[PATCH 3/3] cpufreq: CPPC: Eliminate the impact of cpc_read() latency error

[Zeng Heng]

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@xxxxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20230328193846.8757-1-yang@xxxxxxxxxxxxxxxxxxxxxx/
Signed-off-by: Zeng Heng <zengheng4@xxxxxxxxxx>
---
 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)
 	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 {
+		pr_warn_once("CPU%d: Get rate in atomic context", cpu);
+		udelay(2); /* 2usec delay between sampling */
+	}
 
 	return cppc_get_perf_ctrs(cpu, &fb_ctrs->fb_ctrs_t1);
 }
-- 
2.25.1