回复: 回复: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
发件人: Uladzislau Rezki
发送时间: 2021年2月4日 22:09
收件人: Zhang, Qiang
抄送: Uladzislau Rezki; paul...@kernel.org; j...@joelfernandes.org;
r...@vger.kernel.org; linux-kernel@vger.kernel.org
主题: Re: 回复: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
[Please note: This e-mail is from an EXTERNAL e-mail address]
> 发件人: Uladzislau Rezki
> 发送时间: 2021年2月2日 3:57
> 收件人: Zhang, Qiang
> 抄送: ure...@gmail.com; paul...@kernel.org; j...@joelfernandes.org;
> r...@vger.kernel.org; linux-kernel@vger.kernel.org
> 主题: Re: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
>
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> Hello, Zqiang.
>
> > From: Zqiang
> >
> > Add free per-cpu existing krcp's page cache operation, when
> > the system is under memory pressure.
> >
> > Signed-off-by: Zqiang
> > ---
> > kernel/rcu/tree.c | 26 ++
> > 1 file changed, 26 insertions(+)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index c1ae1e52f638..644b0f3c7b9f 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3571,17 +3571,41 @@ void kvfree_call_rcu(struct rcu_head *head,
> > rcu_callback_t func)
> > }
> > EXPORT_SYMBOL_GPL(kvfree_call_rcu);
> >
> > +static int free_krc_page_cache(struct kfree_rcu_cpu *krcp)
> > +{
> > + unsigned long flags;
> > + struct llist_node *page_list, *pos, *n;
> > + int freed = 0;
> > +
> > + raw_spin_lock_irqsave(>lock, flags);
> > + page_list = llist_del_all(>bkvcache);
> > + krcp->nr_bkv_objs = 0;
> > + raw_spin_unlock_irqrestore(>lock, flags);
> > +
> > + llist_for_each_safe(pos, n, page_list) {
> > + free_page((unsigned long)pos);
> > + freed++;
> > + }
> > +
> > + return freed;
> > +}
> > +
> > static unsigned long
> > kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> > {
> > int cpu;
> > unsigned long count = 0;
> > + unsigned long flags;
> >
> > /* Snapshot count of all CPUs */
> > for_each_possible_cpu(cpu) {
> > struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
> >
> > count += READ_ONCE(krcp->count);
> > +
> > + raw_spin_lock_irqsave(>lock, flags);
> > + count += krcp->nr_bkv_objs;
> > + raw_spin_unlock_irqrestore(>lock, flags);
> > }
> >
> > return count;
> > @@ -3598,6 +3622,8 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct
> > shrink_control *sc)
> > struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
> >
> > count = krcp->count;
> > + count += free_krc_page_cache(krcp);
> > +
> > raw_spin_lock_irqsave(>lock, flags);
> > if (krcp->monitor_todo)
> > kfree_rcu_drain_unlock(krcp, flags);
> > --
> > 2.17.1
> >>
> >Thank you for your patch!
> >
> >I spent some time to see how the patch behaves under low memory condition.
> >To simulate it, i used "rcuscale" tool with below parameters:
> >
> >../rcutorture/bin/kvm.sh --torture rcuscale --allcpus --duration 10
> >--kconfig >CONFIG_NR_CPUS=64 \
> >--bootargs "rcuscale.kfree_rcu_test=1 rcuscale.kfree_nthreads=16
> >>rcuscale.holdoff=20 rcuscale.kfree_loops=1 \
> >torture.disable_onoff_at_boot" --trust-make
> >
> >64 CPUs + 512 MB of memory. In general, my test system was running on edge
> >hitting an out of memory sometimes, but could be considered as stable in
> >regards to a test completion and taken time, so both were pretty solid.
> >
> >You can find a comparison on a plot, that can be downloaded following
> >a link: wget
> >>ftp://vps418301.ovh.net/incoming/release_page_cache_under_low_memory.png
> >
> >In short, i see that a patched version can lead to longer test completion,
> >whereas the default variant is stable on almost all runs. After some analysis
> >and further digging i came to conclusion that a shrinker
> >free_krc_page_cache()
> >concurs with run_page_cache_worker(krcp) running from kvfree_rcu() context.
> >
> >i.e. During the test a page shrinker is pretty active, because of low memory
> >condition. Our callback drains it whereas kvfree_rcu() part refill it right
> >away making kind of vicious circle.
> >
> >
Re: 回复: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
> 发件人: Uladzislau Rezki
> 发送时间: 2021年2月2日 3:57
> 收件人: Zhang, Qiang
> 抄送: ure...@gmail.com; paul...@kernel.org; j...@joelfernandes.org;
> r...@vger.kernel.org; linux-kernel@vger.kernel.org
> 主题: Re: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
>
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> Hello, Zqiang.
>
> > From: Zqiang
> >
> > Add free per-cpu existing krcp's page cache operation, when
> > the system is under memory pressure.
> >
> > Signed-off-by: Zqiang
> > ---
> > kernel/rcu/tree.c | 26 ++
> > 1 file changed, 26 insertions(+)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index c1ae1e52f638..644b0f3c7b9f 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3571,17 +3571,41 @@ void kvfree_call_rcu(struct rcu_head *head,
> > rcu_callback_t func)
> > }
> > EXPORT_SYMBOL_GPL(kvfree_call_rcu);
> >
> > +static int free_krc_page_cache(struct kfree_rcu_cpu *krcp)
> > +{
> > + unsigned long flags;
> > + struct llist_node *page_list, *pos, *n;
> > + int freed = 0;
> > +
> > + raw_spin_lock_irqsave(>lock, flags);
> > + page_list = llist_del_all(>bkvcache);
> > + krcp->nr_bkv_objs = 0;
> > + raw_spin_unlock_irqrestore(>lock, flags);
> > +
> > + llist_for_each_safe(pos, n, page_list) {
> > + free_page((unsigned long)pos);
> > + freed++;
> > + }
> > +
> > + return freed;
> > +}
> > +
> > static unsigned long
> > kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> > {
> > int cpu;
> > unsigned long count = 0;
> > + unsigned long flags;
> >
> > /* Snapshot count of all CPUs */
> > for_each_possible_cpu(cpu) {
> > struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
> >
> > count += READ_ONCE(krcp->count);
> > +
> > + raw_spin_lock_irqsave(>lock, flags);
> > + count += krcp->nr_bkv_objs;
> > + raw_spin_unlock_irqrestore(>lock, flags);
> > }
> >
> > return count;
> > @@ -3598,6 +3622,8 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct
> > shrink_control *sc)
> > struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
> >
> > count = krcp->count;
> > + count += free_krc_page_cache(krcp);
> > +
> > raw_spin_lock_irqsave(>lock, flags);
> > if (krcp->monitor_todo)
> > kfree_rcu_drain_unlock(krcp, flags);
> > --
> > 2.17.1
> >>
> >Thank you for your patch!
> >
> >I spent some time to see how the patch behaves under low memory condition.
> >To simulate it, i used "rcuscale" tool with below parameters:
> >
> >../rcutorture/bin/kvm.sh --torture rcuscale --allcpus --duration 10
> >--kconfig >CONFIG_NR_CPUS=64 \
> >--bootargs "rcuscale.kfree_rcu_test=1 rcuscale.kfree_nthreads=16
> >>rcuscale.holdoff=20 rcuscale.kfree_loops=1 \
> >torture.disable_onoff_at_boot" --trust-make
> >
> >64 CPUs + 512 MB of memory. In general, my test system was running on edge
> >hitting an out of memory sometimes, but could be considered as stable in
> >regards to a test completion and taken time, so both were pretty solid.
> >
> >You can find a comparison on a plot, that can be downloaded following
> >a link: wget
> >>ftp://vps418301.ovh.net/incoming/release_page_cache_under_low_memory.png
> >
> >In short, i see that a patched version can lead to longer test completion,
> >whereas the default variant is stable on almost all runs. After some analysis
> >and further digging i came to conclusion that a shrinker
> >free_krc_page_cache()
> >concurs with run_page_cache_worker(krcp) running from kvfree_rcu() context.
> >
> >i.e. During the test a page shrinker is pretty active, because of low memory
> >condition. Our callback drains it whereas kvfree_rcu() part refill it right
> >away making kind of vicious circle.
> >
> >So, a run_page_cache_worker() should be backoff for some time when a system
> >runs into a low memory condition or high pressure:
> >
> >diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >index 7077d73fcb53..446723b9646b 100644
> >--- a/kernel/rcu/tree.c
> >+++ b/kernel/rcu/tree.c
> >@@ -3163,7 +3163,7 @@ struct kfree_rcu_cpu {
> >bool initialized;
> >int count;
> >
> >- struct work_struct page_cache_work;
> >+ struct delayed_work page_cache_work;
> >atomic_t work_in_progress;
> >struct hrtimer hrtimer;
> >
> >@@ -3419,7 +3419,7 @@ schedule_page_work_fn(struct hrtimer *t)
> >struct kfree_rcu_cpu *krcp =
> >container_of(t, struct kfree_rcu_cpu, hrtimer);
> >
> >- queue_work(system_highpri_wq, >page_cache_work);
> >+ queue_delayed_work(system_highpri_wq, >page_cache_work, 0);
> >return HRTIMER_NORESTART;
> > }
> >
> >@@ -3428,7 +3428,7 @@ static void fill_page_cache_func(struct work_struct
> >*work)
> >struct kvfree_rcu_bulk_data *bnode;
> >
回复: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
发件人: Uladzislau Rezki
发送时间: 2021年2月2日 3:57
收件人: Zhang, Qiang
抄送: ure...@gmail.com; paul...@kernel.org; j...@joelfernandes.org;
r...@vger.kernel.org; linux-kernel@vger.kernel.org
主题: Re: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
[Please note: This e-mail is from an EXTERNAL e-mail address]
Hello, Zqiang.
> From: Zqiang
>
> Add free per-cpu existing krcp's page cache operation, when
> the system is under memory pressure.
>
> Signed-off-by: Zqiang
> ---
> kernel/rcu/tree.c | 26 ++
> 1 file changed, 26 insertions(+)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c1ae1e52f638..644b0f3c7b9f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3571,17 +3571,41 @@ void kvfree_call_rcu(struct rcu_head *head,
> rcu_callback_t func)
> }
> EXPORT_SYMBOL_GPL(kvfree_call_rcu);
>
> +static int free_krc_page_cache(struct kfree_rcu_cpu *krcp)
> +{
> + unsigned long flags;
> + struct llist_node *page_list, *pos, *n;
> + int freed = 0;
> +
> + raw_spin_lock_irqsave(>lock, flags);
> + page_list = llist_del_all(>bkvcache);
> + krcp->nr_bkv_objs = 0;
> + raw_spin_unlock_irqrestore(>lock, flags);
> +
> + llist_for_each_safe(pos, n, page_list) {
> + free_page((unsigned long)pos);
> + freed++;
> + }
> +
> + return freed;
> +}
> +
> static unsigned long
> kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> {
> int cpu;
> unsigned long count = 0;
> + unsigned long flags;
>
> /* Snapshot count of all CPUs */
> for_each_possible_cpu(cpu) {
> struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
>
> count += READ_ONCE(krcp->count);
> +
> + raw_spin_lock_irqsave(>lock, flags);
> + count += krcp->nr_bkv_objs;
> + raw_spin_unlock_irqrestore(>lock, flags);
> }
>
> return count;
> @@ -3598,6 +3622,8 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct
> shrink_control *sc)
> struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
>
> count = krcp->count;
> + count += free_krc_page_cache(krcp);
> +
> raw_spin_lock_irqsave(>lock, flags);
> if (krcp->monitor_todo)
> kfree_rcu_drain_unlock(krcp, flags);
> --
> 2.17.1
>>
>Thank you for your patch!
>
>I spent some time to see how the patch behaves under low memory condition.
>To simulate it, i used "rcuscale" tool with below parameters:
>
>../rcutorture/bin/kvm.sh --torture rcuscale --allcpus --duration 10 --kconfig
>>CONFIG_NR_CPUS=64 \
>--bootargs "rcuscale.kfree_rcu_test=1 rcuscale.kfree_nthreads=16
>>rcuscale.holdoff=20 rcuscale.kfree_loops=1 \
>torture.disable_onoff_at_boot" --trust-make
>
>64 CPUs + 512 MB of memory. In general, my test system was running on edge
>hitting an out of memory sometimes, but could be considered as stable in
>regards to a test completion and taken time, so both were pretty solid.
>
>You can find a comparison on a plot, that can be downloaded following
>a link: wget
>>ftp://vps418301.ovh.net/incoming/release_page_cache_under_low_memory.png
>
>In short, i see that a patched version can lead to longer test completion,
>whereas the default variant is stable on almost all runs. After some analysis
>and further digging i came to conclusion that a shrinker free_krc_page_cache()
>concurs with run_page_cache_worker(krcp) running from kvfree_rcu() context.
>
>i.e. During the test a page shrinker is pretty active, because of low memory
>condition. Our callback drains it whereas kvfree_rcu() part refill it right
>away making kind of vicious circle.
>
>So, a run_page_cache_worker() should be backoff for some time when a system
>runs into a low memory condition or high pressure:
>
>diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>index 7077d73fcb53..446723b9646b 100644
>--- a/kernel/rcu/tree.c
>+++ b/kernel/rcu/tree.c
>@@ -3163,7 +3163,7 @@ struct kfree_rcu_cpu {
>bool initialized;
>int count;
>
>- struct work_struct page_cache_work;
>+ struct delayed_work page_cache_work;
>atomic_t work_in_progress;
>struct hrtimer hrtimer;
>
>@@ -3419,7 +3419,7 @@ schedule_page_work_fn(struct hrtimer *t)
>struct kfree_rcu_cpu *krcp =
>container_of(t, struct kfree_rcu_cpu, hrtimer);
>
>- queue_work(system_highpri_wq, >page_cache_work);
>+ queue_delayed_work(system_highpri_wq, >page_cache_work, 0);
>return HRTIMER_NORESTART;
> }
>
>@@ -3428,7 +3428,7 @@ static void fill_page_cache_func(struct work_struct
>*work)
>struct kvfree_rcu_bulk_data *bnode;
>struct kfree_rcu_cpu *krcp =
>container_of(work, struct kfree_rcu_cpu,
>- page_cache_work);
>+ page_cache_work.work);
>unsigned long flags;
>bool pushed;
回复: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
发件人: Uladzislau Rezki
发送时间: 2021年2月2日 3:57
收件人: Zhang, Qiang
抄送: ure...@gmail.com; paul...@kernel.org; j...@joelfernandes.org;
r...@vger.kernel.org; linux-kernel@vger.kernel.org
主题: Re: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
[Please note: This e-mail is from an EXTERNAL e-mail address]
Hello, Zqiang.
> From: Zqiang
>
> Add free per-cpu existing krcp's page cache operation, when
> the system is under memory pressure.
>
> Signed-off-by: Zqiang
> ---
> kernel/rcu/tree.c | 26 ++
> 1 file changed, 26 insertions(+)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c1ae1e52f638..644b0f3c7b9f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3571,17 +3571,41 @@ void kvfree_call_rcu(struct rcu_head *head,
> rcu_callback_t func)
> }
> EXPORT_SYMBOL_GPL(kvfree_call_rcu);
>
> +static int free_krc_page_cache(struct kfree_rcu_cpu *krcp)
> +{
> + unsigned long flags;
> + struct llist_node *page_list, *pos, *n;
> + int freed = 0;
> +
> + raw_spin_lock_irqsave(>lock, flags);
> + page_list = llist_del_all(>bkvcache);
> + krcp->nr_bkv_objs = 0;
> + raw_spin_unlock_irqrestore(>lock, flags);
> +
> + llist_for_each_safe(pos, n, page_list) {
> + free_page((unsigned long)pos);
> + freed++;
> + }
> +
> + return freed;
> +}
> +
> static unsigned long
> kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> {
> int cpu;
> unsigned long count = 0;
> + unsigned long flags;
>
> /* Snapshot count of all CPUs */
> for_each_possible_cpu(cpu) {
> struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
>
> count += READ_ONCE(krcp->count);
> +
> + raw_spin_lock_irqsave(>lock, flags);
> + count += krcp->nr_bkv_objs;
> + raw_spin_unlock_irqrestore(>lock, flags);
> }
>
> return count;
> @@ -3598,6 +3622,8 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct
> shrink_control *sc)
> struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
>
> count = krcp->count;
> + count += free_krc_page_cache(krcp);
> +
> raw_spin_lock_irqsave(>lock, flags);
> if (krcp->monitor_todo)
> kfree_rcu_drain_unlock(krcp, flags);
> --
> 2.17.1
>>
>Thank you for your patch!
>
>I spent some time to see how the patch behaves under low memory condition.
>To simulate it, i used "rcuscale" tool with below parameters:
>
>../rcutorture/bin/kvm.sh --torture rcuscale --allcpus --duration 10 --kconfig
>>CONFIG_NR_CPUS=64 \
>--bootargs "rcuscale.kfree_rcu_test=1 rcuscale.kfree_nthreads=16
>>rcuscale.holdoff=20 rcuscale.kfree_loops=1 \
>torture.disable_onoff_at_boot" --trust-make
>
>64 CPUs + 512 MB of memory. In general, my test system was running on edge
>hitting an out of memory sometimes, but could be considered as stable in
>regards to a test completion and taken time, so both were pretty solid.
>
>You can find a comparison on a plot, that can be downloaded following
>a link: wget
>>ftp://vps418301.ovh.net/incoming/release_page_cache_under_low_memory.png
>
>In short, i see that a patched version can lead to longer test completion,
>whereas the default variant is stable on almost all runs. After some analysis
>and further digging i came to conclusion that a shrinker free_krc_page_cache()
>concurs with run_page_cache_worker(krcp) running from kvfree_rcu() context.
>
>i.e. During the test a page shrinker is pretty active, because of low memory
>condition. Our callback drains it whereas kvfree_rcu() part refill it right
>away making kind of vicious circle.
>
>So, a run_page_cache_worker() should be backoff for some time when a system
>runs into a low memory condition or high pressure:
>
>diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>index 7077d73fcb53..446723b9646b 100644
>--- a/kernel/rcu/tree.c
>+++ b/kernel/rcu/tree.c
>@@ -3163,7 +3163,7 @@ struct kfree_rcu_cpu {
>bool initialized;
>int count;
>
>- struct work_struct page_cache_work;
>+ struct delayed_work page_cache_work;
>atomic_t work_in_progress;
>struct hrtimer hrtimer;
>
>@@ -3419,7 +3419,7 @@ schedule_page_work_fn(struct hrtimer *t)
>struct kfree_rcu_cpu *krcp =
>container_of(t, struct kfree_rcu_cpu, hrtimer);
>
>- queue_work(system_highpri_wq, >page_cache_work);
>+ queue_delayed_work(system_highpri_wq, >page_cache_work, 0);
>return HRTIMER_NORESTART;
> }
>
>@@ -3428,7 +3428,7 @@ static void fill_page_cache_func(struct work_struct
>*work)
>struct kvfree_rcu_bulk_data *bnode;
>struct kfree_rcu_cpu *krcp =
>container_of(work, struct kfree_rcu_cpu,
>- page_cache_work);
>+ page_cache_work.work);
>unsigned long flags;
>bool pushed;
>
回复: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
发件人: Uladzislau Rezki
发送时间: 2021年2月2日 3:57
收件人: Zhang, Qiang
抄送: ure...@gmail.com; paul...@kernel.org; j...@joelfernandes.org;
r...@vger.kernel.org; linux-kernel@vger.kernel.org
主题: Re: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
[Please note: This e-mail is from an EXTERNAL e-mail address]
Hello, Zqiang.
> From: Zqiang
>
> Add free per-cpu existing krcp's page cache operation, when
> the system is under memory pressure.
>
> Signed-off-by: Zqiang
> ---
> kernel/rcu/tree.c | 26 ++
> 1 file changed, 26 insertions(+)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c1ae1e52f638..644b0f3c7b9f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3571,17 +3571,41 @@ void kvfree_call_rcu(struct rcu_head *head,
> rcu_callback_t func)
> }
> EXPORT_SYMBOL_GPL(kvfree_call_rcu);
>
> +static int free_krc_page_cache(struct kfree_rcu_cpu *krcp)
> +{
> + unsigned long flags;
> + struct llist_node *page_list, *pos, *n;
> + int freed = 0;
> +
> + raw_spin_lock_irqsave(>lock, flags);
> + page_list = llist_del_all(>bkvcache);
> + krcp->nr_bkv_objs = 0;
> + raw_spin_unlock_irqrestore(>lock, flags);
> +
> + llist_for_each_safe(pos, n, page_list) {
> + free_page((unsigned long)pos);
> + freed++;
> + }
> +
> + return freed;
> +}
> +
> static unsigned long
> kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> {
> int cpu;
> unsigned long count = 0;
> + unsigned long flags;
>
> /* Snapshot count of all CPUs */
> for_each_possible_cpu(cpu) {
> struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
>
> count += READ_ONCE(krcp->count);
> +
> + raw_spin_lock_irqsave(>lock, flags);
> + count += krcp->nr_bkv_objs;
> + raw_spin_unlock_irqrestore(>lock, flags);
> }
>
> return count;
> @@ -3598,6 +3622,8 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct
> shrink_control *sc)
> struct kfree_rcu_cpu *krcp = per_cpu_ptr(, cpu);
>
> count = krcp->count;
> + count += free_krc_page_cache(krcp);
> +
> raw_spin_lock_irqsave(>lock, flags);
> if (krcp->monitor_todo)
> kfree_rcu_drain_unlock(krcp, flags);
> --
> 2.17.1
>>
>Thank you for your patch!
>
>I spent some time to see how the patch behaves under low memory condition.
>To simulate it, i used "rcuscale" tool with below parameters:
>
>../rcutorture/bin/kvm.sh --torture rcuscale --allcpus --duration 10 --kconfig
>>CONFIG_NR_CPUS=64 \
>--bootargs "rcuscale.kfree_rcu_test=1 rcuscale.kfree_nthreads=16
>>rcuscale.holdoff=20 rcuscale.kfree_loops=1 \
>torture.disable_onoff_at_boot" --trust-make
>
>64 CPUs + 512 MB of memory. In general, my test system was running on edge
>hitting an out of memory sometimes, but could be considered as stable in
>regards to a test completion and taken time, so both were pretty solid.
>
>You can find a comparison on a plot, that can be downloaded following
>a link: wget
>>ftp://vps418301.ovh.net/incoming/release_page_cache_under_low_memory.png
>
>In short, i see that a patched version can lead to longer test completion,
>whereas the default variant is stable on almost all runs. After some analysis
>and further digging i came to conclusion that a shrinker free_krc_page_cache()
>concurs with run_page_cache_worker(krcp) running from kvfree_rcu() context.
>
>i.e. During the test a page shrinker is pretty active, because of low memory
>condition. Our callback drains it whereas kvfree_rcu() part refill it right
>away making kind of vicious circle.
>
>So, a run_page_cache_worker() should be backoff for some time when a system
>runs into a low memory condition or high pressure:
>
>diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>index 7077d73fcb53..446723b9646b 100644
>--- a/kernel/rcu/tree.c
>+++ b/kernel/rcu/tree.c
>@@ -3163,7 +3163,7 @@ struct kfree_rcu_cpu {
>bool initialized;
>int count;
>
>- struct work_struct page_cache_work;
>+ struct delayed_work page_cache_work;
>atomic_t work_in_progress;
>struct hrtimer hrtimer;
>
>@@ -3419,7 +3419,7 @@ schedule_page_work_fn(struct hrtimer *t)
>struct kfree_rcu_cpu *krcp =
>container_of(t, struct kfree_rcu_cpu, hrtimer);
>
>- queue_work(system_highpri_wq, >page_cache_work);
>+ queue_delayed_work(system_highpri_wq, >page_cache_work, 0);
>return HRTIMER_NORESTART;
> }
>
>@@ -3428,7 +3428,7 @@ static void fill_page_cache_func(struct work_struct
>*work)
>struct kvfree_rcu_bulk_data *bnode;
>struct kfree_rcu_cpu *krcp =
>container_of(work, struct kfree_rcu_cpu,
>- page_cache_work);
>+ page_cache_work.work);
>unsigned long flags;
>bool pushed;
>
