[lttng-dev] [MODULES RFC PATCH] Add PID field to block_* events

Jens Axboe axboe at kernel.dk
Thu May 22 09:50:34 EDT 2014

On 2014-05-21 16:01, Mathieu Desnoyers wrote:
> CCing Jens Axboe, who really know better than I do about the block
> layer! Also adding Steven, since he maintains the Linux upstream block
> layer instrumentation (closely matching the lttng-modules version).

The blktrace parts are still maintained by me, actually not sure why it 
moved location.

> ----- Original Message -----
>> From: "Julien Desfossez" <jdesfossez at efficios.com>
>> To: "mathieu desnoyers" <mathieu.desnoyers at efficios.com>
>> Cc: lttng-dev at lists.lttng.org, "Julien Desfossez" <jdesfossez at efficios.com>
>> Sent: Wednesday, May 21, 2014 5:29:15 PM
>> Subject: [MODULES RFC PATCH] Add PID field to block_* events
>> Most of the block events have the "comm" field, but we have no way to
>> match a block event to a certain PID which makes performing accurate
>> per-process analyses difficult.
> Color me clueless, but isn't the block I/O activity independent of
> the PID which triggers the I/O (e.g. issuing the read(), write(),
> (and so on) system calls) for read ahead, writeback kernel thread
> activity ? Or perhaps are those just rare special case, and for
> the usual case we can use the current PID in which the block layer
> tracepoints are hit as indicator of which PID triggered the I/O ?

It's usually only disconnected for the case of async writes. But even 
for that case, it's interesting to see who is submitting the IO. 
blktrace already issues a pid note, though, for every new process that 
does IO. So what problem are we trying to solve here? The case of 
multiple threads with the same ->comm?

Jens Axboe

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