Re: [v5 PATCH] block: introduce block_rq_error tracepoint

[Yang Shi]

On Thu, Jan 27, 2022 at 12:02 AM Chaitanya Kulkarni
<chaitanyak@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxx>
> > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> > Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
> > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> > ---
> > 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@xxxxxxxxx/.
> >
> > 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 ?

Thanks for the comments, will remove it to follow the convention.

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