Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Sat, 25 Mar 2017 02:39:05 +0100 | Subject | Re: [RFC][PATCH v3 2/2] cpufreq: schedutil: Avoid reducing frequency of busy CPUs prematurely |
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On Sat, Mar 25, 2017 at 2:14 AM, Sai Gurrappadi <sgurrappadi@nvidia.com> wrote: > Hi Rafael, > > On 03/21/2017 04:08 PM, Rafael J. Wysocki wrote: >> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com> >> >> The way the schedutil governor uses the PELT metric causes it to >> underestimate the CPU utilization in some cases. >> >> That can be easily demonstrated by running kernel compilation on >> a Sandy Bridge Intel processor, running turbostat in parallel with >> it and looking at the values written to the MSR_IA32_PERF_CTL >> register. Namely, the expected result would be that when all CPUs >> were 100% busy, all of them would be requested to run in the maximum >> P-state, but observation shows that this clearly isn't the case. >> The CPUs run in the maximum P-state for a while and then are >> requested to run slower and go back to the maximum P-state after >> a while again. That causes the actual frequency of the processor to >> visibly oscillate below the sustainable maximum in a jittery fashion >> which clearly is not desirable. >> >> That has been attributed to CPU utilization metric updates on task >> migration that cause the total utilization value for the CPU to be >> reduced by the utilization of the migrated task. If that happens, >> the schedutil governor may see a CPU utilization reduction and will >> attempt to reduce the CPU frequency accordingly right away. That >> may be premature, though, for example if the system is generally >> busy and there are other runnable tasks waiting to be run on that >> CPU already. >> > > Thinking out loud a bit, I wonder if what you really want to do is basically: > > schedutil_cpu_util(cpu) = max(cpu_rq(cpu)->cfs.util_avg, total_cpu_util_avg); > > Where total_cpu_util_avg tracks the average utilization of the CPU itself over time (% of time the CPU was busy) in the same PELT like manner. The difference here is that it doesn't change instantaneously as tasks migrate in/out but it decays/accumulates just like the per-entity util_avgs. > > Over time, total_cpu_util_avg and cfs_rq(cpu)->util_avg will tend towards each other the lesser the amount of 'overlap' / overloading. > > Yes, the above metric would 'overestimate' in case all tasks have migrated away and we are left with an idle CPU. A fix for that could be to just use the PELT value like so: > > schedutil_cpu_util(cpu) = max(cpu_rq(cpu)->cfs.util_avg, idle_cpu(cpu) ? 0 : total_cpu_util_avg); > > Note that the problem described here in the commit message doesn't need fully runnable threads, it just needs two threads to execute in parallel on the same CPU for a period of time. I don't think looking at just idle_calls necessarily covers all cases. > > Thoughts?
Well, is the total_cpu_util_avg metric readily available?
Thanks, Rafael
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