Date: Fri, 20 May 2011 17:02:28 -0400
From: Vivek Goyal <vgoyal@redhat.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: Namhyung Kim <namhyung@gmail.com>, Shaohua Li <shaohua.li@intel.com>,
        linux-kernel@vger.kernel.org, Divyesh Shah <dpshah@google.com>
Subject: Re: [PATCH] block: call elv_bio_merged() when merged
Message-ID: <20110520210228.GA6789@redhat.com>
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On Fri, May 20, 2011 at 09:21:03PM +0200, Jens Axboe wrote:
> On 2011-05-20 21:19, Vivek Goyal wrote:
> > On Fri, May 20, 2011 at 08:50:47PM +0200, Jens Axboe wrote:
> >> On 2011-05-20 11:51, Namhyung Kim wrote:
> >>> Hello,
> >>>
> >>> 2011-05-20 (금), 16:31 +0800, Shaohua Li:
> >>>> 2011/5/20 Namhyung Kim <namhyung@gmail.com>:
> >>>>> Commit 73c101011926 ("block: initial patch for on-stack per-task plugging")
> >>>>> removed calls to elv_bio_merged() when @bio merged with @req. Re-add them.
> >>>>>
> >>>>> Signed-off-by: Namhyung Kim <namhyung@gmail.com>
> >>>>> Cc: Divyesh Shah <dpshah@google.com>
> >>>>> ---
> >>>>>  block/blk-core.c |    2 ++
> >>>>>  1 files changed, 2 insertions(+), 0 deletions(-)
> >>>>>
> >>>>> diff --git a/block/blk-core.c b/block/blk-core.c
> >>>>> index 3fe00a14822a..4dc02ef5fc82 100644
> >>>>> --- a/block/blk-core.c
> >>>>> +++ b/block/blk-core.c
> >>>>> @@ -1132,6 +1132,7 @@ static bool bio_attempt_back_merge(struct request_queue *q, struct request *req,
> >>>>>        req->ioprio = ioprio_best(req->ioprio, bio_prio(bio));
> >>>>>
> >>>>>        drive_stat_acct(req, 0);
> >>>>> +       elv_bio_merged(q, req, bio);
> >>>>>        return true;
> >>>>>  }
> >>>>>
> >>>>> @@ -1173,6 +1174,7 @@ static bool bio_attempt_front_merge(struct request_queue *q,
> >>>>>        req->ioprio = ioprio_best(req->ioprio, bio_prio(bio));
> >>>>>
> >>>>>        drive_stat_acct(req, 0);
> >>>>> +       elv_bio_merged(q, req, bio);
> >>>>>        return true;
> >>>>>  }
> >>>> Looks you should do this in __make_request. when the routine is called
> >>>> in attempt_plug_merge, the request isn't added to elevator yet.
> >>>>
> >>>
> >>> Hmm.. anyway it is merged. Is there any reason why we shouldn't collect
> >>> the stat - or invoke the callback routine - if the @req is not in the
> >>> elevator? Or we need to add a separate stat item for this case?
> >>
> >> Your patch should be safe, it's essentially only for the cgroup stuff
> >> that does its own accounting and has appropriate protection for it. We'd
> >> want to do this for both the plug and not-plugged merge case.
> >>
> >> It's a bit of a shame to add this though, since now we are hitting the
> >> cgroup lock for each merge.
> > 
> > I think wer can make MERGED per cpu as I have done in my other patch
> > series for following dispatch stats.
> > 
> > BLKIO_STAT_CPU_SECTORS,
> > /* Total bytes transferred */
> > BLKIO_STAT_CPU_SERVICE_BYTES,
> > /* Total IOs serviced, post merge */
> > BLKIO_STAT_CPU_SERVICED,
> 
> Yep, lets please do that and we can re-instate the merge calls with
> that, too.
> 
> > Jens are you planning to merge lockless throttling series? Once that gets
> > merged, we can convert this MERGED stat also per cpu and get rid of need of
> > taking blkg->stats_lock.
> 
> Merged them about an hour ago :-)
> 
> Send further updates relative to for-2.6.40/core.

Sure. I am working on a patch now.

Thanks
Vivek
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