linux-kernel - Re: BLK_TN_PROCESS events not delivered for all devices

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Date:	Tue, 17 Sep 2013 22:31:31 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: BLK_TN_PROCESS events not delivered for all devices

On Tue 17-09-13 12:23:33, Jens Axboe wrote:
> On 09/17/2013 11:10 AM, Jan Kara wrote:
> > On Tue 17-09-13 08:29:07, Jens Axboe wrote:
> >> On 09/16/2013 03:23 AM, Jan Kara wrote:
> >>>   Hi,
> >>>
> >>>   I've been looking into a problem where BLK_TN_PROCESS events are not
> >>> delivered to all devices which are being traced. This results in process
> >>> name being (null) when trace for a single device is parsed.
> >>>
> >>> The reason for this problem is that trace_note_tsk() is called only if
> >>> tsk->btrace_seq != blktrace_seq and it updates tsk->btrace_seq to
> >>> blktrace_seq. Thus after a trace for another device is started
> >>> BLK_TN_PROCESS event is sent only on behalf of the first device with which
> >>> the task interacts. That isn't necessarily the new device thus traces for
> >>> some devices accumulate several BLK_TN_PROCESS events for one task while
> >>> other have none. Is this a known problem and is this intended to work
> >>> better?
> >>>
> >>> I was thinking how to fix that for a while and it doesn't seem to be
> >>> possible without tracking with each block trace which tasks it has been
> >>> notified about. And that is relatively expensive...
> >>
> >> It is unfortunately a known issue... I have not come up with a good way
> >> to fix it either, while keeping it cheap. So if you think of something,
> >> do let me know.
> >   Hum... How about linking all running block traces (struct blk_trace) in a
> > linked list and sending BLK_TN_PROCESS to all the traces? Sure we will be
> > spamming with BLK_TN_PROCESS a bit but starting a trace isn't such a common
> > thing so it shouldn't be too bad. What do you think?
> 
> That might be good enough. I'm not worried about start/stop type
> expenses, those things generally don't matter. And the list wont add any
> fast path overhead when tracing.
  OK, I've sent you a patch which does the trick for me.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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