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

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Date:   Wed, 28 Oct 2020 06:56:16 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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

> The remaining part is a switch to llist which avoids locking (IRQ
> off/on) and it allows invoke the IPI/raise softirq only if something was
> added. The entries are now processed in the reverse order but this
> shouldn't matter right?

For correctness it should not matter, but I think it could have
performance implications.  I think you'll have to throw in a
llist_reverse_order.

> I would split this into two patches (the blk_mq_complete_need_ipi() hunk
> and the llist part) unless there are objections.

Yes, please do.