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

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Date:   Tue, 6 Oct 2020 20:03:22 +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 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

-- 
Mel Gorman
SUSE Labs