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Date:   Wed, 10 Aug 2022 11:04:48 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Hao Jia <jiahao.os@bytedance.com>
Cc:     mingo@redhat.com, peterz@infradead.org, mingo@kernel.org,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] sched/psi: Zero the memory of struct psi_group
Message-ID: <YvPJEDHfOmX90UOz@cmpxchg.org>
References: <20220806120510.96131-1-jiahao.os@bytedance.com>
 <20220806120510.96131-2-jiahao.os@bytedance.com>
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On Sat, Aug 06, 2022 at 08:05:08PM +0800, Hao Jia wrote:
> After commit 5f69a6577bc3 ("psi: dont alloc memory for psi by default"),
> the memory used by struct psi_group is no longer allocated and zeroed
> in cgroup_create().
>
> Since the memory of struct psi_group is not zeroed, the data in this
> memory is random, which will lead to inaccurate psi statistics when
> creating a new cgroup.
> 
> So we use kzlloc() to allocate and zero the struct psi_group and
> remove the redundant zeroing in group_init().
> 
> Steps to reproduce:
> 1. Use cgroup v2 and enable CONFIG_PSI
> 2. Create a new cgroup, and query psi statistics
> mkdir /sys/fs/cgroup/test
> cat /sys/fs/cgroup/test/cpu.pressure
> some avg10=0.00 avg60=0.00 avg300=47927752200.00 total=12884901
> full avg10=561815124.00 avg60=125835394188.00 avg300=1077090462000.00 total=10273561772
> 
> cat /sys/fs/cgroup/test/io.pressure
> some avg10=1040093132823.95 avg60=1203770351379.21 avg300=3862252669559.46 total=4294967296
> full avg10=921884564601.39 avg60=0.00 avg300=1984507298.35 total=442381631
> 
> cat /sys/fs/cgroup/test/memory.pressure
> some avg10=232476085778.11 avg60=0.00 avg300=0.00 total=0
> full avg10=0.00 avg60=0.00 avg300=2585658472280.57 total=12884901
> 
> Fixes: commit 5f69a6577bc3 ("psi: dont alloc memory for psi by default")
> 
> Signed-off-by: Hao Jia <jiahao.os@bytedance.com>

Yikes! Yes, we relied on the embedding cgroup being kzalloc'd, or, in
the case of psi_system, on the psi_group being in NULLed static mem.
The partial zeroing in group_init() obscured that. Thanks for the fix.

Cc: stable@vger.kernel.org # 5.19
Acked-by: Johannes Weiner <hannes@cmpxchg.org>