linux-kernel - Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

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Date:   Fri, 1 Apr 2022 09:58:59 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Yosry Ahmed <yosryahmed@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Jonathan Corbet <corbet@....net>, Yu Zhao <yuzhao@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Wei Xu <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

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.