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

[Roman Gushchin]

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.