Re: [PATCH] x86, sched: Prevent divisions by zero in frequency invariant accounting
From: Peter Zijlstra
Date: Wed Apr 22 2020 - 10:54:06 EST
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@xxxxxxx>
> 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?