linux-kernel - Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme

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Message-ID: <20171212214432.GL3919388@devbig577.frc2.facebook.com>
Date:   Tue, 12 Dec 2017 13:44:32 -0800
From:   "tj@...nel.org" <tj@...nel.org>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "kernel-team@...com" <kernel-team@...com>,
        "oleg@...hat.com" <oleg@...hat.com>, "hch@....de" <hch@....de>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>,
        "osandov@...com" <osandov@...com>
Subject: Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme

Hello, Bart.

On Tue, Dec 12, 2017 at 09:37:11PM +0000, Bart Van Assche wrote:
> Have you considered the following instead of introducing MQ_RQ_IDLE and
> MQ_RQ_IN_FLIGHT? I think this could help to limit the number of new atomic
> operations introduced in the hot path by this patch series.

But nothing in the hot paths is atomic.

> static inline bool blk_mq_rq_in_flight(struct request *rq)
> {
> 	return list_empty(&rq->queuelist);
> }

And the fact that we encode the generation number and state into a
single variable contributes to not needing atomic operations.
Breaking up the state and generation like the above would need more
synchronization, not less.

Thanks.

-- 
tejun