Re: [PATCH v2] block: fix trace completion for chained bio

[Edward Hsieh <edwardh@synology.com>]

Hi Jens and Neil,

Is there any feedback on this patch?

Thank you,
Edward Hsieh

On 3/3/2021 11:22 AM, Edward Hsieh 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.
> 
> 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);
>