— Steph
> On Apr 1, 2016, at 10:03 AM, Peter Zijlstra <[email protected]> wrote: > > On Fri, Apr 01, 2016 at 12:37:00AM -0400, Len Brown wrote: >> diff --git a/arch/x86/kernel/cpu/aperfmperf.c >> b/arch/x86/kernel/cpu/aperfmperf.c >> new file mode 100644 >> index 0000000..9380102 >> --- /dev/null >> +++ b/arch/x86/kernel/cpu/aperfmperf.c >> @@ -0,0 +1,76 @@ >> +/* >> + * x86 APERF/MPERF KHz calculation >> + * Used by /proc/cpuinfo and /sys/.../cpufreq/scaling_cur_freq >> + * >> + * Copyright (C) 2015 Intel Corp. >> + * Author: Len Brown <[email protected]> >> + * >> + * This file is licensed under GPLv2. >> + */ >> + >> +#include <linux/jiffies.h> >> +#include <linux/math64.h> >> +#include <linux/percpu.h> >> +#include <linux/smp.h> >> + >> +struct aperfmperf_sample { >> + unsigned int khz; >> + unsigned long jiffies; >> + unsigned long long aperf; >> + unsigned long long mperf; >> +}; >> + >> +static DEFINE_PER_CPU(struct aperfmperf_sample, samples); >> + >> +/* >> + * aperfmperf_snapshot_khz() >> + * On the current CPU, snapshot APERF, MPERF, and jiffies >> + * unless we already did it within 100ms >> + * calculate kHz, save snapshot >> + */ >> +static void aperfmperf_snapshot_khz(void *dummy) >> +{ >> + unsigned long long aperf, aperf_delta; >> + unsigned long long mperf, mperf_delta; >> + unsigned long long numerator; > > u64 is less typing ;-) > >> + struct aperfmperf_sample *s = &get_cpu_var(samples); >> + >> + /* Cache KHz for 100 ms */ >> + if (time_before(jiffies, s->jiffies + HZ/10)) >> + goto out; > > This puts in a lower bound, but afaict there is no upper bound. Both > users appear to be userspace controlled. > > That is; if userspace doesn't request a freq reading we can go without > reading this for a very long time. > >> + >> + rdmsrl(MSR_IA32_APERF, aperf); >> + rdmsrl(MSR_IA32_MPERF, mperf); >> + >> + aperf_delta = aperf - s->aperf; >> + mperf_delta = mperf - s->mperf; > > That means these delta's can be arbitrarily large, in fact the MSRs can > have wrapped however many times. 64 bits is 18 446 744 073 709 551 615 so even assuming a 10 GHz frequency if my math are good this is more than 58 years before the MSR wrap around, assuming the device ran always at max freq. > >> + >> + /* >> + * There is no architectural guarantee that MPERF >> + * increments faster than we can read it. >> + */ >> + if (mperf_delta == 0) >> + goto out; >> + >> + numerator = cpu_khz * aperf_delta; > > And since delta can be any 64bit value as per the msr range, this > multiplication can overflow. > >> + s->khz = div64_u64(numerator, mperf_delta); >> + s->jiffies = jiffies; >> + s->aperf = aperf; >> + s->mperf = mperf; >> + >> +out: >> + put_cpu_var(samples); >> +} >> + >> +unsigned int aperfmperf_khz_on_cpu(int cpu) >> +{ >> + if (!cpu_khz) >> + return 0; >> + >> + if (!boot_cpu_has(X86_FEATURE_APERFMPERF)) >> + return 0; > > You could do the jiffy compare here; avoiding the IPI. > >> + >> + smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1); >> + >> + return per_cpu(samples.khz, cpu); >> +} > -- > To unsubscribe from this list: send the line "unsubscribe linux-pm" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html
