Messages in this thread |  | | Subject | Re: ACPI _CST introduced performance regresions on Haswll | From | "Rafael J. Wysocki" <> | Date | Tue, 6 Oct 2020 21:29:24 +0200 |
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On 10/6/2020 9:03 PM, Mel Gorman wrote: > On Tue, Oct 06, 2020 at 06:00:18PM +0200, Rafael J. Wysocki wrote: >>> server systems") and enable-cst is the commit. It was not fixed by 5.6 or >>> 5.9-rc8. A lot of bisections ended up here including kernel compilation, >>> tbench, syscall entry/exit microbenchmark, hackbench, Java workloads etc. >>> >>> What I don't understand is why. The latencies for c-state exit states >>> before and after the patch are both as follows >>> >>> /sys/devices/system/cpu/cpu0/cpuidle/state0/latency:0 >>> /sys/devices/system/cpu/cpu0/cpuidle/state1/latency:2 >>> /sys/devices/system/cpu/cpu0/cpuidle/state2/latency:10 >>> /sys/devices/system/cpu/cpu0/cpuidle/state3/latency:33 >>> /sys/devices/system/cpu/cpu0/cpuidle/state4/latency:133 >>> >>> Perf profiles did not show up anything interesting. A diff of >>> /sys/devices/system/cpu/cpu0/cpuidle/state0/ before and after the patch >>> showed up nothing interesting. Any idea why exactly this patch shows up >>> as being hazardous on Haswell in particular? >>> >> Presumably, some of the idle states are disabled by default on the affected >> machines. >> >> Can you check the disable and default_status attributes of each state before >> and after the commit in question? >> > # grep . pre-cst/cpuidle/state*/disable > pre-cst/cpuidle/state0/disable:0 > pre-cst/cpuidle/state1/disable:0 > pre-cst/cpuidle/state2/disable:0 > pre-cst/cpuidle/state3/disable:0 > pre-cst/cpuidle/state4/disable:0 > # grep . enable-cst/cpuidle/state*/disable > enable-cst/cpuidle/state0/disable:0 > enable-cst/cpuidle/state1/disable:0 > enable-cst/cpuidle/state2/disable:0 > enable-cst/cpuidle/state3/disable:0 > enable-cst/cpuidle/state4/disable:0 > # grep . pre-cst/cpuidle/state*/default_status > pre-cst/cpuidle/state0/default_status:enabled > pre-cst/cpuidle/state1/default_status:enabled > pre-cst/cpuidle/state2/default_status:enabled > pre-cst/cpuidle/state3/default_status:enabled > pre-cst/cpuidle/state4/default_status:enabled > > After the commit, the default_status file does not appear in /sys > Something is amiss, then, because the commit doesn't affect the presence of this file.
The only thing it does is to set the use_acpi flag for several processor models in intel_idle.c.
It can be effectively reversed by removing all of the ".use_acpi = true," lines from intel_idle.c.
In particular, please check if changing the value of use_acpi in struct idle_cpu_hsx from 'true' to 'false' alone (without reverting the commit) makes the issue go away in 5.9-rc8 (the default_status file should be present regardless).
Cheers!
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