linux-kernel - Re: [RFC 15/16] sched/fair: Account kthread runtime debt for CFS bandwidth

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Message-ID: <YeFE9j4Qynp9sNXS@hirez.programming.kicks-ass.net>
Date:   Fri, 14 Jan 2022 10:40:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Alexander Duyck <alexanderduyck@...com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Segall <bsegall@...gle.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ingo Molnar <mingo@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Josh Triplett <josh@...htriplett.org>,
        Michal Hocko <mhocko@...e.com>, Nico Pache <npache@...hat.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Steve Sistare <steven.sistare@...cle.com>,
        Tejun Heo <tj@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-mm@...ck.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [RFC 15/16] sched/fair: Account kthread runtime debt for CFS
 bandwidth

On Fri, Jan 14, 2022 at 10:31:55AM +0100, Peter Zijlstra wrote:

> Also, by virtue of this being a start-stop annotation interface, the
> accrued time might be arbitrarily large and arbitrarily delayed. I'm not
> sure that's sensible.
> 
> For tasks it might be better to mark the task and have the tick DTRT
> instead of later trying to 'migrate' the time.

Which is then very close to simply sticking the task into the right
cgroup for a limited duration.

You could do a special case sched_move_task(), that takes a css argument
instead of using the current task_css. Then for cgroups it looks like
nothing changes, but the scheduler will DTRT and act like it is in the
target cgroup. Then at the end, simply move it back to task_css.

This obviously doesn't work for SoftIRQ accounting, but that is
'special' anyway. Softirq stuff is not otherwise under scheduler
control and has preemption disabled.