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Date: Thu, 27 Jan 2022 10:18:57 -0800
From: Yang Shi <shy828301@...il.com>
To: Chaitanya Kulkarni <chaitanyak@...dia.com>
Cc: "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"axboe@...nel.dk" <axboe@...nel.dk>,
"hch@...radead.org" <hch@...radead.org>,
"rostedt@...dmis.org" <rostedt@...dmis.org>
Subject: Re: [v5 PATCH] block: introduce block_rq_error tracepoint
On Thu, Jan 27, 2022 at 12:02 AM Chaitanya Kulkarni
<chaitanyak@...dia.com> 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@...nel.dk>
> > Cc: Christoph Hellwig <hch@...radead.org>
> > Reviewed-by: Steven Rostedt <rostedt@...dmis.org>
> > Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> > Signed-off-by: Yang Shi <shy828301@...il.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 ?
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
> >
>
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