linux-kernel - Re: [External] Re: [PATCH] psi: remove CPU full metric at system level

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Date:   Fri, 4 Mar 2022 22:43:01 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     corbet@....net, mingo@...hat.com, peterz@...radead.org,
        surenb@...gle.com, ebiggers@...gle.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, songmuchun@...edance.com,
        Martin Steigerwald <Martin.Steigerwald@...act.de>
Subject: Re: [External] Re: [PATCH] psi: remove CPU full metric at system
 level

On 2022/3/3 9:26 下午, Johannes Weiner wrote:
> On Thu, Mar 03, 2022 at 01:58:14PM +0800, Chengming Zhou wrote:
>> Martin find it confusing when look at the /proc/pressure/cpu output,
>> and found no hint about that CPU "full" line in psi Documentation.
>>
>> % cat /proc/pressure/cpu
>> some avg10=0.92 avg60=0.91 avg300=0.73 total=933490489
>> full avg10=0.22 avg60=0.23 avg300=0.16 total=358783277
>>
>> The PSI_CPU_FULL state is introduced by commit e7fcd7622823
>> ("psi: Add PSI_CPU_FULL state"), which mainly for cgroup level,
>> but also counted at the system level as a side effect.
>>
>> Naturally, the FULL state doesn't exist for the CPU resource at
>> the system level. These "full" numbers can come from CPU idle
>> schedule latency. For example, t1 is the time when task wakeup
>> on an idle CPU, t2 is the time when CPU pick and switch to it.
>> The delta of (t2 - t1) will be in CPU_FULL state.
>>
>> Another case all processes can be stalled is when all cgroups
>> have been throttled at the same time, which unlikely to happen.
>>
>> Anyway, CPU_FULL metric is meaningless and confusing at the
>> system level. So this patch removed CPU full metric at the
>> system level, and removed it's monitor function too. The psi
>> Documentation has also been updated accordingly.
>>
>> Fixes: e7fcd7622823 ("psi: Add PSI_CPU_FULL state")
>> Reported-by: Martin Steigerwald <Martin.Steigerwald@...act.de>
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>> ---
>>  Documentation/accounting/psi.rst | 18 +++++++++++++++---
>>  kernel/sched/psi.c               | 10 +++++++++-
>>  2 files changed, 24 insertions(+), 4 deletions(-)
>>
>> diff --git a/Documentation/accounting/psi.rst b/Documentation/accounting/psi.rst
>> index 860fe651d645..519652c06d7d 100644
>> --- a/Documentation/accounting/psi.rst
>> +++ b/Documentation/accounting/psi.rst
>> @@ -178,8 +178,20 @@ Cgroup2 interface
>>  In a system with a CONFIG_CGROUP=y kernel and the cgroup2 filesystem
>>  mounted, pressure stall information is also tracked for tasks grouped
>>  into cgroups. Each subdirectory in the cgroupfs mountpoint contains
>> -cpu.pressure, memory.pressure, and io.pressure files; the format is
>> -the same as the /proc/pressure/ files.
>> +cpu.pressure, memory.pressure, and io.pressure files; the format of
>> +memory.pressure and io.pressure is the same as the /proc/pressure/ files.
>> +
>> +But the format of cpu.pressure is as such::
>> +	some avg10=0.00 avg60=0.00 avg300=0.00 total=0
>> +	full avg10=0.00 avg60=0.00 avg300=0.00 total=0
> 
> It's the format of cpu.pressure, except when it's
> /sys/fs/cgroup/cpu.pressure... I think this is getting maybe a tad too
> difficult to write parsers for. Plus, we added the line over a year
> ago so we might break somebody by removing it again.
> 
> How about reporting zeroes at the system level?

Ok, it's really better for userspace parsers, will change to this way
and send later.

Thanks.

> 
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index e14358178849..86824de404bc 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -1062,14 +1062,17 @@ int psi_show(struct seq_file *m, struct psi_group *group, enum psi_res res)
>  	mutex_unlock(&group->avgs_lock);
>  
>  	for (full = 0; full < 2; full++) {
> -		unsigned long avg[3];
> -		u64 total;
> +		unsigned long avg[3] = { 0, };
> +		u64 total = 0;
>  		int w;
>  
> -		for (w = 0; w < 3; w++)
> -			avg[w] = group->avg[res * 2 + full][w];
> -		total = div_u64(group->total[PSI_AVGS][res * 2 + full],
> -				NSEC_PER_USEC);
> +		/* CPU FULL is undefined at the system level */
> +		if (!(group == &psi_system && res == PSI_CPU && full)) {
> +			for (w = 0; w < 3; w++)
> +				avg[w] = group->avg[res * 2 + full][w];
> +			total = div_u64(group->total[PSI_AVGS][res * 2 + full],
> +					NSEC_PER_USEC);
> +		}
>  
>  		seq_printf(m, "%s avg10=%lu.%02lu avg60=%lu.%02lu avg300=%lu.%02lu total=%llu\n",
>  			   full ? "full" : "some",