From: "jianchao.wang" <jianchao.w.wang@oracle.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: axboe@kernel.dk, linux-block@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH V7 2/4] blk-mq: fix issue directly case when q is stopped or quiesced
Date: Thu, 15 Nov 2018 09:37:39 +0800 [thread overview]
Message-ID: <c6d6de0c-0e21-a0b6-a629-9f28814f59b3@oracle.com> (raw)
In-Reply-To: <20181114093544.GD20550@ming.t460p>
On 11/14/18 5:35 PM, Ming Lei wrote:
> On Wed, Nov 14, 2018 at 05:29:54PM +0800, jianchao.wang wrote:
>> Hi Ming
>>
>> On 11/14/18 5:20 PM, Ming Lei wrote:
>>> On Wed, Nov 14, 2018 at 04:45:29PM +0800, Jianchao Wang wrote:
>>>> When try to issue request directly, if the queue is stopped or
>>>> quiesced, 'bypass' will be ignored and return BLK_STS_OK to caller
>>>> to avoid it dispatch request again. Then the request will be
>>>> inserted with blk_mq_sched_insert_request. This is not correct
>>>> for dm-rq case where we should avoid to pass through the underlying
>>>> path's io scheduler.
>>>>
>>>> To fix it, use blk_mq_request_bypass_insert to insert the request
>>>> to hctx->dispatch when we cannot pass through io scheduler but have
>>>> to insert.
>>>
>>> Not sure if the current behaviour is wrong, or worth of a fix.
>>>
>>> Bypassing io scheduler for dm-rq is only for sake of performance
>>> because there has been io scheduler for dm device already, and we
>>> just don't want to schedule these requests twice.
>>
>> As comment of commit 157f377beb710e84bd8bc7a3c4475c0674ebebd7
>> (block: directly insert blk-mq request from blk_insert_cloned_request())
>>
>> All said, a request-based DM multipath device's IO scheduler should be
>> the only one used -- when the original requests are issued to the
>> underlying paths as cloned requests they are inserted directly in the
>> underlying dispatch queue(s) rather than through an additional elevator.
>>
>> But commit bd166ef18 ("blk-mq-sched: add framework for MQ capable IO
>> schedulers") switched blk_insert_cloned_request() from using
>> blk_mq_insert_request() to blk_mq_sched_insert_request(). Which
>> incorrectly added elevator machinery into a call chain that isn't
>> supposed to have any.
>>
>> It sounds like a wrong action.
>
> As I mentioned, it is only for the sake of performance, and IO scheduler
> has to be supported on these devices too, for example, one partition may
> be under dm-rq, and another partition can be accessed directly.
>
> However, you are fixing the handling when queue is quiesced or stopped.
> Under this situation, it is fine to put requests into scheduler queue,
> given no performance need to be worried.
>
OK, I drop this one.
Thanks
Jianchao
next prev parent reply other threads:[~2018-11-15 1:37 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-14 8:45 [PATCH V7 0/4] blk-mq: refactor and fix on issue request directly Jianchao Wang
2018-11-14 8:45 ` [PATCH V7 1/4] blk-mq: refactor the code of " Jianchao Wang
2018-11-14 9:11 ` Ming Lei
2018-11-14 9:23 ` jianchao.wang
2018-11-14 9:43 ` Ming Lei
2018-11-14 15:22 ` Jens Axboe
2018-11-15 1:35 ` jianchao.wang
2018-11-14 8:45 ` [PATCH V7 2/4] blk-mq: fix issue directly case when q is stopped or quiesced Jianchao Wang
2018-11-14 9:20 ` Ming Lei
2018-11-14 9:29 ` jianchao.wang
2018-11-14 9:35 ` Ming Lei
2018-11-15 1:37 ` jianchao.wang [this message]
2018-11-14 8:45 ` [PATCH V7 3/4] blk-mq: issue directly with bypass 'false' in blk_mq_sched_insert_requests Jianchao Wang
2018-11-14 8:45 ` [PATCH V7 4/4] blk-mq: replace and kill blk_mq_request_issue_directly Jianchao Wang
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=c6d6de0c-0e21-a0b6-a629-9f28814f59b3@oracle.com \
--to=jianchao.w.wang@oracle.com \
--cc=axboe@kernel.dk \
--cc=linux-block@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=ming.lei@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).