linux-kernel - Re: [PATCH] psi: reduce min window size to 50ms

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Date:   Fri, 24 Feb 2023 13:47:35 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>,
        David Hildenbrand <david@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mike Rapoport <rppt@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        mark.rutland@....com, will@...nel.org,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org,
        Trilok Soni <quic_tsoni@...cinc.com>,
        Sukadev Bhattiprolu <quic_sukadev@...cinc.com>,
        Srivatsa Vaddagiri <quic_svaddagi@...cinc.com>,
        Patrick Daly <quic_pdaly@...cinc.com>
Subject: Re: [PATCH] psi: reduce min window size to 50ms

On Tue 14-02-23 11:34:30, Suren Baghdasaryan wrote:
[...]
> Your suggestion to have this limit configurable sounds like obvious
> solution. I would like to get some opinions from other maintainers.
> Johannes, WDYT? CC'ing Michal to chime in as well since this is mostly
> related to memory stalls.

I do not think that making this configurable helps much. Many users will
be bound to distribution config and also it would be hard to experiment
with a recompile cycle every time. This seems just too impractical.

Is there any reason why we shouldn't allow any timeout? Shorter
timeouts could be restricted to a priviledged context to avoid an easy
way to swamp system by too frequent polling.

Btw. it seems that there is is only a limit on a single trigger per fd
but no limits per user so it doesn't sound too hard to end up with too
much polling even with a larger timeouts. To me it seems like we need to
contain the polling thread to be bound by the cpu controller.
-- 
Michal Hocko
SUSE Labs