linux-kernel - Re: [v4 PATCH] block: introduce block_rq

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Date:   Wed, 26 Jan 2022 00:21:51 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Yang Shi <shy828301@...il.com>
Cc:     axboe@...nel.dk, rostedt@...dmis.org, xiyou.wangcong@...il.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [v4 PATCH] block: introduce block_rq_error tracepoint

On Tue, Jan 25, 2022 at 12:35:48PM -0800, Yang Shi wrote:
> 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>
> 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.
> 
> 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..bb0593f93675 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, blk_status_to_errno(error), nr_bytes);

Please report the atual block layer status code instead of the errno
mapping here.