[ltt-dev] limiting LTTng to a certain set of cores

[Mathieu Desnoyers]

* Andrew McDermott (andrew.mcdermott at windriver.com) wrote:
> 
> Is there an easy way to limit LTTng to trace a specified set of cores
> only?  I ask because in some 64+ core systems the data set collected
> becomes too large for analysis and the tasks to be analyzed are always
> bound to certain cores (using affinity).
> 
> I'm anticipating that this would require changes to LTTng - what I would
> like to know is how invasive those changes would need to be.
> 

Hi Andrew,

Basically, this could be done on a per tracing-session/per-channel basis
by adding an "active core" bit mask to test within ltt_vtrace and
ltt_serialize_data, in the loop where it iterates on active traces and
calls the "filter" callback. This bitmask would sit within the channel
structure. Setting the active cores should be added to the debugfs
interface :

People would do :
echo tracename > /mnt/debugfs/ltt/setup_trace
echo CPUNR > /mnt/debugfs/ltt/control/tracename/channel/channelname/cpus
cat /mnt/debugfs/ltt/control/tracename/channel/channelname/cpus
...

Maybe we should replace the "enable" test by a cpu mask test with all
cpus active to save some cycles...

And I don't particularly care about the exact debugfs file interface
(file name, the way the cpus are listed). Ideally if we can find
somewhere else in the Linux kernel where it is already done, that would
be a great inspiration source.

Thanks,

Mathieu

> Thanks,
> Andy.
> 
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-- 
Mathieu Desnoyers
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