linux-kernel - bug in elevator/cfq regarding start

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Date:	Sun, 4 Oct 2009 22:54:44 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: bug in elevator/cfq regarding start_time?

Hi Jens,
I found that the start_time is not monotonic in the cfqq fifo_list.
The problem seems to be that:
* attempt_merge overrides one of the start_time:
        if (time_after(req->start_time, next->start_time))
                req->start_time = next->start_time;
* then it calls elv_merge_requests, that in turn invokes cfq's
elevator_merge_req_fn, i.e. cfq_merged_requests
* cfq_merged_requests checks start_times to decide whether requests
need to be repositioned
        if (!list_empty(&rq->queuelist) && !list_empty(&next->queuelist) &&
            time_before(next->start_time, rq->start_time))
                list_move(&rq->queuelist, &next->queuelist);

Other I/O schedulers are not affected, since none of them uses
start_time explicitly, but they have a copy in one of the private
fields.

Corrado

-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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