linux-kernel - Re: [PATCH v7 2/6] memcontrol: add a new function to traverse online-only memcg hierarchy

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Message-ID: <20231129170001.GF135852@cmpxchg.org>
Date:   Wed, 29 Nov 2023 12:00:01 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     akpm@...ux-foundation.org, cerasuolodomenico@...il.com,
        yosryahmed@...gle.com, sjenning@...hat.com, ddstreet@...e.org,
        vitaly.wool@...sulko.com, mhocko@...nel.org,
        roman.gushchin@...ux.dev, shakeelb@...gle.com,
        muchun.song@...ux.dev, chrisl@...nel.org, linux-mm@...ck.org,
        kernel-team@...a.com, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org, shuah@...nel.org
Subject: Re: [PATCH v7 2/6] memcontrol: add a new function to traverse
 online-only memcg hierarchy

On Wed, Nov 29, 2023 at 10:04:05AM -0500, Johannes Weiner wrote:
> On Mon, Nov 27, 2023 at 03:45:56PM -0800, Nhat Pham wrote:
> > The new zswap writeback scheme requires an online-only memcg hierarchy
> > traversal. Add this functionality via the new mem_cgroup_iter_online()
> > function - the old mem_cgroup_iter() is a special case of this new
> > function.
> > 
> > Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: Nhat Pham <nphamcs@...il.com>
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>

I saw Michal's reply on the other thread only after I sent this.

I agree with him it would be better to just check mem_cgroup_online()
in the shinker callsite and leave mem_cgroup_iter() as-is. If it's
offline, just continue to the next mem_cgroup_iter() invocation.