ_FULL=y
>
> which also automatically selects:
>
>CONFIG_RCU_NOCB_CPU=y
>CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
>
> are not expected to bring more overhead than their traditional
> counterparts, unless kernel boot parameters such as "nohz_full="
> or "rcu_nocbs=" are passed.
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
On Thu, Nov 04, 2021 at 10:05:02PM +0100, Henning Schild wrote:
> Am Sat, 30 Oct 2021 16:04:35 +0200
> schrieb Salvatore Bonaccorso :
>
> > Control: tags -1 + moreinfo
> >
> > On Wed, Oct 27, 2021 at 10:16:56AM +0200, Georg Müller wrote:
> > > > But for other configurations it is worse:
> > > >
> > > > config NO_HZ_FULL
> > > > bool "Full dynticks system (tickless)"
> > > > ...
> > > > This is implemented at the expense of some overhead in
> > > > user <-> kernel transitions: syscalls, exceptions and interrupts.
> > > > Even when it's dynamically off.
> > > >
> > > > Say N.
> > > >
> > >
> > >
> > > Upstream commit 176b8906 changed the description regarding
> > > NO_HZ_FULL:
> > >
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=176b8906c399a170886ea4bad5b24763c6713d61
> > >
> > >
> > > > By default, without passing the nohz_full parameter, this behaves
> > > > just like NO_HZ_IDLE.
> > > >
> > > > If you're a distro say Y.
> >
> > While this is changed, and distros encouraged to select it, selecting
> > this would enable both CONFIG_VIRT_CPU_ACCOUNTING_GEN and
> > CONFIG_RCU_NOCB_CPU.
> >
> > For CONFIG_VIRT_CPU_ACCOUNTING_GEN
> >
> > Select this option to enable task and CPU time accounting
> > on full dynticks systems. This accounting is implemented by watching
> > every kernel-user boundaries using the context tracking subsystem.
> > The accounting is thus performed at the expense of some
> > significant overhead.
> >
> > For now this is only useful if you are working on the full
> > dynticks subsystem development.
> >
> > If unsure, say N.
> >
> > which indicates some significant overhead.
>
> I can not answer that from the back of my head. Would have to dig as
> well. Might get back in about two weeks if nobody else finds an answer.
>
> But i took the liberty to include Frederic into Cc, the author of the
> "distro reassure" patch.
>
> Not sure such a change would be allowed for bullseye (5.10) and if the
> answer for 5.10 would be another than for i.e. 5.15
>
> Reading what it is, maybe it can in fact be decoupled from NO_HZ_FULL.
> Which would mean an upstream patch and backporting (if preempt would do
> that, but could be considered a "performance bug" i guess)
>
> > And for CONFIG_RCU_NOCB_CPU
> >
> > Use this option to reduce OS jitter for aggressive HPC or
> > real-time workloads. It can also be used to offload RCU
> > callback invocation to energy-efficient CPUs in
> > battery-powered asymmetric multiprocessors. The price of this
> > reduced jitter is that the overhead of call_rcu() increases and that
> > some workloads will incur significant increases in context-switch
> > rates.
> >
> > This option offloads callback invocation from the set of
> > CPUs specified at boot time by the rcu_nocbs parameter. For each
> > such CPU, a kthread ("rcuox/N") will be created to invoke
> > callbacks, where the "N" is the CPU being offloaded, and
> > where the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
> > RCU-sched (!PREEMPTION kernels). Nothing prevents this
> > kthread from running on the specified CPUs, but (1) the kthreads may
> > be preempted between each callback, and (2) affinity or cgroups can
> > be used to force the kthreads to run on whatever set of
> > CPUs is desired.
> >
> > Say Y here if you need reduced OS jitter, despite added
> > overhead. Say N here if you are unsure.
> >
> > Adding as well overhead.
> >
> > Is this still to be considered true?
>
> probably but only for people that actively choose to use it and only
> for the CPUs they choose. "rcu_nocbs" cmdline param, if not set
> everything will be as it was.
> I already indicated that in the commit message of my MR:
> https://salsa.debian.org/kernel-team/linux/-/merge_requests/385
Ok so the past traditional combo for a distro is:
CONFIG_NO_HZ_IDLE=y
# CONFIG_RCU_NOCB_CPU is not set
CONFIG_TICK_CPU_ACCOUNTING=y
Then nohz_full support has been introduced which allows userspace
tasks to run without being annoyed by tick interrupts. But ths feature
is for extreme workloads. So we arranged for this support not to add
additional overhead when it is not used.
This means that
CONFIG_NO_HZ_FULL=y
which also automatically selects:
CONFIG_RCU_NOCB_CPU=y
CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
are not expected to bring more overhead than their traditional
counterparts, unless kernel boot parameters such as "nohz_full="
or "rcu_nocbs=" are passed.
So you can safely enable CONFIG_NO_HZ_FULL=y. I guess the only corner
case is when you optimize your kernel for size and you are sure you
won't have any user of nohz_full for your kernel, but I suspect some
debian users, like me for example, might be interested in that feature.
I should clarify the help text for
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
Hi Henning,
On Thu, Nov 04, 2021 at 10:05:02PM +0100, Henning Schild wrote:
> Am Sat, 30 Oct 2021 16:04:35 +0200
> schrieb Salvatore Bonaccorso :
>
> > Control: tags -1 + moreinfo
> >
> > On Wed, Oct 27, 2021 at 10:16:56AM +0200, Georg Müller wrote:
> > > > But for other configurations it is worse:
> > > >
> > > > config NO_HZ_FULL
> > > > bool "Full dynticks system (tickless)"
> > > > ...
> > > > This is implemented at the expense of some overhead in
> > > > user <-> kernel transitions: syscalls, exceptions and interrupts.
> > > > Even when it's dynamically off.
> > > >
> > > > Say N.
> > > >
> > >
> > >
> > > Upstream commit 176b8906 changed the description regarding
> > > NO_HZ_FULL:
> > >
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=176b8906c399a170886ea4bad5b24763c6713d61
> > >
> > >
> > > > By default, without passing the nohz_full parameter, this behaves
> > > > just like NO_HZ_IDLE.
> > > >
> > > > If you're a distro say Y.
> >
> > While this is changed, and distros encouraged to select it, selecting
> > this would enable both CONFIG_VIRT_CPU_ACCOUNTING_GEN and
> > CONFIG_RCU_NOCB_CPU.
> >
> > For CONFIG_VIRT_CPU_ACCOUNTING_GEN
> >
> > Select this option to enable task and CPU time accounting
> > on full dynticks systems. This accounting is implemented by watching
> > every kernel-user boundaries using the context tracking subsystem.
> > The accounting is thus performed at the expense of some
> > significant overhead.
> >
> > For now this is only useful if you are working on the full
> > dynticks subsystem development.
> >
> > If unsure, say N.
> >
> > which indicates some significant overhead.
>
> I can not answer that from the back of my head. Would have to dig as
> well. Might get back in about two weeks if nobody else finds an answer.
>
> But i took the liberty to include Frederic into Cc, the author of the
> "distro reassure" patch.
Thanks, lets see what Frederic can comment on it.
> Not sure such a change would be allowed for bullseye (5.10) and if the
> answer for 5.10 would be another than for i.e. 5.15
>
> Reading what it is, maybe it can in fact be decoupled from NO_HZ_FULL.
> Which would mean an upstream patch and backporting (if preempt would do
> that, but could be considered a "performance bug" i guess)
It would for now defintively be anyway for the uploads following 5.15.
Enabling it now for the bullseye kernel is mostly out of scope I
think, unless not further discussed again. But the first aim here
would be to make it for unstable.
> > And for CONFIG_RCU_NOCB_CPU
> >
> > Use this option to reduce OS jitter for aggressive HPC or
> > real-time workloads. It can also be used to offload RCU
> > callback invocation to energy-efficient CPUs in
> > battery-powered asymmetric multiprocessors. The price of this
> > reduced jitter is that the overhead of call_rcu() increases and that
> > some workloads will incur significant increases in context-switch
> > rates.
> >
> > This option offloads callback invocation from the set of
> > CPUs specified at boot time by the rcu_nocbs parameter. For each
> > such CPU, a kthread ("rcuox/N") will be created to invoke
> > callbacks, where the "N" is the CPU being offloaded, and
> > where the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
> > RCU-sched (!PREEMPTION kernels). Nothing prevents this
> > kthread from running on the specified CPUs, but (1) the kthreads may
> > be preempted between each callback, and (2) affinity or cgroups can
> > be used to force the kthreads to run on whatever set of
> > CPUs is desired.
> >
> > Say Y here if you need reduced OS jitter, despite added
> > overhead. Say N here if you are unsure.
> >
> > Adding as well overhead.
> >
> > Is this still to be considered true?
>
> probably but only for people that actively choose to use it and only
> for the CPUs they choose. "rcu_nocbs" cmdline param, if not set
> everything will be as it was.
> I already indicated that in the commit message of my MR:
> https://salsa.debian.org/kernel-team/linux/-/merge_requests/385
Right, but that is not fully clear to me from the above if it's only
an overhead when as well only the parameter or if adds overhead
already by enabling the option as well.
Regards,
Salvatore
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
Am Sat, 30 Oct 2021 16:04:35 +0200
schrieb Salvatore Bonaccorso :
> Control: tags -1 + moreinfo
>
> On Wed, Oct 27, 2021 at 10:16:56AM +0200, Georg Müller wrote:
> > > But for other configurations it is worse:
> > >
> > > config NO_HZ_FULL
> > > bool "Full dynticks system (tickless)"
> > > ...
> > > This is implemented at the expense of some overhead in
> > > user <-> kernel transitions: syscalls, exceptions and interrupts.
> > > Even when it's dynamically off.
> > >
> > > Say N.
> > >
> >
> >
> > Upstream commit 176b8906 changed the description regarding
> > NO_HZ_FULL:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=176b8906c399a170886ea4bad5b24763c6713d61
> >
> >
> > > By default, without passing the nohz_full parameter, this behaves
> > > just like NO_HZ_IDLE.
> > >
> > > If you're a distro say Y.
>
> While this is changed, and distros encouraged to select it, selecting
> this would enable both CONFIG_VIRT_CPU_ACCOUNTING_GEN and
> CONFIG_RCU_NOCB_CPU.
>
> For CONFIG_VIRT_CPU_ACCOUNTING_GEN
>
> Select this option to enable task and CPU time accounting
> on full dynticks systems. This accounting is implemented by watching
> every kernel-user boundaries using the context tracking subsystem.
> The accounting is thus performed at the expense of some
> significant overhead.
>
> For now this is only useful if you are working on the full
> dynticks subsystem development.
>
> If unsure, say N.
>
> which indicates some significant overhead.
I can not answer that from the back of my head. Would have to dig as
well. Might get back in about two weeks if nobody else finds an answer.
But i took the liberty to include Frederic into Cc, the author of the
"distro reassure" patch.
Not sure such a change would be allowed for bullseye (5.10) and if the
answer for 5.10 would be another than for i.e. 5.15
Reading what it is, maybe it can in fact be decoupled from NO_HZ_FULL.
Which would mean an upstream patch and backporting (if preempt would do
that, but could be considered a "performance bug" i guess)
> And for CONFIG_RCU_NOCB_CPU
>
> Use this option to reduce OS jitter for aggressive HPC or
> real-time workloads. It can also be used to offload RCU
> callback invocation to energy-efficient CPUs in
> battery-powered asymmetric multiprocessors. The price of this
> reduced jitter is that the overhead of call_rcu() increases and that
> some workloads will incur significant increases in context-switch
> rates.
>
> This option offloads callback invocation from the set of
> CPUs specified at boot time by the rcu_nocbs parameter. For each
> such CPU, a kthread ("rcuox/N") will be created to invoke
> callbacks, where the "N" is the CPU being offloaded, and
> where the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
> RCU-sched (!PREEMPTION kernels). Nothing prevents this
> kthread from running on the specified CPUs, but (1) the kthreads may
> be preempted between each callback, and (2) affinity or cgroups can
> be used to force the kthreads to run on whatever set of
> CPUs is desired.
>
> Say Y here if you need reduced OS jitter, despite added
> overhead. Say N here if you are unsure.
>
> Adding as well overhead.
>
> Is this still to be considered true?
probably but only for people that actively choose to use it and only
for the CPUs they choose. "rcu_nocbs" cmdline param, if not set
everything will be as it was.
I already indicated that in the commit message of my MR:
https://salsa.debian.org/kernel-team/linux/-/merge_requests/385
regards,
Henning
>
> Regards,
> Salvatore
>
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
Control: tags -1 + moreinfo
On Wed, Oct 27, 2021 at 10:16:56AM +0200, Georg Müller wrote:
> > But for other configurations it is worse:
> >
> > config NO_HZ_FULL
> > bool "Full dynticks system (tickless)"
> > ...
> > This is implemented at the expense of some overhead in user <->
> > kernel
> > transitions: syscalls, exceptions and interrupts. Even when it's
> > dynamically off.
> >
> > Say N.
> >
>
>
> Upstream commit 176b8906 changed the description regarding NO_HZ_FULL:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=176b8906c399a170886ea4bad5b24763c6713d61
>
>
> > By default, without passing the nohz_full parameter, this behaves just
> > like NO_HZ_IDLE.
> >
> > If you're a distro say Y.
While this is changed, and distros encouraged to select it, selecting
this would enable both CONFIG_VIRT_CPU_ACCOUNTING_GEN and
CONFIG_RCU_NOCB_CPU.
For CONFIG_VIRT_CPU_ACCOUNTING_GEN
Select this option to enable task and CPU time accounting on full
dynticks systems. This accounting is implemented by watching every
kernel-user boundaries using the context tracking subsystem.
The accounting is thus performed at the expense of some significant
overhead.
For now this is only useful if you are working on the full
dynticks subsystem development.
If unsure, say N.
which indicates some significant overhead.
And for CONFIG_RCU_NOCB_CPU
Use this option to reduce OS jitter for aggressive HPC or
real-time workloads. It can also be used to offload RCU
callback invocation to energy-efficient CPUs in battery-powered
asymmetric multiprocessors. The price of this reduced jitter
is that the overhead of call_rcu() increases and that some
workloads will incur significant increases in context-switch
rates.
This option offloads callback invocation from the set of CPUs
specified at boot time by the rcu_nocbs parameter. For each
such CPU, a kthread ("rcuox/N") will be created to invoke
callbacks, where the "N" is the CPU being offloaded, and where
the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
from running on the specified CPUs, but (1) the kthreads may be
preempted between each callback, and (2) affinity or cgroups can
be used to force the kthreads to run on whatever set of CPUs is
desired.
Say Y here if you need reduced OS jitter, despite added overhead.
Say N here if you are unsure.
Adding as well overhead.
Is this still to be considered true?
Regards,
Salvatore
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
But for other configurations it is worse: config NO_HZ_FULL bool "Full dynticks system (tickless)" ... This is implemented at the expense of some overhead in user <-> kernel transitions: syscalls, exceptions and interrupts. Even when it's dynamically off. Say N. Upstream commit 176b8906 changed the description regarding NO_HZ_FULL: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=176b8906c399a170886ea4bad5b24763c6713d61 > By default, without passing the nohz_full parameter, this behaves just > like NO_HZ_IDLE. > > If you're a distro say Y.
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
Package: src:linux Followup-For: Bug #804857 Hi, I am just curious, and wanted to know if there is some progress on enableing NO_HZ_FULL in generic debian on most architectures? I wouldn't also mind separate kernel image with CONFIG_PREEMPT (full prempetion) and HZ=1000 for desktop / soft real time work (studio audio work, gaming, etc). Best regards, Witold -- System Information: Debian Release: buster/sid Architecture: amd64 (x86_64) Foreign Architectures: i386 Kernel: Linux 4.18.0-2-amd64 (SMP w/32 CPU cores) Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US.UTF-8 (charmap=UTF-8) Shell: /bin/sh linked to /usr/bin/dash Init: systemd (via /run/systemd/system) LSM: AppArmor: enabled
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
Dear Maintainer, Is it possible to enable this option for the real-time kernel image, like suggested one year ago ? It is a feature that is required for many real-time applications when a CPU is only dedicated to a process. Thank you, Mikael
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
Source: linux Severity: wishlist Dear Maintainer, Could you please enable kernel options CONFIG_NO_HZ_FULL, CONFIG_RCU_NOCB_CPU and CONFIG_RCU_NOCB_CPU_NONE. As explained in: https://www.kernel.org/doc/Documentation/timers/NO_HZ.txt This options: - Don't modify default kernel behavior - Enable kernel command line nohz_full to enable tickless cpus - Enable kernel command line rcu_nocbs to offload rcu callbacks The combination of these parameters + cpu-pinning of virtual machines greatly improve performance. It also reduces latency in these virtual machines as the host system doesn't try to use it. (if your cgroup is correctly made) These parameters are disabled in all debian kernels. These parameters are enabled for various linux distribution by default without problem. (rhel 7 being one of them) (note that rhel 7 uses CONFIG_RCU_NOCB_CPU_ALL and user can pin offload kthreads to a core) The usage of CONFIG_RCU_NOCB_CPU_NONE doesn't offload by default. But possibly this behavior is not the best. Quentin. -- System Information: Debian Release: stretch/sid APT prefers stable-updates APT policy: (500, 'stable-updates'), (500, 'unstable'), (500, 'testing'), (500, 'stable'), (500, 'oldstable'), (1, 'experimental') Architecture: amd64 (x86_64) Foreign Architectures: i386 Kernel: Linux 4.3.0-rc7-amd64 (SMP w/8 CPU cores) Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8) Shell: /bin/sh linked to /bin/dash Init: systemd (via /run/systemd/system)
Bug#804857: linux: New feature: enable CONFIG_NO_HZ_FULL and CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU_NONE
On Thu, 2015-11-12 at 14:13 +0100, Quentin Deldycke wrote: > Source: linux > Severity: wishlist > > Dear Maintainer, > > Could you please enable kernel options CONFIG_NO_HZ_FULL, > CONFIG_RCU_NOCB_CPU and CONFIG_RCU_NOCB_CPU_NONE. > > As explained in: https://www.kernel.org/doc/Documentation/timers/NO_HZ.txt > > This options: > - Don't modify default kernel behavior > - Enable kernel command line nohz_full to enable tickless cpus > - Enable kernel command line rcu_nocbs to offload rcu callbacks > > The combination of these parameters + cpu-pinning of virtual machines > greatly improve performance. > It also reduces latency in these virtual machines as the host system > doesn't try to use it. (if your cgroup is correctly made) [...] But for other configurations it is worse: config NO_HZ_FULL bool "Full dynticks system (tickless)" ... This is implemented at the expense of some overhead in user <-> kernel transitions: syscalls, exceptions and interrupts. Even when it's dynamically off. Say N. I think we should enable this for the rt featureset when it's next available, but not by default. Ben. -- Ben Hutchings Logic doesn't apply to the real world. - Marvin Minsky signature.asc Description: This is a digitally signed message part
