linux-kernel - Re: [PATCH v1 5/6] blk-mq: fix for trace_block

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Message-ID: <CACVXFVNuLBTUQFDMdkykJvbKLV_e0EPGuM1gJ71FRVO0b6ujYg@mail.gmail.com>
Date:	Mon, 19 Oct 2015 23:58:39 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ming Lin <ming.l@....samsung.com>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v1 5/6] blk-mq: fix for trace_block_plug()

On Mon, Oct 19, 2015 at 11:38 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Ming Lei <ming.lei@...onical.com> writes:
>
>> The trace point is for tracing plug event of each request
>> queue instead of each task, so we should check the request
>> count in the plug list from current queue instead of
>> current task.
>
> If blk_queue_nomerges returns true, request_count won't be updated, so
> you've essentially broken plugging for those queues.

Yes, you are right, and blk_queue_bio() has the same problem too,
and looks automatic flush plug can't work for nomerges queue at all.

Thanks,
Ming

>
> Cheers,
> Jeff
>
>>
>> Signed-off-by: Ming Lei <ming.lei@...onical.com>
>> ---
>>  block/blk-mq.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index d55d022..a5e33bc 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -1377,7 +1377,7 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio)
>>       plug = current->plug;
>>       if (plug) {
>>               blk_mq_bio_to_request(rq, bio);
>> -             if (list_empty(&plug->mq_list))
>> +             if (!request_count)
>>                       trace_block_plug(q);
>>               else if (request_count >= BLK_MAX_REQUEST_COUNT) {
>>                       blk_flush_plug_list(plug, false);
--
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