回复: 回复: [PATCH v3] kvfree_rcu: Release page cache under memory pressure
From: Zhang, Qiang
Date: Sat Feb 06 2021 - 02:27:25 EST
________________________________________
发件人: Uladzislau Rezki <urezki@xxxxxxxxx>
发送时间: 2021年2月4日 22:09
收件人: Zhang, Qiang
抄送: Uladzislau Rezki; paulmck@xxxxxxxxxx; joel@xxxxxxxxxxxxxxxxx; rcu@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
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 <urezki@xxxxxxxxx>
> 发送时间: 2021年2月2日 3:57
> 收件人: Zhang, Qiang
> 抄送: urezki@xxxxxxxxx; paulmck@xxxxxxxxxx; joel@xxxxxxxxxxxxxxxxx; rcu@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> 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 <qiang.zhang@xxxxxxxxxxxxx>
> >
> > Add free per-cpu existing krcp's page cache operation, when
> > the system is under memory pressure.
> >
> > Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
> > ---
> > 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(&krcp->lock, flags);
> > + page_list = llist_del_all(&krcp->bkvcache);
> > + krcp->nr_bkv_objs = 0;
> > + raw_spin_unlock_irqrestore(&krcp->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(&krc, cpu);
> >
> > count += READ_ONCE(krcp->count);
> > +
> > + raw_spin_lock_irqsave(&krcp->lock, flags);
> > + count += krcp->nr_bkv_objs;
> > + raw_spin_unlock_irqrestore(&krcp->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(&krc, cpu);
> >
> > count = krcp->count;
> > + count += free_krc_page_cache(krcp);
> > +
> > raw_spin_lock_irqsave(&krcp->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=10000 \
> >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, &krcp->page_cache_work);
> >+ queue_delayed_work(system_highpri_wq, &krcp->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;
> > int i;
> >@@ -3452,15 +3452,22 @@ static void fill_page_cache_func(struct work_struct >*work)
> > atomic_set(&krcp->work_in_progress, 0);
> > }
> >
> >+static bool backoff_page_cache_fill;
> >+
> > static void
> > run_page_cache_worker(struct kfree_rcu_cpu *krcp)
> > {
> > if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
> > !atomic_xchg(&krcp->work_in_progress, 1)) {
> >- hrtimer_init(&krcp->hrtimer, CLOCK_MONOTONIC,
> >- HRTIMER_MODE_REL);
> >- krcp->hrtimer.function = schedule_page_work_fn;
> >- hrtimer_start(&krcp->hrtimer, 0, HRTIMER_MODE_REL);
> >+ if (READ_ONCE(backoff_page_cache_fill)) {
> >+ queue_delayed_work(system_highpri_wq, &krcp->page_cache_work, >HZ);
> >+ WRITE_ONCE(backoff_page_cache_fill, false);
> >+ } else {
> >+ hrtimer_init(&krcp->hrtimer, CLOCK_MONOTONIC,
> >+ HRTIMER_MODE_REL);
> >+ krcp->hrtimer.function = schedule_page_work_fn;
> >+ hrtimer_start(&krcp->hrtimer, 0, HRTIMER_MODE_REL);
> >+ }
> > }
> > }
> >
> >@@ -3644,6 +3651,8 @@ kfree_rcu_shrink_count(struct shrinker *shrink, struct >shrink_control *sc)
> > raw_spin_unlock_irqrestore(&krcp->lock, flags);
> > }
> >
> >+ // Low memory condition, limit a page cache worker activity.
> >+ WRITE_ONCE(backoff_page_cache_fill, true);
> > return count;
> > }
>
> >@@ -4634,7 +4643,7 @@ static void __init kfree_rcu_batch_init(void)
> > }
> >
> > INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
> >- INIT_WORK(&krcp->page_cache_work, fill_page_cache_func);
> >+ INIT_DELAYED_WORK(&krcp->page_cache_work, fill_page_cache_func);
> > krcp->initialized = true;
> > }
> > if (register_shrinker(&kfree_rcu_shrinker))
> >
> >below patch fixes it. We should backoff the page fill worker keeping it empty
> >until the situation with memory consumption is normalized.
>
> Maybe can cancel the timer and cancel work in the rcu_shrink_count function, after
> the rcu_shrink_scan function is executed and recover timer.
> what do you think?
>I think it is just easier to schedule a periodic delayed work >with HZ
>interval. If a system is in a tight memory situation, that >periodic
>work will not be harmful from CPU cycles point of view. >Once it is
>settled, the hrtimer takes over to speedup the process of >cache filling.
It looks like it's easier. I'll resend a patch
Thanks
Qiang
>
> The other question is, why not call queue_work function directly to fill page cache,
> but through a hrtimer, what is the purpose of this design?
>
>It is done that way because the kvfree_rcu() is supposed that >it could
>be invoked from the scheduler part. It means that if a CPU >holds rq->lock
>queuing the work or placing it into rq will lead to deadlock.
>
>--
>Vlad Rezki