Re: Sending some form of notifaction when sched_fifo throttling kicks in...

[Bharata B Rao]

On Thu, Dec 23, 2010 at 6:47 PM, Dario Faggioli <raistlin@xxxxxxxx> wrote:
> On Thu, 2010-12-23 at 12:39 +0100, torbenh wrote:
>> hi...
>>
> Hi,
>
>> when the rt_runtime budget is exceeded, the kernel silently stops
>> scheduling RT tasks. there is no way to distinguish this from
>> a task taking very long to complete.
>>
> Well, this depends on how you do the accounting in your (user-level)
> code.
>
>> it would be very nice, if the kernel would send some form of notifaction
>> when it starts throttling things.
>>
> This might be tricky, if the meaning is signals or something to the
> throttled tasks, since you can have a (or more!) runqueue full of
> them... Are we signalling them all? Moreover, they'll get the
> notification only after resuming, and it's not guaranteed that this
> helps in finding out who is "responsible for" the throttling and... By
> the way...
>
>> recording the timestamp of the last occurence of throttling
>> in a /proc file would be sufficient, if there were no cgroups.
>>
>> would it be possible to add a readonly property to the cpu subsystem ?
>>
> ... If you think you're fine with some /proc (and perhaps cpuacct, if
> cgroups are being used) readable, I can try to come up with something.
>
> I think that the number of times that throttling fired and the last
> throttling instant could be exported this way without much issues.
>
> Do others have some idea and/or comments about that? This is
> ABI/interface, and that really scares me! :-P

You might want to note that CFS bandwidth control patches export some
such stats via a new per-cgroup file cpu.stat. It exports 3 statistics:

nr_periods: Number of enforcement intervals that have elapsed.
nr_throttled: Number of times the group has been throttled/limited.
throttled_time: The total time duration (in nanoseconds) for which the group
remained throttled.

Regards,
Bharata.
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