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SubjectRe: [PATCH] x86: use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz" again
Date
On Thursday, November 9, 2017 11:30:54 PM CET Rafael J. Wysocki wrote:
> On Thu, Nov 9, 2017 at 5:06 PM, Rafael J. Wysocki
> <rafael.j.wysocki@intel.com> wrote:
> > Hi Linus,
> >
> > On 11/9/2017 11:38 AM, WANG Chao wrote:
> >>
> >> Commit 941f5f0f6ef5 (x86: CPU: Fix up "cpu MHz" in /proc/cpuinfo) caused
> >> a serious performance issue when reading from /proc/cpuinfo on system
> >> with aperfmperf.
> >>
> >> For each cpu, arch_freq_get_on_cpu() sleeps 20ms to get its frequency.
> >> On a system with 64 cpus, it takes 1.5s to finish running `cat
> >> /proc/cpuinfo`, while it previously was done in 15ms.
> >
> > Honestly, I'm not sure what to do to address this ATM.
> >
> > The last requested frequency is only available in the non-HWP case, so it
> > cannot be used universally.
>
> OK, here's an idea.
>
> c_start() can run aperfmperf_snapshot_khz() on all CPUs upfront (say
> in parallel), then wait for a while (say 5 ms; the current 20 ms wait
> is overkill) and then aperfmperf_snapshot_khz() can be run once on
> each CPU in show_cpuinfo() without taking the "stale cache" threshold
> into account.
>
> I'm going to try that and see how far I can get with it.

Below is what I have.

I ended up using APERFMPERF_REFRESH_DELAY_MS for the delay in
aperfmperf_snapshot_all(), because 5 ms tended to add too much
variation to the results on my test box.

I think it may be reduced to 10 ms, though.

Chao, can you please try this one and report back?


---
arch/x86/kernel/cpu/aperfmperf.c | 42 ++++++++++++++++++++++++++++-----------
arch/x86/kernel/cpu/cpu.h | 4 +++
arch/x86/kernel/cpu/proc.c | 5 +++-
3 files changed, 39 insertions(+), 12 deletions(-)

Index: linux-pm/arch/x86/kernel/cpu/aperfmperf.c
===================================================================
--- linux-pm.orig/arch/x86/kernel/cpu/aperfmperf.c
+++ linux-pm/arch/x86/kernel/cpu/aperfmperf.c
@@ -14,6 +14,8 @@
#include <linux/percpu.h>
#include <linux/smp.h>

+#include "cpu.h"
+
struct aperfmperf_sample {
unsigned int khz;
ktime_t time;
@@ -38,8 +40,6 @@ static void aperfmperf_snapshot_khz(void
u64 aperf, aperf_delta;
u64 mperf, mperf_delta;
struct aperfmperf_sample *s = this_cpu_ptr(&samples);
- ktime_t now = ktime_get();
- s64 time_delta = ktime_ms_delta(now, s->time);
unsigned long flags;

local_irq_save(flags);
@@ -57,15 +57,10 @@ static void aperfmperf_snapshot_khz(void
if (mperf_delta == 0)
return;

- s->time = now;
+ s->time = ktime_get();
s->aperf = aperf;
s->mperf = mperf;
-
- /* If the previous iteration was too long ago, discard it. */
- if (time_delta > APERFMPERF_STALE_THRESHOLD_MS)
- s->khz = 0;
- else
- s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta);
+ s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta);
}

unsigned int arch_freq_get_on_cpu(int cpu)
@@ -82,16 +77,41 @@ unsigned int arch_freq_get_on_cpu(int cp
/* Don't bother re-computing within the cache threshold time. */
time_delta = ktime_ms_delta(ktime_get(), per_cpu(samples.time, cpu));
khz = per_cpu(samples.khz, cpu);
- if (khz && time_delta < APERFMPERF_CACHE_THRESHOLD_MS)
+ if (time_delta < APERFMPERF_CACHE_THRESHOLD_MS)
return khz;

smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1);
khz = per_cpu(samples.khz, cpu);
- if (khz)
+ if (time_delta <= APERFMPERF_STALE_THRESHOLD_MS)
return khz;

+ /* If the previous iteration was too long ago, take a new data point. */
msleep(APERFMPERF_REFRESH_DELAY_MS);
smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1);

return per_cpu(samples.khz, cpu);
}
+
+void aperfmperf_snapshot_all(void)
+{
+ if (!cpu_khz)
+ return;
+
+ if (!static_cpu_has(X86_FEATURE_APERFMPERF))
+ return;
+
+ smp_call_function_many(cpu_online_mask, aperfmperf_snapshot_khz, NULL, 1);
+ msleep(APERFMPERF_REFRESH_DELAY_MS);
+}
+
+unsigned int aperfmperf_snapshot_cpu(int cpu)
+{
+ if (!cpu_khz)
+ return 0;
+
+ if (!static_cpu_has(X86_FEATURE_APERFMPERF))
+ return 0;
+
+ smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1);
+ return per_cpu(samples.khz, cpu);
+}
Index: linux-pm/arch/x86/kernel/cpu/cpu.h
===================================================================
--- linux-pm.orig/arch/x86/kernel/cpu/cpu.h
+++ linux-pm/arch/x86/kernel/cpu/cpu.h
@@ -47,4 +47,8 @@ extern const struct cpu_dev *const __x86

extern void get_cpu_cap(struct cpuinfo_x86 *c);
extern void cpu_detect_cache_sizes(struct cpuinfo_x86 *c);
+
+extern unsigned int aperfmperf_snapshot_cpu(int cpu);
+extern void aperfmperf_snapshot_all(void);
+
#endif /* ARCH_X86_CPU_H */
Index: linux-pm/arch/x86/kernel/cpu/proc.c
===================================================================
--- linux-pm.orig/arch/x86/kernel/cpu/proc.c
+++ linux-pm/arch/x86/kernel/cpu/proc.c
@@ -5,6 +5,8 @@
#include <linux/seq_file.h>
#include <linux/cpufreq.h>

+#include "cpu.h"
+
/*
* Get CPU information for use by the procfs.
*/
@@ -78,7 +80,7 @@ static int show_cpuinfo(struct seq_file
seq_printf(m, "microcode\t: 0x%x\n", c->microcode);

if (cpu_has(c, X86_FEATURE_TSC)) {
- unsigned int freq = arch_freq_get_on_cpu(cpu);
+ unsigned int freq = aperfmperf_snapshot_cpu(cpu);

if (!freq)
freq = cpufreq_quick_get(cpu);
@@ -141,6 +143,7 @@ static int show_cpuinfo(struct seq_file

static void *c_start(struct seq_file *m, loff_t *pos)
{
+ aperfmperf_snapshot_all();
*pos = cpumask_next(*pos - 1, cpu_online_mask);
if ((*pos) < nr_cpu_ids)
return &cpu_data(*pos);
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