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Date:   Tue, 12 Jan 2021 10:59:58 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Roman Gushchin <guro@...com>,
        Michal Hocko <mhocko@...e.com>, Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH] mm: memcontrol: prevent starvation when writing memory.high

On Tue, Jan 12, 2021 at 9:12 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> When a value is written to a cgroup's memory.high control file, the
> write() context first tries to reclaim the cgroup to size before
> putting the limit in place for the workload. Concurrent charges from
> the workload can keep such a write() looping in reclaim indefinitely.
>

Is this observed on real workload?

>
> In the past, a write to memory.high would first put the limit in place
> for the workload, then do targeted reclaim until the new limit has
> been met - similar to how we do it for memory.max. This wasn't prone
> to the described starvation issue. However, this sequence could cause
> excessive latencies in the workload, when allocating threads could be
> put into long penalty sleeps on the sudden memory.high overage created
> by the write(), before that had a chance to work it off.
>
> Now that memory_high_write() performs reclaim before enforcing the new
> limit, reflect that the cgroup may well fail to converge due to
> concurrent workload activity. Bail out of the loop after a few tries.
>
> Fixes: 536d3bf261a2 ("mm: memcontrol: avoid workload stalls when lowering memory.high")
> Cc: <stable@...r.kernel.org> # 5.8+
> Reported-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> ---
>  mm/memcontrol.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 605f671203ef..63a8d47c1cd3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6275,7 +6275,6 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>
>         for (;;) {
>                 unsigned long nr_pages = page_counter_read(&memcg->memory);
> -               unsigned long reclaimed;
>
>                 if (nr_pages <= high)
>                         break;
> @@ -6289,10 +6288,10 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>                         continue;
>                 }
>
> -               reclaimed = try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
> -                                                        GFP_KERNEL, true);
> +               try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
> +                                            GFP_KERNEL, true);
>
> -               if (!reclaimed && !nr_retries--)

Any particular reason to remove !reclaimed?

> +               if (!nr_retries--)
>                         break;
>         }
>
> --
> 2.30.0
>

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