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

From: Roman Gushchin <roman.gushchin@linux.dev>
To: Michal Hocko <mhocko@suse.com>
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
Date: Fri, 1 Apr 2022 09:58:59 -0700	[thread overview]
Message-ID: <YkcvU2hosJV3cL8F@carbon.dhcp.thefacebook.com> (raw)
In-Reply-To: <YkcC3z5ReeQ5vdg9@dhcp22.suse.cz>

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.