linux-kernel - Re: [RFC PATCH 6/7] workingset: memcg: sleep when flushing stats in workingset

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Message-ID: <20230323160030.GD739026@cmpxchg.org>
Date:   Thu, 23 Mar 2023 12:00:30 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vasily Averin <vasily.averin@...ux.dev>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org
Subject: Re: [RFC PATCH 6/7] workingset: memcg: sleep when flushing stats in
 workingset_refault()

On Thu, Mar 23, 2023 at 04:00:36AM +0000, Yosry Ahmed wrote:
> In workingset_refault(), we call mem_cgroup_flush_stats_delayed() to
> flush stats within an RCU read section and with sleeping disallowed.
> Move the call to mem_cgroup_flush_stats_delayed() above the RCU read
> section and allow sleeping to avoid unnecessarily performing a lot of
> work without sleeping.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---
> 
> A lot of code paths call into workingset_refault(), so I am not
> generally sure at all whether it's okay to sleep in all contexts or not.
> Feedback here would be very helpful.
> 
> ---
>  mm/workingset.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/workingset.c b/mm/workingset.c
> index 042eabbb43f6..410bc6684ea7 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -406,6 +406,8 @@ void workingset_refault(struct folio *folio, void *shadow)
>  	unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);
>  	eviction <<= bucket_order;
>  
> +	/* Flush stats (and potentially sleep) before holding RCU read lock */
> +	mem_cgroup_flush_stats_delayed(true);

Btw, it might be a good time to rename this while you're in the
area. delayed suggests this is using a delayed_work, but this is
actually sometimes flushing directly from the callsite.

What it's doing is ratelimited calls. A better name would be:

	mem_cgroup_flush_stats_ratelimited()