Date:   Sat, 6 Feb 2021 07:26:17 +0000
From:   "Zhang, Qiang" <Qiang.Zhang@...driver.com>
To:     Uladzislau Rezki <urezki@...il.com>
CC:     "paulmck@...nel.org" <paulmck@...nel.org>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: 回复: 回复: [PATCH v3] kvfree_rcu: Release page cache under memory pressure



________________________________________
发件人: Uladzislau Rezki <urezki@...il.com>
发送时间: 2021年2月4日 22:09
收件人: Zhang, Qiang
抄送: Uladzislau Rezki; paulmck@...nel.org; joel@...lfernandes.org; rcu@...r.kernel.org; linux-kernel@...r.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 <urezki@...il.com>
> 发送时间: 2021年2月2日 3:57
> 收件人: Zhang, Qiang
> 抄送: urezki@...il.com; paulmck@...nel.org; joel@...lfernandes.org; rcu@...r.kernel.org; linux-kernel@...r.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 <qiang.zhang@...driver.com>
> >
> > Add free per-cpu existing krcp's page cache operation, when
> > the system is under memory pressure.
> >
> > Signed-off-by: Zqiang <qiang.zhang@...driver.com>
> > ---
> >  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

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