linux-kernel - Re: [RFC][PATCH v2 08/10] smp,irq_work: Use the new irq

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Date:   Thu, 20 Aug 2020 15:08:12 +0200
From:   peterz@...radead.org
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [RFC][PATCH v2 08/10] smp,irq_work: Use the new irq_work API

On Wed, Aug 19, 2020 at 03:04:56PM -0700, Linus Torvalds wrote:
> On Wed, Aug 19, 2020 at 12:41 PM <peterz@...radead.org> wrote:
> >
> > I'm not sure I get the "expensive irq_work queues" argument, I fully
> > agree with you that adding the atomic op is fairly crap.
> 
> There's an atomic op on the actual runing side too, because of the
> whole IRQ_WORK_PENDING thing.
> 
> So you get that double hit.
> 
> Maybe it doesn't matter. I just remember us being very careful to
> avoid any unnecessary atomics in the smp_call_function area, but
> admittedly I haven't worked on that code for a few years, so ..

Patch #3 trades that atomic for a full barrier. Not a massive win on
x86, but still.