linux-kernel - Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu

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Date:   Sat, 31 Oct 2020 18:09:11 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Christoph Hellwig <hch@...radead.org>,
        linux-block@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        David Runge <dave@...epmap.de>, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Wagner <dwagner@...e.de>, Mike Galbraith <efault@....de>
Subject: Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu_done

On Sat, Oct 31, 2020 at 09:01:45AM -0600, Jens Axboe wrote:
> On 10/31/20 9:00 AM, Jens Axboe wrote:
> > On 10/31/20 4:41 AM, Sebastian Andrzej Siewior wrote:
> >> On 2020-10-29 14:07:59 [-0700], Sagi Grimberg wrote:
> >>>> in which context?
> >>>
> >>> Not sure what is the question.
> >>
> >> The question is in which context do you complete your requests. My guess
> >> by now is "usually softirq/NAPI and context in rare error case".
> > 
> > There really aren't any rules for this, and it's perfectly legit to
> > complete from process context. Maybe you're a kthread driven driver and
> > that's how you handle completions. The block completion path has always
> > been hard IRQ safe, but possible to call from anywhere.
> 
> A more recent example is polled IO, which will always complete from
> process/task context and very much is fast path.

But we never IPI for that anyway, so it is the easy case.