Re: [RFC PATCH 3/4] cpufreq: Add Active Stats calls tracking frequency changes

From: Lukasz Luba
Date: Tue Jun 22 2021 - 11:03:00 EST




On 6/22/21 3:51 PM, Rafael J. Wysocki wrote:
On Tue, Jun 22, 2021 at 4:09 PM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:



On 6/22/21 2:51 PM, Rafael J. Wysocki wrote:
On Tue, Jun 22, 2021 at 3:42 PM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:



On 6/22/21 1:28 PM, Rafael J. Wysocki wrote:
On Tue, Jun 22, 2021 at 9:59 AM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:

The Active Stats framework tracks and accounts the activity of the CPU
for each performance level. It accounts the real residency, when the CPU
was not idle, at a given performance level. This patch adds needed calls
which provide the CPU frequency transition events to the Active Stats
framework.

Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
---
drivers/cpufreq/cpufreq.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 802abc925b2a..d79cb9310572 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -14,6 +14,7 @@

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

+#include <linux/active_stats.h>
#include <linux/cpu.h>
#include <linux/cpufreq.h>
#include <linux/cpu_cooling.h>
@@ -387,6 +388,8 @@ static void cpufreq_notify_transition(struct cpufreq_policy *policy,

cpufreq_stats_record_transition(policy, freqs->new);
policy->cur = freqs->new;
+
+ active_stats_cpu_freq_change(policy->cpu, freqs->new);
}
}

@@ -2085,6 +2088,8 @@ unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
policy->cpuinfo.max_freq);
cpufreq_stats_record_transition(policy, freq);

+ active_stats_cpu_freq_fast_change(policy->cpu, freq);
+

This is quite a bit of overhead and so why is it needed in addition to
the code below?

The code below is tracing, which is good for post-processing. We use in
our tool LISA, when we analyze the EAS decision, based on captured
trace data.

This new code is present at run time, so subsystems like our thermal
governor IPA can use it and get better estimation about CPU used power
for any arbitrary period, e.g. 50ms, 100ms, 300ms, ...

So can it be made not run when the IPA is not using it?

I can make a Kconfig for IPA to select this ACTIVE_STATS.
Also, I can add description that this framework is mostly needed
for IPA, so don't enable it if you don't use IPA (default is 'n'
so it shouldn't harm others).

This Active Stats shouldn't be stopped when thermal zone is switching
between governors at run time, e.g. IPA -> step_wise -> IPA
because when IPA is set next time, it might not have correct CPU
stats (what is the current frequency and for how long it has been
actively used).

But after a while it will collect enough useful data I suppose?

True, it will get enough data after a first freq switch made by
cpufreq governor. I don't want to race with schedutil and check
the current freq, but I will check that option too.


Beside, switching governors at run time is not a good idea
(apart from stress testing them ;) ).



And pretty much the same goes for the idle loop change. There is
quite a bit of instrumentation in that code already and it avoids
adding new locking for a reason. Why is it a good idea to add more
locking to that code?

This active_stats_cpu_freq_fast_change() doesn't use the locking, it
relies on schedutil lock in [1].

Ah, OK.

But it still adds overhead AFAICS.

Agree, it's an extra code. For platforms which use IPA it's a
justifiable cost, weighted by better estimation thanks to this calls.
For other platforms, this framework will be set to default 'n' option.

A general problem with build-time configuration is for distros that
want to ship one kernel binary to run on multiple hardware platforms.
They need to enable those options anyway and then get the full cost on
the platforms that don't need it, but want to use the common binary
kernel.

I see your point. Fair enough.


Again, please consider making this new code run only when it is needed
even if configured in and if it runs, make it as low-overhead as
possible.


Sure thing. I'll improve this.