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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 18 May 2021 11:08:06 -0700
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Subject: Re: [PATCH v2 1/1] cgroup: make per-cgroup pressure stall tracking configurable
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>,
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On Mon, May 17, 2021 at 7:02 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> PSI accounts stalls for each cgroup separately and aggregates it at each
> level of the hierarchy. This causes additional overhead with psi_avgs_work
> being called for each cgroup in the hierarchy. psi_avgs_work has been
> highly optimized, however on systems with large number of cgroups the
> overhead becomes noticeable.
> Systems which use PSI only at the system level could avoid this overhead
> if PSI can be configured to skip per-cgroup stall accounting.
> Add "cgroup_disable=pressure" kernel command-line option to allow
> requesting system-wide only pressure stall accounting. When set, it
> keeps system-wide accounting under /proc/pressure/ but skips accounting
> for individual cgroups and does not expose PSI nodes in cgroup hierarchy.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

I am assuming that this is for Android and at the moment Android is
only interested in system level pressure. I am wondering if there is
any plan for Android to have cgroup hierarchies with explicit limits
in future?

If yes, then I think we should follow up (this patch is fine
independently) with making this feature more general by explicitly
enabling psi for each cgroup level similar to how we enable
controllers through cgroup.subtree_control.

Something like:

$ echo "+psi" > cgroup.subtree_control

This definitely would be helpful for server use cases where jobs do
sub-containers but might not be interested in psi but the admin is
interested in the top level job's psi.