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Date:   Wed, 22 Apr 2020 16:53:34 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Giovanni Gherdovich <ggherdovich@suse.cz>
Cc:     Ingo Molnar <mingo@redhat.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>, x86@kernel.org,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@suse.de>, Len Brown <lenb@kernel.org>
Subject: Re: [PATCH] x86, sched: Prevent divisions by zero in frequency
 invariant accounting
Message-ID: <20200422145334.GM20730@hirez.programming.kicks-ass.net>
References: <20200422144055.18171-1-ggherdovich@suse.cz>
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On Wed, Apr 22, 2020 at 04:40:55PM +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.
> 
> Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
> ---
>  arch/x86/kernel/smpboot.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 8c89e4d9ad28..fb71395cbcad 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -2055,14 +2055,14 @@ 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 (!mcnt)
> +		return;

Should we not pr_warn() and disable the whole thing when this happens?