Re: [PATCH] block: call elv_bio_merged() when merged

[Jens Axboe]

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 (ê), 16:31 +0800, Shaohua Li:
>>>> 2011/5/20 Namhyung Kim <namhyung@xxxxxxxxx>:
>>>>> 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@xxxxxxxxx>
>>>>> Cc: Divyesh Shah <dpshah@xxxxxxxxxx>
>>>>> ---
>>>>>  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.

-- 
Jens Axboe

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