回复: [PATCH v3] kvfree_rcu: Release page cache under memory pressure

["Zhang, Qiang" <Qiang.Zhang@windriver.com>]

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

Thanks
Qiang





>
>Any thoughts ideas?
>
>--
>Vlad Rezki