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

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Message-ID: <20201027175920.otcm2ox7vxce5k3c@linutronix.de>
Date:   Tue, 27 Oct 2020 18:59:20 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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 2020-10-27 17:23:09 [+0000], Christoph Hellwig wrote:
> On Tue, Oct 27, 2020 at 06:05:15PM +0100, Thomas Gleixner wrote:
> > > 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.
> 
> Ok.  I was hoping we could hide this in core code somehow, especially
> a peterz didn't like the use of smp_call_function_single_async in the
> blk-mq completion code very much.
> 
> Sebastian, would this solve your preempt-rt and lockdep issues?

second. I'm cooking something.

Sebastian