linux-kernel - Re: [PATCH 14/31] sched_ext: Implement BPF extensible scheduler class

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Date:   Tue, 6 Dec 2022 11:42:27 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Barret Rhoden <brho@...gle.com>
Cc:     torvalds@...ux-foundation.org, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, martin.lau@...nel.org,
        joshdon@...gle.com, pjt@...gle.com, derkling@...gle.com,
        haoluo@...gle.com, dvernet@...a.com, dschatzberg@...a.com,
        dskarlat@...cmu.edu, riel@...riel.com,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        kernel-team@...a.com
Subject: Re: [PATCH 14/31] sched_ext: Implement BPF extensible scheduler class

Hello, Barret.

On Fri, Dec 02, 2022 at 08:01:37AM -1000, Tejun Heo wrote:
> > you still end up grabbing both locks, but just not at the same time.
> 
> Yeah, this probably would look better than the current double lock dancing,
> especially in the finish_dispatch() path.
>
> > plus, task_rq_lock() takes the guesswork out of whether you're getting p's
> > rq lock or not.  it looks like you're using the holding_cpu to handle the
> > race where p moves cpus after you read task_rq(p) but before you lock that
> > task_rq.  maybe you can drop the whole concept of the holding_cpu?
> 
> ->holding_cpu is there to basically detect intervening dequeues, so if we
> lock them out with TASK_ON_RQ_MIGRATING, we might be able to drop it. I need
> to look into it more tho. Things get pretty subtle around there, so I could
> easily be missing something. I'll try this and let you know how it goes.

I tried both and I'm pretty ambivalent. The problem is that the
finish_dispatch() path can't use TASK_ON_RQ_MIGRATING the way the consume
path does because the dispatch path isn't starting with the task locked. The
only claim it has to the task is through p->scx.ops_state.

It can be argued that getting rid of double locking is still nice but given
that holding_cpu is needed anyway and can play the same role, I'm not sure
how attractive it is. I suppose we can go either way.

Thanks.

-- 
tejun