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

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Message-ID: <x497fmioom0.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 19 Oct 2015 11:38:15 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Jens Axboe <axboe@...nel.dk>, 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()

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.

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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