Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20200416054745.740-1-ggherdovich@suse.cz> <20200416054745.740-2-ggherdovich@suse.cz>
In-Reply-To: <20200416054745.740-2-ggherdovich@suse.cz>
From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 16 Apr 2020 17:41:47 +0200
Message-ID: <CAJZ5v0hypB1cYAis6+tg5_ckY6Z4-Xbif8ZUj5ZP+M9SFHM4fw@mail.gmail.com>
Subject: Re: [PATCH 1/4] x86, sched: Bail out of frequency invariance if base
 frequency is unknown
To:     Giovanni Gherdovich <ggherdovich@suse.cz>
Cc:     Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@suse.de>, Len Brown <lenb@kernel.org>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Doug Smythies <dsmythies@telus.net>,
        Like Xu <like.xu@linux.intel.com>,
        Neil Rickert <nwr10cst-oslnx@yahoo.com>,
        Chris Wilson <chris@chris-wilson.co.uk>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Apr 16, 2020 at 7:48 AM Giovanni Gherdovich <ggherdovich@suse.cz> wrote:
>
> Some hypervisors such as VMWare ESXi 5.5 advertise support for
> X86_FEATURE_APERFMPERF but then fill all MSR's with zeroes. In particular,
> MSR_PLATFORM_INFO set to zero tricks the code that wants to know the base
> clock frequency of the CPU (highest non-turbo frequency), producing a
> division by zero when computing the ratio turbo_freq/base_freq necessary
> for frequency invariant accounting.
>
> It is to be noted that even if MSR_PLATFORM_INFO contained the appropriate
> data, APERF and MPERF are constantly zero on ESXi 5.5, thus freq-invariance
> couldn't be done in principle (not that it would make a lot of sense in a
> VM anyway). The real problem is advertising X86_FEATURE_APERFMPERF. This
> appears to be fixed in more recent versions: ESXi 6.7 doesn't advertise
> that feature.
>
> Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")

Please feel free to add

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

to all patches in the series and I'm expecting them to be routed through tip.

Thanks!

> ---
>  arch/x86/kernel/smpboot.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index fe3ab9632f3b..3a318ec9bc17 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1985,6 +1985,15 @@ static bool intel_set_max_freq_ratio(void)
>         return false;
>
>  out:
> +       /*
> +        * Some hypervisors advertise X86_FEATURE_APERFMPERF
> +        * but then fill all MSR's with zeroes.
> +        */
> +       if (!base_freq) {
> +               pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
> +               return false;
> +       }
> +
>         arch_turbo_freq_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE,
>                                         base_freq);
>         arch_set_max_freq_ratio(turbo_disabled());
> --
> 2.16.4
>