linux-kernel - Re: blk-softirq vs smp_call_function_single

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Message-ID: <20200609133837.GA2514@hirez.programming.kicks-ass.net>
Date:   Tue, 9 Jun 2020 15:38:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, frederic@...nel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: blk-softirq vs smp_call_function_single_async()

On Mon, Jun 08, 2020 at 09:42:54AM -0700, Christoph Hellwig wrote:
> On Mon, Jun 08, 2020 at 06:40:09PM +0200, Peter Zijlstra wrote:
> > There isn't one, it was meant to be used with static allocations.
> > 
> > Frederic proposed changing all these to irq_work, and I think I'll go do
> > that. First dinner though.

OK, after having looked at this more, I think my initial analysis is
actually wrong and this code should work as-is.

The thing that I missed yesterday is that we only add the request to the
blk_cpu_done list in the IPI, this means that the race I described
earlier is not in fact possible.

The IPI must happen for progress to be made.

And the same is true for blk_mq_force_complete_rq(), which also uses
this csd.

> The irq_work API looks reasonable.  What are the tradeoffs for
> smp_call_single_async vs irq_work?

To still answer your question; irq_work_queue*() has an atomic op extra
that allows for more convenient semantics -- but is in your case
strictly superfluous.

Still, Jens' point about irq_work being smaller stands, and I think more
users could benefit from something intermediate. Let me continue with
the cleanups / audit and see what comes out at the end.

Thanks!