linux-kernel - Re: [PATCH] blk-mq: make synchronous hw

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Message-ID: <YbhQ5b2qZN0Jpk5i@infradead.org>
Date:   Tue, 14 Dec 2021 00:08:05 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Christoph Hellwig <hch@...radead.org>, axboe@...nel.dk,
        bigeasy@...utronix.de, tglx@...utronix.de,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] blk-mq: make synchronous hw_queue runs RT friendly

On Mon, Dec 13, 2021 at 11:05:29AM -0800, Davidlohr Bueso wrote:
> On Mon, 13 Dec 2021, Christoph Hellwig wrote:
> > But more importantly:  why isn't migrate_disable/enable doing the right
> > thing for !PREEMPT_RT to avoid this mess?
> 
> Please see Peter's description of the situation in af449901b84.

That explains why migrate_disable is a bad idea in PREEMPT_RT, not why it
can't do something sensible for !PREEMPT_RT…

> 
> While I'm not at all a fan of sprinkling migrate_disabling around code,
> I didn't want to add any overhead for the common case. If this, however,
> were not an issue (if most cases are async runs, for example) the ideal
> solution I think would be to just pin current to the hctx->cpumask.

sync running is the performance case.