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Date:   Mon, 4 Apr 2022 10:44:04 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Yosry Ahmed <yosryahmed@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Jonathan Corbet <corbet@lwn.net>, Yu Zhao <yuzhao@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Wei Xu <weixugc@google.com>, Greg Thelen <gthelen@google.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface
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On Fri 01-04-22 09:58:59, Roman Gushchin wrote:
> On Fri, Apr 01, 2022 at 03:49:19PM +0200, Michal Hocko wrote:
> > On Thu 31-03-22 10:25:23, Roman Gushchin wrote:
> > > On Thu, Mar 31, 2022 at 08:41:51AM +0000, Yosry Ahmed wrote:
> > [...]
> > > > - A similar per-node interface can also be added to support proactive
> > > >   reclaim and reclaim-based demotion in systems without memcg.
> > > 
> > > Maybe an option to specify a timeout? That might simplify the userspace part.
> > 
> > What do you mean by timeout here? Isn't
> > timeout $N echo $RECLAIM > ....
> > 
> > enough?
> 
> It's nice and simple when it's a bash script, but when it's a complex
> application trying to do the same, it quickly becomes less simple and
> likely will require a dedicated thread to avoid blocking the main app
> for too long and a mechanism to unblock it by timer/when the need arises.
> 
> In my experience using correctly such semi-blocking interfaces (semi- because
> it's not clearly defined how much time the syscall can take and whether it
> makes sense to wait longer) is tricky.

We have the same approach to setting other limits which need to perform
the reclaim. Have we ever hit that as a limitation that would make
userspace unnecessarily too complex?
-- 
Michal Hocko
SUSE Labs