Re: [Xen-devel] Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
Dan Hecht wrote: Yes, and regardless of whether you run your periodic timer slower than HZ, calibrating time in a VM is always difficult due to the fact the kernel is time sharing the physical cpu. Why not just ask the underlying hypervisor? Upstream Xen does just that. I'm guessing we'll want something similar for KVM paravirt. -- Politics is the struggle between those who want to make their country the best in the world, and those who believe it already is. Each group calls the other unpatriotic. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Xen-devel] Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
On 02/16/2007 01:51 PM, Zachary Amsden wrote: Keir Fraser wrote: On 16/2/07 17:46, "Jeremy Fitzhardinge" <[EMAIL PROTECTED]> wrote: Keir Fraser wrote: This initial patchset does not include save/restore support anyway, so in fact it would be consistent to have CONFIG_PREEMPT configurable. I'm sure that we are going to have some nasty bugs to fix up as a result, but we can't fix them until we find them! Then we can convert our save/restore code to use the freezer before submitting it for inclusion. OK. So that makes the only config restriction the 100Hz ticks. We can extend the Xen timer interface quite easily and get rid of this one too. In fact it doesn't *much* matter if the CONFIG_HZ differs from the Xen ticker rate -- we modified the Linux timer ISR to handle timer interrupts at arbitrary times already. The only drawback is that jiffies updates in burts if CONFIG_HZ is higher than the actual tick rate, and this can affect some calibration constants and cause Linux to print out some weird values at start-of-day. That's why we'd very much like to get a get_cpu_speed paravirt-op implemented. I think this would be useful to work around these problems for Xen as well. Yes, and regardless of whether you run your periodic timer slower than HZ, calibrating time in a VM is always difficult due to the fact the kernel is time sharing the physical cpu. Why not just ask the underlying hypervisor? Really, Time is one of those areas where paravirtualization makes a lot of sense, no matter how much virtualization hardware you have crammed into your cpu - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Xen-devel] Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
Keir Fraser wrote: On 16/2/07 17:46, "Jeremy Fitzhardinge" <[EMAIL PROTECTED]> wrote: Keir Fraser wrote: This initial patchset does not include save/restore support anyway, so in fact it would be consistent to have CONFIG_PREEMPT configurable. I'm sure that we are going to have some nasty bugs to fix up as a result, but we can't fix them until we find them! Then we can convert our save/restore code to use the freezer before submitting it for inclusion. OK. So that makes the only config restriction the 100Hz ticks. We can extend the Xen timer interface quite easily and get rid of this one too. In fact it doesn't *much* matter if the CONFIG_HZ differs from the Xen ticker rate -- we modified the Linux timer ISR to handle timer interrupts at arbitrary times already. The only drawback is that jiffies updates in burts if CONFIG_HZ is higher than the actual tick rate, and this can affect some calibration constants and cause Linux to print out some weird values at start-of-day. That's why we'd very much like to get a get_cpu_speed paravirt-op implemented. I think this would be useful to work around these problems for Xen as well. Zach - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Xen-devel] Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
Keir Fraser wrote: > We can extend the Xen timer interface quite easily and get rid of this one > too. In fact it doesn't *much* matter if the CONFIG_HZ differs from the Xen > ticker rate -- we modified the Linux timer ISR to handle timer interrupts at > arbitrary times already. The only drawback is that jiffies updates in burts > if CONFIG_HZ is higher than the actual tick rate, and this can affect some > calibration constants and cause Linux to print out some weird values at > start-of-day. > I was wondering what would happen if we tried that. I guess its equivalent to having stolen time between each Xen tick. J - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Xen-devel] Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
On 16/2/07 17:46, "Jeremy Fitzhardinge" <[EMAIL PROTECTED]> wrote: > Keir Fraser wrote: >> This initial patchset does not include save/restore support anyway, so in >> fact it would be consistent to have CONFIG_PREEMPT configurable. I'm sure >> that we are going to have some nasty bugs to fix up as a result, but we >> can't fix them until we find them! Then we can convert our save/restore code >> to use the freezer before submitting it for inclusion. > OK. So that makes the only config restriction the 100Hz ticks. We can extend the Xen timer interface quite easily and get rid of this one too. In fact it doesn't *much* matter if the CONFIG_HZ differs from the Xen ticker rate -- we modified the Linux timer ISR to handle timer interrupts at arbitrary times already. The only drawback is that jiffies updates in burts if CONFIG_HZ is higher than the actual tick rate, and this can affect some calibration constants and cause Linux to print out some weird values at start-of-day. -- Keir - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Xen-devel] Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
Keir Fraser wrote: > This initial patchset does not include save/restore support anyway, so in > fact it would be consistent to have CONFIG_PREEMPT configurable. I'm sure > that we are going to have some nasty bugs to fix up as a result, but we > can't fix them until we find them! Then we can convert our save/restore code > to use the freezer before submitting it for inclusion. OK. So that makes the only config restriction the 100Hz ticks. J - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
On 16/2/07 10:54, "Andrew Morton" <[EMAIL PROTECTED]> wrote: > OK, you're screwed. I agree that the process freezer is the way out of that > one. > > Ingo said that he's clocked the freezer at a few milliseconds. But if it's > any higher than that it'll need to get sped up once we convert cpu hotplug > to use it. A few milliseconds is acceptable. Presumably that is under some stress workload? What we'd really like is to be able to gang-hotplug CPUs to amortise the freezer overhead. This initial patchset does not include save/restore support anyway, so in fact it would be consistent to have CONFIG_PREEMPT configurable. I'm sure that we are going to have some nasty bugs to fix up as a result, but we can't fix them until we find them! Then we can convert our save/restore code to use the freezer before submitting it for inclusion. -- Keir - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
On Fri, 16 Feb 2007 10:47:11 + Keir Fraser <[EMAIL PROTECTED]> wrote:
> On 16/2/07 10:09, "Andrew Morton" <[EMAIL PROTECTED]> wrote:
>
> > Are the places where the domU code references machine addresses splattered
> > all over the code? If not, they can just be wrapped with
> > preempt_disable/preempt_enable?
>
> The main places where machine addresses are 'visible' are any code that
> holds a pte_t,pmd_t,pud_t,pgd_t. We hide the machine-to-pseudophysical and
> pseudophysical-to-machine translations inside e.g., pte_val() and __pte()
> (i.e., constructors and extractors for page table entries). Obviously the
> users of these macros are open coded all over the place, quite apart from
> the performance cost of sprinkling preempt_{enable,disable} so liberally.
OK, you're screwed. I agree that the process freezer is the way out of that
one.
Ingo said that he's clocked the freezer at a few milliseconds. But if it's
any higher than that it'll need to get sped up once we convert cpu hotplug
to use it.
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Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
On 16/2/07 10:09, "Andrew Morton" <[EMAIL PROTECTED]> wrote:
> Are the places where the domU code references machine addresses splattered
> all over the code? If not, they can just be wrapped with
> preempt_disable/preempt_enable?
The main places where machine addresses are 'visible' are any code that
holds a pte_t,pmd_t,pud_t,pgd_t. We hide the machine-to-pseudophysical and
pseudophysical-to-machine translations inside e.g., pte_val() and __pte()
(i.e., constructors and extractors for page table entries). Obviously the
users of these macros are open coded all over the place, quite apart from
the performance cost of sprinkling preempt_{enable,disable} so liberally.
-- Keir
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RE: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
Jeremy Fitzhardinge wrote: > Andrew Morton wrote: > > On Thu, 15 Feb 2007 22:14:45 -0800 Dan Hecht > <[EMAIL PROTECTED]> wrote: > > > > > >>> config PREEMPT > >>> bool "Preemptible Kernel (Low-Latency Desktop)" > >>> + depends on !XEN > >>> help > >>> This option reduces the latency of the kernel by making > >>> all kernel code (that is not executing in a critical section) > >>> > >>> > > > > Oh, so that's why it doesn't break when CONFIG_PREEMPT=y. > In which case > > that preempt_disable() I spotted is wrong-and-unneeded. > > > > Why doesn't Xen work with preemption?? > > > > I've forgotten the details. Ian? Keir? Steven? Maybe it > can be done. With CONFIG_PREEMPT, we can have preempted threads reference machine addresses across save/restore. After restore, these machine addresses will be incorrect. Christian - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [patch 14/21] Xen-paravirt: Add XEN config options and disableunsupported config options.
On Fri, 16 Feb 2007 02:00:39 -0800 "Christian Limpach" <[EMAIL PROTECTED]> wrote: > Jeremy Fitzhardinge wrote: > > Andrew Morton wrote: > > > On Thu, 15 Feb 2007 22:14:45 -0800 Dan Hecht > > <[EMAIL PROTECTED]> wrote: > > > > > > > > >>> config PREEMPT > > >>> bool "Preemptible Kernel (Low-Latency Desktop)" > > >>> + depends on !XEN > > >>> help > > >>> This option reduces the latency of the kernel by making > > >>> all kernel code (that is not executing in a critical section) > > >>> > > >>> > > > > > > Oh, so that's why it doesn't break when CONFIG_PREEMPT=y. > > In which case > > > that preempt_disable() I spotted is wrong-and-unneeded. > > > > > > Why doesn't Xen work with preemption?? > > > > > > > I've forgotten the details. Ian? Keir? Steven? Maybe it > > can be done. > > With CONFIG_PREEMPT, we can have preempted threads reference machine > addresses across save/restore. After restore, these machine addresses > will be incorrect. > It would help if you could define the terms "save/restore" and "machine addresses". One can guess, but there are probably subtleties here.. Are the places where the domU code references machine addresses splattered all over the code? If not, they can just be wrapped with preempt_disable/preempt_enable? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
