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

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Date:   Wed, 7 Oct 2020 17:45:30 +0200
From:   "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Takashi Iwai <tiwai@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: ACPI _CST introduced performance regresions on Haswll

On 10/6/2020 11:18 PM, Mel Gorman wrote:
> On Tue, Oct 06, 2020 at 09:29:24PM +0200, Rafael J. Wysocki wrote:
>>> 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.
>>
> This was cleared up in another mail.
>
>> 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).
>>
> Thanks.  I applied the following
>
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index 9a810e4a7946..6478347669a9 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -1044,7 +1044,7 @@ static const struct idle_cpu idle_cpu_hsw __initconst = {
>   static const struct idle_cpu idle_cpu_hsx __initconst = {
>   	.state_table = hsw_cstates,
>   	.disable_promotion_to_c1e = true,
> -	.use_acpi = true,
> +	.use_acpi = false,
>   };
>
> netperf UDP_STREAM
>                                        pre                 enable                 enable                5.9-rc8                5.9-rc8
>                                        cst                    cst        cst-no-hsx-acpi                vanilla            no-hsx-acpi
> Hmean     send-64       203.96 (   0.00%)      179.23 * -12.13%*      201.04 (  -1.44%)      203.24 (  -0.36%)      233.43 *  14.45%*
> Hmean     send-128      403.66 (   0.00%)      355.99 * -11.81%*      402.28 (  -0.34%)      387.65 *  -3.97%*      461.47 *  14.32%*
> Hmean     send-256      784.39 (   0.00%)      697.78 * -11.04%*      782.15 (  -0.29%)      758.49 *  -3.30%*      895.31 *  14.14%*
> Hmean     recv-64       203.96 (   0.00%)      179.23 * -12.13%*      201.04 (  -1.44%)      203.24 (  -0.36%)      233.43 *  14.45%*
> Hmean     recv-128      403.66 (   0.00%)      355.99 * -11.81%*      402.28 (  -0.34%)      387.65 *  -3.97%*      461.47 *  14.32%*
> Hmean     recv-256      784.39 (   0.00%)      697.78 * -11.04%*      782.15 (  -0.29%)      758.49 *  -3.30%*      895.28 *  14.14%*
>
> This is a more limited run to save time but is enough to illustrate
> the point.
>
> pre-cst is just before your patch
> enable-cst is your patch that was bisected
> enable-cst-no-hsx-acpi is your patch with use_acpi disabled
> 5.9-rc8-vanilla is what it sounds like
> 5.9-rc8-no-hsx-acpi disables use_acpi
>
> The enable-cst-no-hsx-acpi result indicates that use_acpi was the issue for
> Haswell (at least these machines). Looking just at 5.9-rc8-vanillaa might
> have been misleading because its performance is not far off the baseline
> due to unrelated changes that mostly offset the performance penalty.
>
> The key question is -- how appropriate would it be to disable acpi for
> Haswell? Would that be generally safe or could it hide other surprises?
>
It should be safe, but let's try to do something more fine-grained.

There is the CPUIDLE_FLAG_ALWAYS_ENABLE flag that is set for C1E.  Can 
you please try to set it for C6 in hsw_cstates instead of clearing 
use_acpi in idle_cpu_hsx and retest?