Date:   Fri, 16 Oct 2020 17:29:34 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Takashi Iwai <tiwai@...e.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Len Brown <lenb@...nel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: ACPI _CST introduced performance regresions on Haswll

On Fri, Oct 16, 2020 at 4:09 PM Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Fri, Oct 16, 2020 at 03:41:12PM +0200, Rafael J. Wysocki wrote:
> > > Turns out I didn't even have that. On another machine (same model,
> > > same cpu, different BIOS that cannot be updated), enabling the C6 state
> > > still did not enable it on boot and dmesg complained about CST not being
> > > usable. This is weird because one would expect that if CST was unusable
> > > that it would be the same as use_acpi == false.
> > >
> > > This could potentially be if the ACPI tables are unsuitable due to bad
> > > bad FFH information for a lower c-state. If _CST is not found or usable,
> > > should acpi_state_table.count be reset to go back to the old behaviour?
> >
> > Yes, it should, although I would reset it in intel_idle_cst_usable()
> > right away before returning 'false'.
> >
>
> Good stuff.
>
> > I can send a patch to do the above or please submit the one below as
> > it works too.
> >
>
> I'm happy to go with your alternative if you prefer. For a finish,
> I decided it was worth reporting if the _CST was ignored regardless of
> the reason. It performs roughly the same as setting use_acpi = false on
> the affected machines.
>
> ---8<---
> intel_idle: Ignore _CST if control cannot be taken from the platform
>
> e6d4f08a6776 ("intel_idle: Use ACPI _CST on server systems") avoids
> enabling c-states that have been disabled by the platform with the
> exception of C1E.
>
> Unfortunately, BIOS implementations are not always consistent in terms
> of how capabilities are advertised and control cannot always be handed
> over. If control cannot be handed over then intel_idle reports that "ACPI
> _CST not found or not usable" but does not clear acpi_state_table.count
> meaning the information is still partially used.
>
> This patch ignores ACPI information if CST control cannot be requested from
> the platform. This was only observed on a number of Haswell platforms that
> had identical CPUs but not identical BIOS versions.  While this problem
> may be rare overall, 24 separate test cases bisected to this specific
> commit across 4 separate test machines and is worth addressing. If the
> situation occurs, the kernel behaves as it did before commit e6d4f08a6776
> and uses any c-states that are discovered.
>
> The affected test cases were all ones that involved a small number of
> processes -- exec microbenchmark, pipe microbenchmark, git test suite,
> netperf, tbench with one client and system call microbenchmark. Each
> case benefits from being able to use turboboost which is prevented if the
> lower c-states are unavailable. This may mask real regressions specific
> to older hardware so it is worth addressing.
>
> C-state status before and after the patch
>
> 5.9.0-vanilla            POLL     latency:0      disabled:0 default:enabled
> 5.9.0-vanilla            C1       latency:2      disabled:0 default:enabled
> 5.9.0-vanilla            C1E      latency:10     disabled:0 default:enabled
> 5.9.0-vanilla            C3       latency:33     disabled:1 default:disabled
> 5.9.0-vanilla            C6       latency:133    disabled:1 default:disabled
> 5.9.0-ignore-cst-v1r1    POLL     latency:0      disabled:0 default:enabled
> 5.9.0-ignore-cst-v1r1    C1       latency:2      disabled:0 default:enabled
> 5.9.0-ignore-cst-v1r1    C1E      latency:10     disabled:0 default:enabled
> 5.9.0-ignore-cst-v1r1    C3       latency:33     disabled:0 default:enabled
> 5.9.0-ignore-cst-v1r1    C6       latency:133    disabled:0 default:enabled
>
> Patch enables C3/C6.
>
> Netperf UDP_STREAM
>
> netperf-udp
>                                       5.5.0                  5.9.0
>                                     vanilla        ignore-cst-v1r1
> Hmean     send-64         193.41 (   0.00%)      226.54 *  17.13%*
> Hmean     send-128        392.16 (   0.00%)      450.54 *  14.89%*
> Hmean     send-256        769.94 (   0.00%)      881.85 *  14.53%*
> Hmean     send-1024      2994.21 (   0.00%)     3468.95 *  15.85%*
> Hmean     send-2048      5725.60 (   0.00%)     6628.99 *  15.78%*
> Hmean     send-3312      8468.36 (   0.00%)    10288.02 *  21.49%*
> Hmean     send-4096     10135.46 (   0.00%)    12387.57 *  22.22%*
> Hmean     send-8192     17142.07 (   0.00%)    19748.11 *  15.20%*
> Hmean     send-16384    28539.71 (   0.00%)    30084.45 *   5.41%*
>
> Fixes: e6d4f08a6776 ("intel_idle: Use ACPI _CST on server systems")
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> ---
>  drivers/idle/intel_idle.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index 9a810e4a7946..4af2d3f2c8aa 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -1212,14 +1212,13 @@ static bool __init intel_idle_acpi_cst_extract(void)
>                 if (!intel_idle_cst_usable())
>                         continue;
>
> -               if (!acpi_processor_claim_cst_control()) {
> -                       acpi_state_table.count = 0;
> -                       return false;
> -               }
> +               if (!acpi_processor_claim_cst_control())
> +                       break;
>
>                 return true;
>         }
>
> +       acpi_state_table.count = 0;
>         pr_debug("ACPI _CST not found or not usable\n");
>         return false;
>  }

Applied as a fix for 5.10-rc1, thanks!

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