linux-kernel - Re: Flush requests not going through IO scheduler

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Message-ID: <x498u6f2e97.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 03 Nov 2015 12:18:12 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Flush requests not going through IO scheduler

Jens Axboe <axboe@...nel.dk> writes:

>>> Certainly, the current behavior is undoubtedly broken. The least
>>> intrusive fix would be to kick off scheduling when we add it to the
>>> request, but the elevator should handle it. Are you going to be up
>>> for hacking up a fix?
>>
>> I have some trouble understanding what do you mean exactly. Do you think we
>> should just call __blk_run_queue() after we add the request to
>> q->queue_head?
>
> No, that won't be enough, as it won't always break out of the idle
> logic. We need to ensure that the new request is noticed, so that CFQ
> knows and can decide to kick off things.

Hmm?  __blk_run_queue calls the request_fn, which will call
blk_peek_request, which calls __elv_next_request, which will find the
request on queue_head.  Right?

        while (1) {
                if (!list_empty(&q->queue_head)) {
                        rq = list_entry_rq(q->queue_head.next);
                        return rq;

Cheers,
Jeff
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