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Subject: Re: ACPI _CST introduced performance regresions on Haswll
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Takashi Iwai <tiwai@suse.de>, linux-kernel@vger.kernel.org
References: <20201006083639.GJ3227@techsingularity.net>
 <c3566d2b-3da1-917b-2df6-f7dcfb33c8ed@intel.com>
 <20201006190322.GL3227@techsingularity.net>
From:   "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Organization: Intel Technology Poland Sp. z o. o., KRS 101882, ul. Slowackiego
 173, 80-298 Gdansk
Message-ID: <25f31d3e-7a67-935f-93ba-32216a5084e2@intel.com>
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!