linux-kernel - BLK_TN_PROCESS events not delivered for all devices

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Message-ID: <20130916092347.GB19027@quack.suse.cz>
Date:	Mon, 16 Sep 2013 11:23:47 +0200
From:	Jan Kara <jack@...e.cz>
To:	axboe@...nel.dk
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: BLK_TN_PROCESS events not delivered for all devices

  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...

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