Messages in this thread |  | | Date | Mon, 13 Aug 2018 11:12:21 +0100 | From | Patrick Bellasi <> | Subject | Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks |
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Hi Vincent!
On 09-Aug 18:03, Vincent Guittot wrote: > > On 07-Aug 15:26, Juri Lelli wrote:
[...]
> > > > + util_cfs = cpu_util_cfs(rq); > > > > + util_rt = cpu_util_rt(rq); > > > > + if (sched_feat(UCLAMP_SCHED_CLASS)) { > > > > + util = 0; > > > > + if (util_cfs) > > > > + util += uclamp_util(cpu_of(rq), util_cfs); > > > > + if (util_rt) > > > > + util += uclamp_util(cpu_of(rq), util_rt); > > > > + } else { > > > > + util = cpu_util_cfs(rq); > > > > + util += cpu_util_rt(rq); > > > > + util = uclamp_util(cpu_of(rq), util); > > > > + } > > > > Regarding the two policies, do you have any comment? > > Does the policy for (sched_feat(UCLAMP_SCHED_CLASS)== true) really > make sense as it is ? > I mean, uclamp_util doesn't make any difference between rt and cfs > tasks when clamping the utilization so why should be add twice the > returned value ? > IMHO, this policy would make sense if there were something like > uclamp_util_rt() and a uclamp_util_cfs()
The idea for the UCLAMP_SCHED_CLASS policy is to improve fairness on low-priority classese, especially when we have high RT utilization.
Let say we have:
util_rt = 40%, util_min=0% util_cfs = 10%, util_min=50%
the two policies will select:
UCLAMP_SCHED_CLASS: util = uclamp(40) + uclamp(10) = 50 + 50 = 100% !UCLAMP_SCHED_CLASS: util = uclamp(40 + 10) = uclmp(50) = 50%
Which means that, despite the CPU's util_min will be set to 50% when CFS is running, these tasks will have almost no boost at all, since their bandwidth margin is eclipsed by RT tasks.
> > We had an internal discussion and we found pro/cons for both... but
The UCLAMP_SCHED_CLASS policy is thus less energy efficiency but it should grant a better "isolation" in terms of what is the expected speed-up a task will get at run-time, independently from higher priority classes.
Does that make sense?
> > I'm not sure keeping the sched_feat is a good solution on the long > > run, i.e. mainline merge ;)
This problem still stands...
-- #include <best/regards.h>
Patrick Bellasi
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