Messages in this thread | | | Date | Tue, 17 Sep 2013 12:23:33 -0600 | From | Jens Axboe <> | Subject | Re: BLK_TN_PROCESS events not delivered for all devices |
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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.
-- Jens Axboe
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