Re: ACPI _CST introduced performance regresions on Haswll

[Rafael J. Wysocki]

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!