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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 28 Mar 2023 08:18:11 -0700
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Subject: Re: [PATCH v1 7/9] workingset: memcg: sleep when flushing stats in workingset_refault()
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Josef Bacik <josef@toxicpanda.com>,
        Jens Axboe <axboe@kernel.dk>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
        Vasily Averin <vasily.averin@linux.dev>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        bpf@vger.kernel.org
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On Mon, Mar 27, 2023 at 11:16=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com=
> wrote:
>
> In workingset_refault(), we call mem_cgroup_flush_stats_ratelimited()
> to flush stats within an RCU read section and with sleeping disallowed.
> Move the call to mem_cgroup_flush_stats_ratelimited() above the RCU read
> section and allow sleeping to avoid unnecessarily performing a lot of
> work without sleeping.
>
> Since workingset_refault() is the only caller of
> mem_cgroup_flush_stats_ratelimited(), just make it call the non-atomic
> mem_cgroup_flush_stats().
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>

A nit below:

Acked-by: Shakeel Butt <shakeelb@google.com>

> ---
>  mm/memcontrol.c | 12 ++++++------
>  mm/workingset.c |  4 ++--
>  2 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 57e8cbf701f3..0c0e74188e90 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -674,12 +674,6 @@ void mem_cgroup_flush_stats_atomic(void)
>                 __mem_cgroup_flush_stats_atomic();
>  }
>
> -void mem_cgroup_flush_stats_ratelimited(void)
> -{
> -       if (time_after64(jiffies_64, READ_ONCE(flush_next_time)))
> -               mem_cgroup_flush_stats_atomic();
> -}
> -
>  /* non-atomic functions, only safe from sleepable contexts */
>  static void __mem_cgroup_flush_stats(void)
>  {
> @@ -695,6 +689,12 @@ void mem_cgroup_flush_stats(void)
>                 __mem_cgroup_flush_stats();
>  }
>
> +void mem_cgroup_flush_stats_ratelimited(void)
> +{
> +       if (time_after64(jiffies_64, READ_ONCE(flush_next_time)))
> +               mem_cgroup_flush_stats();
> +}
> +
>  static void flush_memcg_stats_dwork(struct work_struct *w)
>  {
>         __mem_cgroup_flush_stats();
> diff --git a/mm/workingset.c b/mm/workingset.c
> index af862c6738c3..7d7ecc46521c 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -406,6 +406,8 @@ void workingset_refault(struct folio *folio, void *sh=
adow)
>         unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);
>         eviction <<=3D bucket_order;
>
> +       /* Flush stats (and potentially sleep) before holding RCU read lo=
ck */

I think the only reason we use rcu lock is due to
mem_cgroup_from_id(). Maybe we should add mem_cgroup_tryget_from_id().
The other caller of mem_cgroup_from_id() in vmscan is already doing
the same and could use mem_cgroup_tryget_from_id().

Though this can be done separately to this series (if we decide to do
it at all).