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    SubjectRe: bug in elevator/cfq regarding start_time?
    On Sun, Oct 04 2009, Corrado Zoccolo wrote:
    > 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.

    Good spotting! So we can either add the same copy in cfq, or we can just
    move the ->start_time check and assignment in attempt_merge() to post
    the elv_merge_requests() call. I think the latter would work fine, but
    I'll check and fix tomorrow.

    --
    Jens Axboe



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