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SubjectRe: [v5 PATCH] block: introduce block_rq_error tracepoint
Date
On 1/26/22 10:51 AM, Yang Shi wrote:
> External email: Use caution opening links or attachments
>
>
> Currently, rasdaemon uses the existing tracepoint block_rq_complete
> and filters out non-error cases in order to capture block disk errors.
>
> But there are a few problems with this approach:
>
> 1. Even kernel trace filter could do the filtering work, there is
> still some overhead after we enable this tracepoint.
>
> 2. The filter is merely based on errno, which does not align with kernel
> logic to check the errors for print_req_error().
>
> 3. block_rq_complete only provides dev major and minor to identify
> the block device, it is not convenient to use in user-space.
>
> So introduce a new tracepoint block_rq_error just for the error case
> and provides the device name for convenience too. With this patch,
> rasdaemon could switch to block_rq_error.
>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Christoph Hellwig <hch@infradead.org>
> Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
> The v3 patch was submitted in Feb 2020, and Steven reviewed the patch, but
> it was not merged to upstream. See
> https://lore.kernel.org/lkml/20200203053650.8923-1-xiyou.wangcong@gmail.com/.
>
> The problems fixed by that patch still exist and we do need it to make
> disk error handling in rasdaemon easier. So this resurrected it and
> continued the version number.
>
> v4 --> v5:
> * Report the actual block layer status code instead of the errno per
> Christoph Hellwig.
> v3 --> v4:
> * Rebased to v5.17-rc1.
> * Collected reviewed-by tag from Steven.
>
> block/blk-mq.c | 4 +++-
> include/trace/events/block.h | 41 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 44 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index f3bf3358a3bb..4ca72ea917d4 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -789,8 +789,10 @@ bool blk_update_request(struct request *req, blk_status_t error,
> #endif
>
> if (unlikely(error && !blk_rq_is_passthrough(req) &&
> - !(req->rq_flags & RQF_QUIET)))
> + !(req->rq_flags & RQF_QUIET))) {
> + trace_block_rq_error(req, error, nr_bytes);
> blk_print_req_error(req, error);
> + }
>
> blk_account_io_completion(req, nr_bytes);
>
> diff --git a/include/trace/events/block.h b/include/trace/events/block.h
> index 27170e40e8c9..918b190718d5 100644
> --- a/include/trace/events/block.h
> +++ b/include/trace/events/block.h
> @@ -144,6 +144,47 @@ TRACE_EVENT(block_rq_complete,
> __entry->nr_sector, __entry->error)
> );
>
> +/**
> + * block_rq_error - block IO operation error reported by device driver
> + * @rq: block operations request
> + * @error: status code
> + * @nr_bytes: number of completed bytes
> + *
> + * The block_rq_error tracepoint event indicates that some portion
> + * of operation request has failed as reported by the device driver.
> + */
> +TRACE_EVENT(block_rq_error,
> +
> + TP_PROTO(struct request *rq, blk_status_t error, unsigned int nr_bytes),
> +
> + TP_ARGS(rq, error, nr_bytes),
> +
> + TP_STRUCT__entry(
> + __field( dev_t, dev )
> + __string( name, rq->q->disk ? rq->q->disk->disk_name : "?")
> + __field( sector_t, sector )
> + __field( unsigned int, nr_sector )
> + __field( int, error )
> + __array( char, rwbs, RWBS_LEN )
> + ),
> +
> + TP_fast_assign(
> + __entry->dev = rq->q->disk ? disk_devt(rq->q->disk) : 0;
> + __assign_str(name, rq->q->disk ? rq->q->disk->disk_name : "?");

Did you verify that rq->q->disk NULL checks are must in this checkout
for blk_update_request() ?

> + __entry->sector = blk_rq_pos(rq);
> + __entry->nr_sector = nr_bytes >> 9;
> + __entry->error = blk_status_to_errno(error);
> +
> + blk_fill_rwbs(__entry->rwbs, rq->cmd_flags);
> + ),
> +
> + TP_printk("%d,%d %s %s %llu + %u [%d]",
> + MAJOR(__entry->dev), MINOR(__entry->dev),
> + __get_str(name), __entry->rwbs,

Since printing major number and minor number is sufficient to identify
the block device. We don't have a notion of printing disk_name for the
block tracepoints what makes this tracepoint special ?

> + (unsigned long long)__entry->sector,
> + __entry->nr_sector, __entry->error)
> +);
> +
> DECLARE_EVENT_CLASS(block_rq,
>
> TP_PROTO(struct request *rq),
> --
> 2.26.3
>

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