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

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Date:	Tue, 17 Sep 2013 08:29:07 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Jan Kara <jack@...e.cz>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: BLK_TN_PROCESS events not delivered for all devices

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.

-- 
Jens Axboe

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