linux-kernel - Re: [Patch v2] block: introduce block_rq

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Date:   Tue, 21 Jan 2020 19:00:59 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [Patch v2] block: introduce block_rq_error tracepoint

On Mon, 20 Jan 2020 14:26:18 -0800
Cong Wang <xiyou.wangcong@...il.com> wrote:

> ---
>  block/blk-core.c             |  4 +++-
>  include/trace/events/block.h | 43 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 46 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 089e890ab208..0c7ad70d06be 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1450,8 +1450,10 @@ 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, blk_status_to_errno(error), nr_bytes);
>  		print_req_error(req, error, __func__);
> +	}
>  
>  	blk_account_io_completion(req, nr_bytes);
>  
> diff --git a/include/trace/events/block.h b/include/trace/events/block.h
> index 81b43f5bdf23..37e99be19536 100644
> --- a/include/trace/events/block.h
> +++ b/include/trace/events/block.h
> @@ -145,6 +145,49 @@ 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, int error, unsigned int nr_bytes),
> +
> +	TP_ARGS(rq, error, nr_bytes),
> +
> +	TP_STRUCT__entry(
> +		__field(  dev_t,	dev			)
> +		__dynamic_array( char,  name,	DISK_NAME_LEN	)

Hmm, looks like I need to go and do a clean up of the kernel, and
educate people on how to use dynamic arrays :-/

The "len" field of a __dynamic_array() is to be a function to determine
the length needed for each instance of an event. By having a constant
there, it will be the same for every events, plus the meta data to hold
the "dynamic" part of the array. This would be much better to simple
use __array() instead.

But as you use "__assign_str()" below, then it's expected that name is
a nul terminated string. In which case, you want to define this as:

		__string( name, rq->rq_disk ? rq->rq_disk->disk_name : "?"  )


> +		__field(  sector_t,	sector			)
> +		__field(  unsigned int,	nr_sector		)
> +		__field(  int,		error			)
> +		__array(  char,		rwbs,	RWBS_LEN	)
> +		__dynamic_array( char,	cmd,	1		)

Not sure what you are doing with cmd. It appears to be always hard
coded as an empty string?

-- Steve


> +	),
> +
> +	TP_fast_assign(
> +		__entry->dev	   = rq->rq_disk ? disk_devt(rq->rq_disk) : 0;
> +		__assign_str(name,   rq->rq_disk ? rq->rq_disk->disk_name : "?");
> +		__entry->sector    = blk_rq_pos(rq);
> +		__entry->nr_sector = nr_bytes >> 9;
> +		__entry->error     = error;
> +
> +		blk_fill_rwbs(__entry->rwbs, rq->cmd_flags, nr_bytes);
> +		__get_str(cmd)[0] = '\0';
> +	),
> +
> +	TP_printk("%d,%d %s %s (%s) %llu + %u [%d]",
> +		  MAJOR(__entry->dev), MINOR(__entry->dev),
> +		  __get_str(name), __entry->rwbs, __get_str(cmd),
> +		  (unsigned long long)__entry->sector,
> +		  __entry->nr_sector, __entry->error)
> +);
> +
>  DECLARE_EVENT_CLASS(block_rq,
>  
>  	TP_PROTO(struct request_queue *q, struct request *rq),