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Date:   Wed, 9 Mar 2022 14:14:35 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Chengming Zhou <zhouchengming@bytedance.com>
Cc:     corbet@lwn.net, mingo@redhat.com, peterz@infradead.org,
        surenb@google.com, ebiggers@google.com, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, songmuchun@bytedance.com,
        Martin Steigerwald <Martin.Steigerwald@proact.de>
Subject: Re: [PATCH v2] psi: report zeroes for CPU full at the system level
Message-ID: <Yij8m9qHtvNKyGMK@cmpxchg.org>
References: <20220309111445.86484-1-zhouchengming@bytedance.com>
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On Wed, Mar 09, 2022 at 07:14:45PM +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 will report zeroes for CPU full
> at the system level, and update psi Documentation accordingly.
> 
> Fixes: e7fcd7622823 ("psi: Add PSI_CPU_FULL state")
> Reported-by: Martin Steigerwald <Martin.Steigerwald@proact.de>
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Peter, would you mind picking this up for 5.18?

Thanks!