From: Peter Zijlstra <peterz@infradead.org>
To: Giovanni Gherdovich <ggherdovich@suse.cz>
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: Fri, 1 May 2020 15:30:42 +0200 [thread overview]
Message-ID: <20200501133042.GE3762@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200428132450.24901-2-ggherdovich@suse.cz>
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 ;-)
--- 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:[~2020-05-01 13:30 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 [this message]
2020-05-02 14:25 ` Giovanni Gherdovich
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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