linux-kernel - Re: [PATCH v1 12/14] sched: psi: implement psi trigger handling using bpf

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Message-ID: <87cy8qx50g.fsf@linux.dev>
Date: Tue, 19 Aug 2025 16:31:43 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: linux-mm@...ck.org,  bpf@...r.kernel.org,  Johannes Weiner
 <hannes@...xchg.org>,  Michal Hocko <mhocko@...e.com>,  David Rientjes
 <rientjes@...gle.com>,  Matt Bobrowski <mattbobrowski@...gle.com>,  Song
 Liu <song@...nel.org>,  Kumar Kartikeya Dwivedi <memxor@...il.com>,
  Alexei Starovoitov <ast@...nel.org>,  Andrew Morton
 <akpm@...ux-foundation.org>,  linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 12/14] sched: psi: implement psi trigger handling
 using bpf

Roman Gushchin <roman.gushchin@...ux.dev> writes:

> Suren Baghdasaryan <surenb@...gle.com> writes:
>
>> On Mon, Aug 18, 2025 at 10:02 AM Roman Gushchin
>> <roman.gushchin@...ux.dev> wrote:
>
>>
>>> +
>>> +       /* Cgroup Id */
>>> +       u64 cgroup_id;
>>
>> This cgroup_id field is weird. It's not initialized and not used here,
>> then it gets initialized in the next patch and used in the last patch
>> from a selftest. This is quite confusing. Also logically I don't think
>> a cgroup attribute really belongs to psi_trigger... Can we at least
>> move it into bpf_psi where it might fit a bit better?
>
> I can't move it to bpf_psi, because a single bpf_psi might own multiple
> triggers with different cgroup_id's.
> For sure I can move it to the next patch, if it's preferred.
>
> If you really don't like it here, other option is to replace it with
> a new bpf helper (kfunc) which calculates the cgroup_id by walking the
> trigger->group->cgroup->cgroup_id path each time.

Actually there is no easy path from psi_group to cgroup, so there is
no such option available, unfortunately. Or we need a back-link from
the psi_group to cgroup.