Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1387010 > unrolled thread

[PATCH] MD: make bio mergeable

Back to article view | Back to linux.kernel

Contents

  [PATCH] MD: make bio mergeable Shaohua Li <shli@fb.com> - 2016-04-26 02:00 +0200
    Re: [PATCH] MD: make bio mergeable Jens Axboe <axboe@kernel.dk> - 2016-04-26 03:00 +0200
      Re: [PATCH] MD: make bio mergeable Jens Axboe <axboe@kernel.dk> - 2016-04-26 03:20 +0200
    Re: [PATCH] MD: make bio mergeable Ming Lei <ming.lei@canonical.com> - 2016-04-26 12:00 +0200
      Re: [PATCH] MD: make bio mergeable Jens Axboe <axboe@kernel.dk> - 2016-04-26 16:30 +0200
        Re: [PATCH] MD: make bio mergeable Ming Lei <ming.lei@canonical.com> - 2016-04-26 17:20 +0200

#1387010 — [PATCH] MD: make bio mergeable

blk_queue_split marks bio unmergeable, which makes sense for normal bio.
But if dispatching the bio to underlayer disk, the blk_queue_split
checks are invalid, hence it's possible the bio becomes mergeable.

In the reported bug, this bug causes trim against raid0 performance slash
https://bugzilla.kernel.org/show_bug.cgi?id=117051

Reported-by: Park Ju Hyung <qkrwngud825@gmail.com>
Fixes: 6ac45aeb6bca(block: avoid to merge splitted bio)
Cc: stable@vger.kernel.org (v4.3+)
Cc: Ming Lei <ming.lei@canonical.com>
Cc: Jens Axboe <axboe@fb.com>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Shaohua Li <shli@fb.com>
---
 drivers/md/md.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 194580f..14d3b37 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -284,6 +284,8 @@ static blk_qc_t md_make_request(struct request_queue *q, struct bio *bio)
 	 * go away inside make_request
 	 */
 	sectors = bio_sectors(bio);
+	/* bio could be mergeable after passing to underlayer */
+	bio->bi_rw &= ~REQ_NOMERGE;
 	mddev->pers->make_request(mddev, bio);
 
 	cpu = part_stat_lock();
-- 
2.8.0.rc2

[toc] | [next] | [standalone]

#1387042

On 04/25/2016 05:52 PM, Shaohua Li wrote:
> blk_queue_split marks bio unmergeable, which makes sense for normal bio.
> But if dispatching the bio to underlayer disk, the blk_queue_split
> checks are invalid, hence it's possible the bio becomes mergeable.
>
> In the reported bug, this bug causes trim against raid0 performance slash
> https://bugzilla.kernel.org/show_bug.cgi?id=117051

Good catch! Will apply for this series, thanks Shaohua.

-- 
Jens Axboe

[toc] | [prev] | [next] | [standalone]

#1387049

On 04/25/2016 06:59 PM, Jens Axboe wrote:
> On 04/25/2016 05:52 PM, Shaohua Li wrote:
>> blk_queue_split marks bio unmergeable, which makes sense for normal bio.
>> But if dispatching the bio to underlayer disk, the blk_queue_split
>> checks are invalid, hence it's possible the bio becomes mergeable.
>>
>> In the reported bug, this bug causes trim against raid0 performance slash
>> https://bugzilla.kernel.org/show_bug.cgi?id=117051
>
> Good catch! Will apply for this series, thanks Shaohua.

Actually, let's let that go through the md tree instead. But you can add 
my Reviewed-by, and it'd be nice to get this into 4.6.

-- 
Jens Axboe

[toc] | [prev] | [next] | [standalone]

#1387313

On Tue, Apr 26, 2016 at 7:52 AM, Shaohua Li <shli@fb.com> wrote:
> blk_queue_split marks bio unmergeable, which makes sense for normal bio.
> But if dispatching the bio to underlayer disk, the blk_queue_split
> checks are invalid, hence it's possible the bio becomes mergeable.

If the bio from md is splitted and marked as NOMERGE, it means some
queue limits are reached. So looks the raid's queue limit is set as not
big enough, could your find which limit causes the splitting and nomerge?

>
> In the reported bug, this bug causes trim against raid0 performance slash
> https://bugzilla.kernel.org/show_bug.cgi?id=117051
>
> Reported-by: Park Ju Hyung <qkrwngud825@gmail.com>
> Fixes: 6ac45aeb6bca(block: avoid to merge splitted bio)
> Cc: stable@vger.kernel.org (v4.3+)
> Cc: Ming Lei <ming.lei@canonical.com>
> Cc: Jens Axboe <axboe@fb.com>
> Cc: Neil Brown <neilb@suse.de>
> Signed-off-by: Shaohua Li <shli@fb.com>
> ---
>  drivers/md/md.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 194580f..14d3b37 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -284,6 +284,8 @@ static blk_qc_t md_make_request(struct request_queue *q, struct bio *bio)
>          * go away inside make_request
>          */
>         sectors = bio_sectors(bio);
> +       /* bio could be mergeable after passing to underlayer */
> +       bio->bi_rw &= ~REQ_NOMERGE;

IMO it isn't a good fix, eigher we need to set a correct queue limit, or
we simply don't set nomerge for all stackable block device. But I prefer
to the former a bit.

Thanks,

>         mddev->pers->make_request(mddev, bio);
>
>         cpu = part_stat_lock();
> --
> 2.8.0.rc2
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-block" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

[toc] | [prev] | [next] | [standalone]

#1387533

On 04/26/2016 03:56 AM, Ming Lei wrote:
> On Tue, Apr 26, 2016 at 7:52 AM, Shaohua Li <shli@fb.com> wrote:
>> blk_queue_split marks bio unmergeable, which makes sense for normal bio.
>> But if dispatching the bio to underlayer disk, the blk_queue_split
>> checks are invalid, hence it's possible the bio becomes mergeable.
>
> If the bio from md is splitted and marked as NOMERGE, it means some
> queue limits are reached. So looks the raid's queue limit is set as not
> big enough, could your find which limit causes the splitting and nomerge?

raid0 sets a limit of the stripe size for IO. Once the IO has passed md, 
there's no reason why we can't merge for the lower driver. This is 
(potentially) a huge performance issue on trim, since a lot of devices 
are trim ops / sec limited rather than throughput limited.

-- 
Jens Axboe

[toc] | [prev] | [next] | [standalone]

#1387606

On Tue, Apr 26, 2016 at 10:21 PM, Jens Axboe <axboe@kernel.dk> wrote:
> On 04/26/2016 03:56 AM, Ming Lei wrote:
>>
>> On Tue, Apr 26, 2016 at 7:52 AM, Shaohua Li <shli@fb.com> wrote:
>>>
>>> blk_queue_split marks bio unmergeable, which makes sense for normal bio.
>>> But if dispatching the bio to underlayer disk, the blk_queue_split
>>> checks are invalid, hence it's possible the bio becomes mergeable.
>>
>>
>> If the bio from md is splitted and marked as NOMERGE, it means some
>> queue limits are reached. So looks the raid's queue limit is set as not
>> big enough, could your find which limit causes the splitting and nomerge?
>
>
> raid0 sets a limit of the stripe size for IO. Once the IO has passed md,
> there's no reason why we can't merge for the lower driver. This is
> (potentially) a huge performance issue on trim, since a lot of devices are
> trim ops / sec limited rather than throughput limited.

Just found raid0 maps the chunk sectors into max hw sectors of queue,
and dm uses blk_stack_limits() to set up the limits.

So looks a raid specific issue, then the fix is correct, sorry for the noise.

thanks,
Ming

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web