linux-kernel - RE: [patch 00/10] x86/cpu: Consolidate APERF/MPERF code

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Date:   Mon, 25 Apr 2022 17:45:42 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Doug Smythies <dsmythies@...us.net>,
        "'Rafael J. Wysocki'" <rafael@...nel.org>
Cc:     'the arch/x86 maintainers' <x86@...nel.org>,
        "'Rafael J. Wysocki'" <rafael@...nel.org>,
        'Linux PM' <linux-pm@...r.kernel.org>,
        'Eric Dumazet' <edumazet@...gle.com>,
        "'Paul E. McKenney'" <paulmck@...nel.org>,
        'LKML' <linux-kernel@...r.kernel.org>,
        Doug Smythies <dsmythies@...us.net>
Subject: RE: [patch 00/10] x86/cpu: Consolidate APERF/MPERF code

On Wed, Apr 20 2022 at 15:08, Doug Smythies wrote:
> On 2022.04.19 14:11 Thomas Gleixner wrote:
>>> That's because after the changes in this series scaling_cur_freq
>>> returns 0 if the given CPU is idle.
>>
>> Which is sensible IMO as there is really no point in waking an idle CPU
>> just to read those MSRs, then wait 20ms wake it up again to read those
>> MSRs again.
>
> I totally agree.
> It is the inconsistency for what is displayed as a function of driver/governor
> that is my concern.

Raphael suggested to move the show_cpuinfo() logic into the a/mperf
code. See below.

Thanks,

        tglx
---
Subject: x86/aperfmperf: Integrate the fallback code from show_cpuinfo()
From: Thomas Gleixner <tglx@...utronix.de>
Date: Mon, 25 Apr 2022 15:19:29 +0200

Due to the avoidance of IPIs to idle CPUs arch_freq_get_on_cpu() can return
0 when the last sample was too long ago.

show_cpuinfo() has a fallback to cpufreq_quick_get() and if that fails to
return cpu_khz, but the readout code for the per CPU scaling frequency in
sysfs does not.

Move that fallback into arch_freq_get_on_cpu() so the behaviour is the same
when reading /proc/cpuinfo and /sys/..../cur_scaling_freq.

Suggested-by: "Rafael J. Wysocki" <rafael@...nel.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 arch/x86/kernel/cpu/aperfmperf.c |   10 +++++++---
 arch/x86/kernel/cpu/proc.c       |    7 +------
 2 files changed, 8 insertions(+), 9 deletions(-)

--- a/arch/x86/kernel/cpu/aperfmperf.c
+++ b/arch/x86/kernel/cpu/aperfmperf.c
@@ -405,12 +405,12 @@ void arch_scale_freq_tick(void)
 unsigned int arch_freq_get_on_cpu(int cpu)
 {
 	struct aperfmperf *s = per_cpu_ptr(&cpu_samples, cpu);
+	unsigned int seq, freq;
 	unsigned long last;
-	unsigned int seq;
 	u64 acnt, mcnt;
 
 	if (!cpu_feature_enabled(X86_FEATURE_APERFMPERF))
-		return 0;
+		goto fallback;
 
 	do {
 		seq = raw_read_seqcount_begin(&s->seq);
@@ -424,9 +424,13 @@ unsigned int arch_freq_get_on_cpu(int cp
 	 * which covers idle and NOHZ full CPUs.
 	 */
 	if (!mcnt || (jiffies - last) > MAX_SAMPLE_AGE)
-		return 0;
+		goto fallback;
 
 	return div64_u64((cpu_khz * acnt), mcnt);
+
+fallback:
+	freq = cpufreq_quick_get(cpu);
+	return freq ? freq : cpu_khz;
 }
 
 static int __init bp_init_aperfmperf(void)
--- a/arch/x86/kernel/cpu/proc.c
+++ b/arch/x86/kernel/cpu/proc.c
@@ -86,12 +86,7 @@ static int show_cpuinfo(struct seq_file
 	if (cpu_has(c, X86_FEATURE_TSC)) {
 		unsigned int freq = arch_freq_get_on_cpu(cpu);
 
-		if (!freq)
-			freq = cpufreq_quick_get(cpu);
-		if (!freq)
-			freq = cpu_khz;
-		seq_printf(m, "cpu MHz\t\t: %u.%03u\n",
-			   freq / 1000, (freq % 1000));
+		seq_printf(m, "cpu MHz\t\t: %u.%03u\n", freq / 1000, (freq % 1000));
 	}
 
 	/* Cache size */