From: Ming Lei <ming.lei@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, Sagi Grimberg <sagi@grimberg.me>,
Baolin Wang <baolin.wang7@gmail.com>
Subject: Re: [PATCH 3/9] blk-mq: don't predicate last flag in blk_mq_dispatch_rq_list
Date: Thu, 14 May 2020 10:09:55 +0800 [thread overview]
Message-ID: <20200514020955.GH2073570@T590> (raw)
In-Reply-To: <20200513122753.GC23958@infradead.org>
On Wed, May 13, 2020 at 05:27:53AM -0700, Christoph Hellwig wrote:
> On Wed, May 13, 2020 at 05:54:37PM +0800, Ming Lei wrote:
> > .commit_rqs() is supposed to handle partial dispatch when driver may not
> > see .last of flag passed to .queue_rq().
> >
> > We have added .commit_rqs() in case of partial dispatch and all consumers
> > of bd->last have implemented .commit_rqs() callback, so it is perfect to
> > pass real .last flag of the request list to .queue_rq() instead of faking
> > it by trying to allocate driver tag for next request in the batching list.
>
> The current case still seems like a nice optimization to avoid an extra
> indirect function call. So if you want to get rid of it I think it at
> least needs a better rationale.
Forget to mention, trying to predicate the last request via allocating
tag for next request can't avoid extra .commit_rqs() because this
indirect call is always called when the rq list isn't done.
Also no matter .last is set or not, every implementation of .commit_rqs
always grabs one lock, so looks this patch can get real win without any
performance loss.
On the other side, .commit_rqs() can be avoided iff the last queued(successful)
rq is marked as .last, and the cost is to keep current estimate on .last.
However, why is .commit_rqs() required? Why doesn't .queue_rq() handle the batching
submission before non-STS_OK is returned? And the inline handling can be quite
efficient because one more spin lock acquire can be avoided usually. Then
.commit_rqs() can be killed.
Thanks,
Ming
next prev parent reply other threads:[~2020-05-14 2:10 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-13 9:54 [PATCH 0/9] blk-mq: support batching dispatch from scheduler Ming Lei
2020-05-13 9:54 ` [PATCH 1/9] blk-mq: pass request queue into get/put budget callback Ming Lei
2020-05-13 10:06 ` Johannes Thumshirn
2020-05-13 12:24 ` Christoph Hellwig
2020-05-13 14:36 ` Doug Anderson
2020-05-13 22:48 ` Sagi Grimberg
2020-05-13 9:54 ` [PATCH 2/9] blk-mq: pass hctx to blk_mq_dispatch_rq_list Ming Lei
2020-05-13 12:26 ` Christoph Hellwig
2020-05-13 22:49 ` Sagi Grimberg
2020-05-13 9:54 ` [PATCH 3/9] blk-mq: don't predicate last flag in blk_mq_dispatch_rq_list Ming Lei
2020-05-13 12:27 ` Christoph Hellwig
2020-05-14 0:50 ` Ming Lei
2020-05-14 5:50 ` Christoph Hellwig
2020-05-14 2:09 ` Ming Lei [this message]
2020-05-14 2:19 ` Ming Lei
2020-05-14 3:21 ` Keith Busch
2020-05-14 8:28 ` Ming Lei
2020-05-13 9:54 ` [PATCH 4/9] blk-mq: move getting driver tag and bugget into one helper Ming Lei
2020-05-13 12:37 ` Christoph Hellwig
2020-05-13 22:54 ` Sagi Grimberg
2020-05-13 9:54 ` [PATCH 5/9] blk-mq: move .queue_rq code " Ming Lei
2020-05-13 12:38 ` Christoph Hellwig
2020-05-13 9:54 ` [PATCH 6/9] blk-mq: move code for handling partial dispatch " Ming Lei
2020-05-13 12:56 ` Christoph Hellwig
2020-05-13 13:01 ` Christoph Hellwig
2020-05-14 1:25 ` Ming Lei
2020-05-13 9:54 ` [PATCH 7/9] blk-mq: remove dead check from blk_mq_dispatch_rq_list Ming Lei
2020-05-13 12:57 ` Christoph Hellwig
2020-05-13 23:24 ` Sagi Grimberg
2020-05-13 9:54 ` [PATCH 8/9] blk-mq: pass obtained budget count to blk_mq_dispatch_rq_list Ming Lei
2020-05-13 13:26 ` Christoph Hellwig
2020-05-13 9:54 ` [PATCH 9/9] blk-mq: support batching dispatch in case of io scheduler Ming Lei
2020-05-23 7:45 ` [PATCH 0/9] blk-mq: support batching dispatch from scheduler Baolin Wang
2020-05-25 2:17 ` Ming Lei
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