Messages in this thread | | | Date | Wed, 22 Apr 2020 16:53:34 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86, sched: Prevent divisions by zero in frequency invariant accounting |
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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?
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