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

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Date:   Fri, 14 Jan 2022 06:38:09 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        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>,
        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

Hello,

On Fri, Jan 14, 2022 at 10:40:06AM +0100, Peter Zijlstra wrote:
> 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.

So, if this particular use case doesn't fit the backcharge model (I'm not
sure yet). I'd much prefer it to maintain dynamic per-cgroup helper threads
than move tasks around dynamically. Nothing else is using migration this way
and we don't even need migration for seeding cgroups w/ CLONE_INTO_CGROUP.
In the future, this should allow further optimizations and likely
simplifications. It'd suck to have an odd exception usage.

Thanks.

-- 
tejun