Messages in this thread | | | Subject | Re: [Question]: about 'cpuinfo_cur_freq' shown in sysfs when the CPU is in idle state | From | Xiongfeng Wang <> | Date | Thu, 4 Jun 2020 09:32:41 +0800 |
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Hi Rafael,
Thanks for your reply !
On 2020/6/3 21:39, Rafael J. Wysocki wrote: > On Wed, Jun 3, 2020 at 9:52 AM Viresh Kumar <viresh.kumar@linaro.org> wrote: >> >> On 02-06-20, 11:34, Xiongfeng Wang wrote: >>> Hi Viresh, >>> >>> Sorry to disturb you about another problem as follows. >>> >>> CPPC use the increment of Desired Performance counter and Reference Performance >>> counter to get the CPU frequency and show it in sysfs through >>> 'cpuinfo_cur_freq'. But ACPI CPPC doesn't specifically define the behavior of >>> these two counters when the CPU is in idle state, such as stop incrementing when >>> the CPU is in idle state. >>> >>> ARMv8.4 Extension inctroduced support for the Activity Monitors Unit (AMU). The >>> processor frequency cycles and constant frequency cycles in AMU can be used as >>> Delivered Performance counter and Reference Performance counter. These two >>> counter in AMU does not increase when the PE is in WFI or WFE. So the increment >>> is zero when the PE is in WFI/WFE. This cause no issue because >>> 'cppc_get_rate_from_fbctrs()' in cppc_cpufreq driver will check the increment >>> and return the desired performance if the increment is zero. >>> >>> But when the CPU goes into power down idle state, accessing these two counters >>> in AMU by memory-mapped address will return zero. Such as CPU1 went into power >>> down idle state and CPU0 try to get the frequency of CPU1. In this situation, >>> will display a very big value for 'cpuinfo_cur_freq' in sysfs. Do you have some >>> advice about this problem ? >>> >>> I was thinking about an idea as follows. We can run 'cppc_cpufreq_get_rate()' on >>> the CPU to be measured, so that we can make sure the CPU is in C0 state when we >>> access the two counters. Also we can return the actual frequency rather than >>> desired performance when the CPU is in WFI/WFE. But this modification will >>> change the existing logical and I am not sure if this will cause some bad effect. >>> >>> >>> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c >>> index 257d726..ded3bcc 100644 >>> --- a/drivers/cpufreq/cppc_cpufreq.c >>> +++ b/drivers/cpufreq/cppc_cpufreq.c >>> @@ -396,9 +396,10 @@ static int cppc_get_rate_from_fbctrs(struct cppc_cpudata *cpu, >>> return cppc_cpufreq_perf_to_khz(cpu, delivered_perf); >>> } >>> >>> -static unsigned int cppc_cpufreq_get_rate(unsigned int cpunum) >>> +static int cppc_cpufreq_get_rate_cpu(void *info) >>> { >>> struct cppc_perf_fb_ctrs fb_ctrs_t0 = {0}, fb_ctrs_t1 = {0}; >>> + unsigned int cpunum = *(unsigned int *)info; >>> struct cppc_cpudata *cpu = all_cpu_data[cpunum]; >>> int ret; >>> >>> @@ -418,6 +419,22 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpunum) >>> return cppc_get_rate_from_fbctrs(cpu, fb_ctrs_t0, fb_ctrs_t1); >>> } >>> >>> +static unsigned int cppc_cpufreq_get_rate(unsigned int cpunum) >>> +{ >>> + unsigned int ret; >>> + >>> + ret = smp_call_on_cpu(cpunum, cppc_cpufreq_get_rate_cpu, &cpunum, true); >>> + >>> + /* >>> + * convert negative error code to zero, otherwise we will display >>> + * an odd value for 'cpuinfo_cur_freq' in sysfs >>> + */ >>> + if (ret < 0) >>> + ret = 0; >>> + >>> + return ret; >>> +} >>> + >>> static int cppc_cpufreq_set_boost(struct cpufreq_policy *policy, int state) >>> { >>> struct cppc_cpudata *cpudata; >> >> I don't see any other sane solution, even if this brings the CPU back >> to normal state and waste power. We should be able to reliably provide >> value to userspace. >> >> Rafael / Sudeep: What you do say ? > > The frequency value obtained by kicking the CPU out of idle > artificially is bogus, though. You may as well return a random number > instead.
Yes, it may return a randowm number as well.
> > The frequency of a CPU in an idle state is in fact unknown in the case > at hand, so returning 0 looks like the cleanest option to me.
I am not sure about how the user will use 'cpuinfo_cur_freq' in sysfs. If I return 0 when the CPU is idle, when I run a light load on the CPU, I will get a zero value for 'cpuinfo_cur_freq' when the CPU is idle. When the CPU is not idle, I will get a non-zero value. The user may feel odd about 'cpuinfo_cur_frreq' switching between a zero value and a non-zero value. They may hope it can return the frequency when the CPU execute instructions, namely in C0 state. I am not so sure about the user will look at 'cpuinfo_cur_freq'.
Thanks, Xiongfeng
> > Thanks! > > . >
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