linux-kernel - Re: [RFC PATCH 0/5] softirq: Per vector threading v2

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Date:   Wed, 17 Jan 2018 15:56:43 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Mauro Carvalho Chehab <mchehab@...pensource.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Levin Alexander <alexander.levin@...izon.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Dmitry Safonov <dima@...sta.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Radu Rendec <rrendec@...sta.com>,
        Ingo Molnar <mingo@...nel.org>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH 0/5] softirq: Per vector threading v2

On Wed, Jan 17, 2018 at 10:07 AM, Frederic Weisbecker
<frederic@...nel.org> wrote:
>
> I see, so you may want to test (possibly much) higher values of MAX_SOFTIRQ_RESTART,
> such as 50 or 100.

I suspect the "number of softiqs per jiffy" is hardly interesting at all.

We used to allow up to 2mS or ten iterations per _invocation_, never
mind per timer tick.

I thought you were going to actally account for time, but I don't
think you ended up doing that.

Maybe time isn't necessarily the thing to do, but just pure "count per
jiffy" seems very bad.

What I might suggest using instead:

 - do it by time. This may be too expensive, though. Keeping track of
ns-level timing per invocation can be nasty.

 - do it by "we got a new softirq event while handling another softirq
event". That was our old count per invocation, except you could do it
per softirq, and just allow *one* (ie keep a bitmask of "I've already
handled this softirq", and if the restart results in it being
triggered *again* you say "ok, I'll just move this to a workqueue"

 - .. something else?

I'd suggest trying the "if we get a new softirq event that we've
already seen while we were already handling softirq events" thing.
That should really take care of the networking case of "90% time spend
in softirq handling during packet storms" thing. If we spend that much
time on softirqs, we *will* get a new softirq while handling an old
one occasionally.

            Linus