linux-kernel - [PATCH] cpufreq: CPPC: use 10ms delay instead of 2us to avoid high error

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Message-Id: <20230328193846.8757-1-yang@os.amperecomputing.com>
Date:   Tue, 28 Mar 2023 12:38:46 -0700
From:   Yang Shi <yang@...amperecomputing.com>
To:     rafael@...nel.org, viresh.kumar@...aro.org
Cc:     yang@...amperecomputing.com, scott@...amperecomputing.com,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] cpufreq: CPPC: use 10ms delay instead of 2us to avoid high error

When testing CPPC cpufreq on our platform, we noticed the error may be quite
high and the high error may happen quite often.  For example, on a platform
with a maximum frequency of 2.8GHz when the CPUs were fully loaded (100% load),
we saw cpuinfo_cur_freq may show 4GHz, it means the error is > 40%.  And the
high error (> 1%) happened 256 times out of 2127 samples (sampled every 3
seconds) in an approximate 2hrs test.

We tried to enlarge the delay, and tested with 100us, 1ms and 10ms.  The
below is the results.

100us:
The highest error is 4GHz, 22 times out of 3623 samples

1ms:
The highest error is 3.3GHz, 3 times out of 2814 samples

10ms:
No high error anymore

Increase the measurement delay in cppc_cpufreq_get_rate to 10ms to avoid
high measurement errors.

Signed-off-by: Yang Shi <yang@...amperecomputing.com>
---
 drivers/cpufreq/cppc_cpufreq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index 022e3555407c..c2bf65448d3d 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -851,7 +851,7 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
 	if (ret)
 		return ret;

-	udelay(2); /* 2usec delay between sampling */
+	mdelay(10); /* 10msec delay between sampling */

 	ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t1);
 	if (ret)
-- 
2.39.2