Re: [RFC PATCH v3 10/10] sched/fair: Throttle CFS tasks on return to userspace

Valentin Schneider
2024-07-24T07:22:17Z

On 23/07/24 18:34, Benjamin Segall wrote:
> Valentin Schneider <vschneid@redhat.com> writes:
>
>> On 18/07/24 17:25, Benjamin Segall wrote:
>>> Valentin Schneider <vschneid@redhat.com> writes:
>>>
>>>> I've tested a 10ms runtime / 100ms period cgroup with an always running
>>>> task: upstream gets a "clean" periodic pattern of 10ms runtime every 100ms,
>>>> whereas this gets something more like 40ms runtime every 400ms.
>>>
>>> Hmm, this seems a little odd since TWA_RESUME does a kick_process.
>>
>> I didn't ponder too much on the workload used here, but the point I wanted
>> to bring up is: if you give a cgroup X amount of runtime, it may still
>> consume more than that within a single period because execution in
>> kernelspace isn't immediately stopped/throttled.
>>
>> It means the "standard" bandwidth control behaviour becomes a bit more
>> bursty.
>
> Yeah, more bursty behavior when doing cpu-burning syscalls is expected.
> With the check on exit to user I wouldn't expect anything worse than the
> duration of the syscall though, so it depends on what your test was.
>

That would also be my expectations. That was rt-app, so the userspace part
is mostly floating point operations IIRC. I'll do a bit of tracing to make
sure nothing funny is happening in the kernel.