From: Giovanni Gherdovich <ggherdovich@suse.cz>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>,
Len Brown <lenb@kernel.org>,
"Rafael J . Wysocki" <rjw@rjwysocki.net>,
x86@kernel.org, linux-pm@vger.kernel.org,
linux-kernel@vger.kernel.org,
Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting
Date: Sat, 02 May 2020 16:25:00 +0200 [thread overview]
Message-ID: <1588429500.8505.29.camel@suse.cz> (raw)
In-Reply-To: <20200501133042.GE3762@hirez.programming.kicks-ass.net>
On Fri, 2020-05-01 at 15:30 +0200, Peter Zijlstra wrote:
> On Tue, Apr 28, 2020 at 03:24:49PM +0200, Giovanni Gherdovich wrote:
> > The product mcnt * arch_max_freq_ratio could be zero if it overflows u64.
> >
> > For context, a large value for arch_max_freq_ratio would be 5000,
> > corresponding to a turbo_freq/base_freq ratio of 5 (normally it's more like
> > 1500-2000). A large increment frequency for the MPERF counter would be 5GHz
> > (the base clock of all CPUs on the market today is less than that). With
> > these figures, a CPU would need to go without a scheduler tick for around 8
> > days for the u64 overflow to happen. It is unlikely, but the check is
> > warranted.
> >
> > In that case it's also appropriate to disable frequency invariant
> > accounting: the feature relies on measures of the clock frequency done at
> > every scheduler tick, which need to be "fresh" to be at all meaningful.
> >
> > Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> > Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
> > acnt <<= 2*SCHED_CAPACITY_SHIFT;
> > mcnt *= arch_max_freq_ratio;
> > + if (!mcnt) {
>
> The problem is; this doesn't do what you claim it does.
>
> > + pr_warn("Scheduler tick missing for long time, disabling scale-invariant accounting.\n");
> > + /* static_branch_disable() acquires a lock and may sleep */
> > + schedule_work(&disable_freq_invariance_work);
> > + return;
> > + }
> >
> > freq_scale = div64_u64(acnt, mcnt);
>
> I've changed the patch like so.. OK?
>
> (ok, perhaps I went a little overboard with the paranoia ;-)
Right, I wasn't really checking for overflow, only for when the product
"mcnt * arch_max_freq_ratio" becomes zero.
Thanks for your edit (I took note of the macros check_*_overflow, didn't know
them). I fully subscribe to the paranoid approach.
I understand you've already edited the patches in your tree, so I am not
resending, just confirming my
Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
>
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -55,6 +55,7 @@
> #include <linux/gfp.h>
> #include <linux/cpuidle.h>
> #include <linux/numa.h>
> +#include <linux/overflow.h>
>
> #include <asm/acpi.h>
> #include <asm/desc.h>
> @@ -2057,11 +2058,19 @@ static void init_freq_invariance(bool se
> }
> }
>
> +static void disable_freq_invariance_workfn(struct work_struct *work)
> +{
> + static_branch_disable(&arch_scale_freq_key);
> +}
> +
> +static DECLARE_WORK(disable_freq_invariance_work,
> + disable_freq_invariance_workfn);
> +
> DEFINE_PER_CPU(unsigned long, arch_freq_scale) = SCHED_CAPACITY_SCALE;
>
> void arch_scale_freq_tick(void)
> {
> - u64 freq_scale;
> + u64 freq_scale = SCHED_CAPACITY_SCALE;
> u64 aperf, mperf;
> u64 acnt, mcnt;
>
> @@ -2073,19 +2082,27 @@ void arch_scale_freq_tick(void)
>
> acnt = aperf - this_cpu_read(arch_prev_aperf);
> mcnt = mperf - this_cpu_read(arch_prev_mperf);
> - if (!mcnt)
> - return;
>
> this_cpu_write(arch_prev_aperf, aperf);
> this_cpu_write(arch_prev_mperf, mperf);
>
> - acnt <<= 2*SCHED_CAPACITY_SHIFT;
> - mcnt *= arch_max_freq_ratio;
> + if (check_shl_overflow(acnt, 2*SCHED_CAPACITY_SHIFT, &acnt))
> + goto error;
> +
> + if (check_mul_overflow(mcnt, arch_max_freq_ratio, &mcnt) || !mcnt)
> + goto error;
>
> freq_scale = div64_u64(acnt, mcnt);
> + if (!freq_scale)
> + goto error;
>
> if (freq_scale > SCHED_CAPACITY_SCALE)
> freq_scale = SCHED_CAPACITY_SCALE;
>
> this_cpu_write(arch_freq_scale, freq_scale);
> + return;
> +
> +error:
> + pr_warn("Scheduler frequency invariance went wobbly, disabling!\n");
> + schedule_work(&disable_freq_invariance_work);
> }
next prev parent reply other threads:[~2020-05-02 14:25 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-28 13:24 [PATCH 0/2] More frequency invariance fixes for x86 Giovanni Gherdovich
2020-04-28 13:24 ` [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting Giovanni Gherdovich
2020-04-29 11:30 ` Rafael J. Wysocki
2020-05-01 13:30 ` Peter Zijlstra
2020-05-02 14:25 ` Giovanni Gherdovich [this message]
2020-05-18 22:20 ` Ricardo Neri
2020-05-19 16:46 ` Giovanni Gherdovich
2020-04-28 13:24 ` [PATCH 2/2] x86, sched: Bail out of frequency invariance if turbo frequency is unknown Giovanni Gherdovich
2020-04-29 11:30 ` Rafael J. Wysocki
2020-05-01 13:04 ` Peter Zijlstra
2020-05-02 0:06 ` Ricardo Neri
2020-05-02 14:26 ` Giovanni Gherdovich
2020-05-02 0:04 ` Ricardo Neri
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