Messages in this thread | | | Date | Wed, 25 Oct 2023 12:01:16 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 3/3] cpufreq: CPPC: Eliminate the impact of cpc_read() latency error |
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On Wed, Oct 25, 2023 at 05:38:47PM +0800, Zeng Heng wrote: > We have found significant differences in the latency of cpc_read() between > regular scenarios and scenarios with high memory access pressure. Ignoring > this error can result in getting rate interface occasionally returning > absurd values. > > Here provides a high memory access sample test by stress-ng. My local > testing platform includes 160 CPUs, the CPC registers is accessed by mmio > method, and the cpuidle feature is disabled (the AMU always works online): > > ~~~ > ./stress-ng --memrate 160 --timeout 180 > ~~~ > > The following data is sourced from ftrace statistics towards > cppc_get_perf_ctrs(): > > Regular scenarios || High memory access pressure scenarios > 104) | cppc_get_perf_ctrs() { || 133) | cppc_get_perf_ctrs() { > 104) 0.800 us | cpc_read.isra.0(); || 133) 4.580 us | cpc_read.isra.0(); > 104) 0.640 us | cpc_read.isra.0(); || 133) 7.780 us | cpc_read.isra.0(); > 104) 0.450 us | cpc_read.isra.0(); || 133) 2.550 us | cpc_read.isra.0(); > 104) 0.430 us | cpc_read.isra.0(); || 133) 0.570 us | cpc_read.isra.0(); > 104) 4.610 us | } || 133) ! 157.610 us | } > 104) | cppc_get_perf_ctrs() { || 133) | cppc_get_perf_ctrs() { > 104) 0.720 us | cpc_read.isra.0(); || 133) 0.760 us | cpc_read.isra.0(); > 104) 0.720 us | cpc_read.isra.0(); || 133) 4.480 us | cpc_read.isra.0(); > 104) 0.510 us | cpc_read.isra.0(); || 133) 0.520 us | cpc_read.isra.0(); > 104) 0.500 us | cpc_read.isra.0(); || 133) + 10.100 us | cpc_read.isra.0(); > 104) 3.460 us | } || 133) ! 120.850 us | } > 108) | cppc_get_perf_ctrs() { || 87) | cppc_get_perf_ctrs() { > 108) 0.820 us | cpc_read.isra.0(); || 87) ! 255.200 us | cpc_read.isra.0(); > 108) 0.850 us | cpc_read.isra.0(); || 87) 2.910 us | cpc_read.isra.0(); > 108) 0.590 us | cpc_read.isra.0(); || 87) 5.160 us | cpc_read.isra.0(); > 108) 0.610 us | cpc_read.isra.0(); || 87) 4.340 us | cpc_read.isra.0(); > 108) 5.080 us | } || 87) ! 315.790 us | } > 108) | cppc_get_perf_ctrs() { || 87) | cppc_get_perf_ctrs() { > 108) 0.630 us | cpc_read.isra.0(); || 87) 0.800 us | cpc_read.isra.0(); > 108) 0.630 us | cpc_read.isra.0(); || 87) 6.310 us | cpc_read.isra.0(); > 108) 0.420 us | cpc_read.isra.0(); || 87) 1.190 us | cpc_read.isra.0(); > 108) 0.430 us | cpc_read.isra.0(); || 87) + 11.620 us | cpc_read.isra.0(); > 108) 3.780 us | } || 87) ! 207.010 us | } > > My local testing platform works under 3000000hz, but the cpuinfo_cur_freq > interface returns values that are not even close to the actual frequency: > > [root@localhost ~]# cd /sys/devices/system/cpu > [root@localhost cpu]# for i in {0..159}; do cat cpu$i/cpufreq/cpuinfo_cur_freq; done > 5127812 > 2952127 > 3069001 > 3496183 > 922989768 > 2419194 > 3427042 > 2331869 > 3594611 > 8238499 > ... > > The reason is when under heavy memory access pressure, the execution of > cpc_read() delay has increased from sub-microsecond to several hundred > microseconds. Moving the cpc_read function into a critical section by irq > disable/enable has minimal impact on the result. > > cppc_get_perf_ctrs()[0] cppc_get_perf_ctrs()[1] > / \ / \ > cpc_read cpc_read cpc_read cpc_read > ref[0] delivered[0] ref[1] delivered[1] > | | | | > v v v v > -----------------------------------------------------------------------> time > <--delta[0]--> <------sample_period------> <-----delta[1]-----> > > Since that, > freq = ref_freq * (delivered[1] - delivered[0]) / (ref[1] - ref[0]) > and > delivered[1] - delivered[0] = freq * (delta[1] + sample_period), > ref[1] - ref[0] = ref_freq * (delta[0] + sample_period) > > To eliminate the impact of system memory access latency, setting a > sampling period of 2us is far from sufficient. Consequently, we suggest > cppc_cpufreq_get_rate() only can be called in the process context, and > adopt a longer sampling period to neutralize the impact of random latency. > > Here we call the cond_resched() function instead of sleep-like functions > to ensure that `taskset -c $i cat cpu$i/cpufreq/cpuinfo_cur_freq` could > work when cpuidle feature is enabled. > > Reported-by: Yang Shi <yang@os.amperecomputing.com> > Link: https://lore.kernel.org/all/20230328193846.8757-1-yang@os.amperecomputing.com/ > Signed-off-by: Zeng Heng <zengheng4@huawei.com> > --- > drivers/cpufreq/cppc_cpufreq.c | 16 +++++++++++++++- > 1 file changed, 15 insertions(+), 1 deletion(-) > > diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c > index 321a9dc9484d..a7c5418bcda7 100644 > --- a/drivers/cpufreq/cppc_cpufreq.c > +++ b/drivers/cpufreq/cppc_cpufreq.c > @@ -851,12 +851,26 @@ static int cppc_get_perf_ctrs_pair(void *val)
The previous patch added this function, and calls it with smp_call_on_cpu(), where it'll run in IRQ context with IRQs disabled...
> struct fb_ctr_pair *fb_ctrs = val; > int cpu = fb_ctrs->cpu; > int ret; > + unsigned long timeout; > > ret = cppc_get_perf_ctrs(cpu, &fb_ctrs->fb_ctrs_t0); > if (ret) > return ret; > > - udelay(2); /* 2usec delay between sampling */ > + if (likely(!irqs_disabled())) { > + /* > + * Set 1ms as sampling interval, but never schedule > + * to the idle task to prevent the AMU counters from > + * stopping working. > + */ > + timeout = jiffies + msecs_to_jiffies(1); > + while (!time_after(jiffies, timeout)) > + cond_resched(); > + > + } else {
... so we'll enter this branch of the if-else ...
> + pr_warn_once("CPU%d: Get rate in atomic context", cpu);
... and pr_warn_once() for something that's apparently normal and outside of the user's control?
That doesn't make much sense to me.
Mark.
> + udelay(2); /* 2usec delay between sampling */ > + } > > return cppc_get_perf_ctrs(cpu, &fb_ctrs->fb_ctrs_t1); > } > -- > 2.25.1 >
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