linux-kernel - Re: ACPI _CST introduced performance regresions on Haswll

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Date:   Tue, 6 Oct 2020 20:47:45 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:     Takashi Iwai <tiwai@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: ACPI _CST introduced performance regresions on Haswll

On Tue, Oct 06, 2020 at 08:03:22PM +0100, 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

Sorry, second attempt after thinking the results made no sense at all.
Turns out I fat fingered setting up the enable-cst kernel the second time
to collect what you asked for and the patch was not applied at all.

# 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 . 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
# 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:1
enable-cst/cpuidle/state4/disable:1
# grep . enable-cst/cpuidle/state*/default_status
enable-cst/cpuidle/state0/default_status:enabled
enable-cst/cpuidle/state1/default_status:enabled
enable-cst/cpuidle/state2/default_status:enabled
enable-cst/cpuidle/state3/default_status:disabled
enable-cst/cpuidle/state4/default_status:disabled

That looks like C3 and C6 are disabled after the patch.

# grep . enable-cst/cpuidle/state*/name
enable-cst/cpuidle/state0/name:POLL
enable-cst/cpuidle/state1/name:C1
enable-cst/cpuidle/state2/name:C1E
enable-cst/cpuidle/state3/name:C3
enable-cst/cpuidle/state4/name:C6

-- 
Mel Gorman
SUSE Labs