linux-kernel - Re: [RFC PATCH 0/4] softirq: Per vector threading v3

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Date:   Tue, 23 Jan 2018 12:32:13 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Paolo Abeni <pabeni@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Levin Alexander <alexander.levin@...izon.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Radu Rendec <rrendec@...sta.com>,
        Ingo Molnar <mingo@...nel.org>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH 0/4] softirq: Per vector threading v3

On Tue, 2018-01-23 at 11:13 +0100, Paolo Abeni wrote:
> Hi,
> 
> On Fri, 2018-01-19 at 16:46 +0100, Frederic Weisbecker wrote:
> > As per Linus suggestion, this take doesn't limit the number of
> > occurences
> > per jiffy anymore but instead defers a vector to workqueues as soon
> > as
> > it gets re-enqueued on IRQ tail.
> > 
> > No tunable here, so testing should be easier.
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-
> > dynticks.git
> > 	softirq/thread-v3
> > 
> > HEAD: 6835e92cbd70ef4a056987d2e1ed383b294429d4
> 
> I tested this series in the UDP flood scenario, binding the user
> space
> process receiving the packets on the same CPU processing the related
> IRQ, and the tput sinks nearly to 0, like before Eric's patch.
> 
> The perf tool says that almost all the softirq processing is done
> inside the workqueue, but the user space process is scheduled very
> rarely, while before this series, in this scenario, ksoftirqd and the
> user space process got a fair share of the CPU time.

It get a fair share of the CPU time only on some lucky platforms (maybe
on the most). On other not-so-lucky platforms it was also unfair - as
I've previously described by the reason of slow raising softirqs :-/

-- 
Thanks,
             Dmitry