From: NeilBrown <neilb@suse.de>
To: edwardh <edwardh@synology.com>, axboe@kernel.dk, neilb@suse.com
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
s3t@synology.com, bingjingc@synology.com, cccheng@synology.com,
Edward Hsieh <edwardh@synology.com>
Subject: Re: [PATCH v2] block: fix trace completion for chained bio
Date: Tue, 23 Mar 2021 08:22:59 +1100 [thread overview]
Message-ID: <87zgyudgss.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <1614741726-28074-1-git-send-email-edwardh@synology.com>
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On Wed, Mar 03 2021, edwardh wrote:
> From: Edward Hsieh <edwardh@synology.com>
>
> For chained bio, trace_block_bio_complete in bio_endio is currently called
> only by the parent bio once upon all chained bio completed.
> However, the sector and size for the parent bio are modified in bio_split.
> Therefore, the size and sector of the complete events might not match the
> queue events in blktrace.
>
> The original fix of bio completion trace <fbbaf700e7b1> ("block: trace
> completion of all bios.") wants multiple complete events to correspond
> to one queue event but missed this.
>
> md/raid5 read with bio cross chunks can reproduce this issue.
>
> To fix, move trace completion into the loop for every chained bio to call.
Thanks. I think this is correct as far as tracing goes.
However the code still looks a bit odd.
The comment for the handling of bio_chain_endio suggests that the *only*
purpose for that is to avoid deep recursion. That suggests it should be
at the end of the function.
As it is blk_throtl_bio_endio() and bio_unint() are only called on the
last bio in a chain.
That seems wrong.
I'd be more comfortable if the patch moved the bio_chain_endio()
handling to the end, after all of that.
So the function would end.
if (bio->bi_end_io == bio_chain_endio) {
bio = __bio_chain_endio(bio);
goto again;
} else if (bio->bi_end_io)
bio->bi_end_io(bio);
Jens: can you see any reason why that functions must only be called on
the last bio in the chain?
Thanks,
NeilBrown
>
> Fixes: fbbaf700e7b1 ("block: trace completion of all bios.")
> Reviewed-by: Wade Liang <wadel@synology.com>
> Reviewed-by: BingJing Chang <bingjingc@synology.com>
> Signed-off-by: Edward Hsieh <edwardh@synology.com>
> ---
> block/bio.c | 13 ++++++-------
> 1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index a1c4d29..2ff72cb 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1397,8 +1397,7 @@ static inline bool bio_remaining_done(struct bio *bio)
> *
> * bio_endio() can be called several times on a bio that has been chained
> * using bio_chain(). The ->bi_end_io() function will only be called the
> - * last time. At this point the BLK_TA_COMPLETE tracing event will be
> - * generated if BIO_TRACE_COMPLETION is set.
> + * last time.
> **/
> void bio_endio(struct bio *bio)
> {
> @@ -1411,6 +1410,11 @@ void bio_endio(struct bio *bio)
> if (bio->bi_bdev)
> rq_qos_done_bio(bio->bi_bdev->bd_disk->queue, bio);
>
> + if (bio->bi_bdev && bio_flagged(bio, BIO_TRACE_COMPLETION)) {
> + trace_block_bio_complete(bio->bi_bdev->bd_disk->queue, bio);
> + bio_clear_flag(bio, BIO_TRACE_COMPLETION);
> + }
> +
> /*
> * Need to have a real endio function for chained bios, otherwise
> * various corner cases will break (like stacking block devices that
> @@ -1424,11 +1428,6 @@ void bio_endio(struct bio *bio)
> goto again;
> }
>
> - if (bio->bi_bdev && bio_flagged(bio, BIO_TRACE_COMPLETION)) {
> - trace_block_bio_complete(bio->bi_bdev->bd_disk->queue, bio);
> - bio_clear_flag(bio, BIO_TRACE_COMPLETION);
> - }
> -
> blk_throtl_bio_endio(bio);
> /* release cgroup info */
> bio_uninit(bio);
> --
> 2.7.4
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next prev parent reply other threads:[~2021-03-22 21:23 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-03 3:22 [PATCH v2] block: fix trace completion for chained bio edwardh
2021-03-16 10:30 ` Edward Hsieh
2021-03-22 21:22 ` NeilBrown [this message]
2021-04-23 8:04 ` Edward Hsieh
2021-05-10 2:06 ` Edward Hsieh
2021-05-25 9:37 ` Edward Hsieh
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