linux-kernel - Re: [PATCH 3/3 v2] blk-mq: Use llist_head for blk_cpu

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Date:   Wed, 27 Jan 2021 12:23:31 +0100
From:   Daniel Wagner <dwagner@...e.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3/3 v2] blk-mq: Use llist_head for blk_cpu_done

On Mon, Jan 25, 2021 at 10:54:12AM +0100, Sebastian Andrzej Siewior wrote:
> With llist_head it is possible to avoid the locking (the irq-off region)
> when items are added. This makes it possible to add items on a remote
> CPU without additional locking.
> llist_add() returns true if the list was previously empty. This can be
> used to invoke the SMP function call / raise sofirq only if the first
> item was added (otherwise it is already pending).
> This simplifies the code a little and reduces the IRQ-off regions.
> 
> blk_mq_raise_softirq() needs a preempt-disable section to ensure the
> request is enqueued on the same CPU as the softirq is raised.
> Some callers (USB-storage) invoke this path in preemptible context.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

I did a quick test run with the whole series. Looks good.

Reviewed-by: Daniel Wagner <dwagner@...e.de>