On Wed, Mar 03 2021, edwardh wrote:
> From: Edward Hsieh <edwa...@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 <wa...@synology.com>
> Reviewed-by: BingJing Chang <bingji...@synology.com>
> Signed-off-by: Edward Hsieh <edwa...@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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