linux-kernel - Re: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT

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Message-ID: <87eelj1tx0.fsf@nanos.tec.linutronix.de>
Date:   Tue, 27 Oct 2020 18:05:15 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Christoph Hellwig <hch@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Christoph Hellwig <hch@...radead.org>,
        David Runge <dave@...epmap.de>, linux-rt-users@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Wagner <dwagner@...e.de>
Subject: Re: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT

On Tue, Oct 27 2020 at 16:07, Christoph Hellwig wrote:
> On Tue, Oct 27, 2020 at 11:11:02AM +0100, Sebastian Andrzej Siewior wrote:
>> Right. I found this David Runge's log:
>
> True, ->bi_end_io instances can do a lot of things as long as they
> are hardirq safe.
>
> And in the end the IPI case isn't the super fast path anyway, as it
> means we don't use a queue per CPU.
>
> Is there a way to raise a softirq and preferably place it on a given
> CPU without our IPI dance?  That should be a win-win situation for
> everyone.

Not really. Softirq pending bits are strictly per cpu and we don't have
locking or atomics to set them remotely. Even if we had that, then you'd
still need a mechanism to make sure that the remote CPU actually
processes them. So you'd still need an IPI of some sorts.

Thanks,

        tglx