Hi Ionela, Thanks for your reply !
On 2020/6/10 17:40, Ionela Voinescu wrote: > Hi guys, > > Sorry for showing up late to the party, I was on holiday last week. > > On Thursday 04 Jun 2020 at 13:58:22 (+0100), Sudeep Holla wrote: >> On Thu, Jun 04, 2020 at 12:42:06PM +0200, Rafael J. Wysocki wrote: >>> On Thu, Jun 4, 2020 at 6:41 AM Viresh Kumar <viresh.ku...@linaro.org> wrote: >>>> >>>> On 04-06-20, 09:32, Xiongfeng Wang wrote: >>>>> On 2020/6/3 21:39, Rafael J. Wysocki wrote: >>>>>> 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'. >>>> >>>> This is what I was worried about as well. The interface to sysfs needs >>>> to be robust. Returning frequency on some readings and 0 on others >>>> doesn't look right to me as well. This will break scripts (I am not >>>> sure if some scripts are there to look for these values) with the >>>> randomness of values returned by it. >>> >>> The only thing the scripts need to do is to skip zeros (or anything >>> less than the minimum hw frequency for that matter) coming from that >>> attribute. >>> >>>> On reading values locally from the CPU, I thought about the case where >>>> userspace can prevent a CPU going into idle just by reading its >>>> frequency from sysfs (and so waste power), but the same can be done by >>>> userspace to run arbitrary load on the CPUs. >>>> >>>> Can we do some sort of caching of the last frequency the CPU was >>>> running at before going into idle ? Then we can just check if cpu is >>>> idle and so return cached value. >>> >>> That is an option, but it looks like in this case the cpuinfo_cur_freq >>> attribute should not be present at all, as per the documentation. >>> >> >> +1 for dropping the attribute. >> > > I've been experimenting with some code quite recently that uses the > scheduler frequency scale factor to compute this hardware current rate > for CPPC. > > On the scheduler tick, the scale factor is computed in > arch_scale_freq_tick() to give an indication on delivered performance, > using AMUs on arm64 [1] and APERF/MPERF on x86 [2]. Basically, this scale > factor has the cached value of the average delivered performance between > the last two scheduler ticks, on a capacity scale: 0-1024. All that would > be needed is to convert from the scheduler frequency scale to the CPPC > expected performance scale. > > The gist of the code would be: > > delivered_perf = topology_get_freq_scale(cpu); > delivered_perf *= fb_ctrs.reference_perf; > delivered_perf = div64_u64(delivered_perf << SCHED_CAPACITY_SHIFT, > per_cpu(arch_max_freq_scale, cpu)); > > While this solution is not perfect, it would provide the best view of > the hardware "current" rate without the cost of waking up the CPU when > idle, scheduling additional work on the CPU, doing checks on whether > the CPU is idle and/or providing other caching mechanisms. I think it's a good idea. It's just that the value is a average frequency in the last two scheduler ticks, not an instantaneous frequency. What 'cppc_cpufreq_get_rate()' get is also not an instantaneous frequency. It's a average frequency in 2us. I check the interval between two frequency updates on my machine. It's about 10ms. So the frequency will change at least one time in two scheduler ticks if HZ is 1000. I am not sure how people will use 'cpuinfo_cur_freq'. They may not expect a very accurate frequency. How about we return this value when CPU is idle? Or just return 0 in idle is better ? > > Do you think such an implementation could make cpuinfo_cur_freq worth > keeping? > > I'm happy to push the patches for this and discuss the details there. Thanks. I'm happy to see the patches and give some help. Thanks, Xiongfeng > > Thanks, > Ionela. > > > [1] https://lkml.org/lkml/2020/3/5/183 > [2] https://lkml.org/lkml/2020/1/22/1039 > >> -- >> Regards, >> Sudeep > > . >