linux-kernel - Re: [RFC][PATCH 0/3] sched: User Managed Concurrency Groups

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Date:   Wed, 15 Dec 2021 11:44:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Oskolkov <posk@...k.io>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, juri.lelli@...hat.com,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, mgorman@...e.de,
        bristot@...hat.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-api@...r.kernel.org, x86@...nel.org,
        Paul Turner <pjt@...gle.com>, Peter Oskolkov <posk@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Jann Horn <jannh@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: Re: [RFC][PATCH 0/3] sched: User Managed Concurrency Groups

On Tue, Dec 14, 2021 at 07:46:25PM -0800, Peter Oskolkov wrote:

> Anyway, I'll test your patchset over the next week or so and let you
> know if anything really needed is missing (other than waking an idle
> server if there is one on a worker wakeup; this piece is definitely
> needed).

Right, so the problem I'm having is that a single idle server ptr like
before can trivially miss waking annother idle server.

Suppose:

	umcg::idle_server_tid_ptr

Then the enqueue_and_wake() thing from the last patch would:

	idle_server_tid = xchg((pid_t __user *)self->idle_server_tid_ptr, 0);

to consume the tid, and then use that to enqueue and wake. But what if a
second wakeup happens right after that? There might be a second idle
server, but we'll never find it, because userspace hasn't had time to
update the field again.

Alternatively, we do a linked list of servers, but then every such
wakeup needs to iterate the whole list, looking for one that has
UMCG_TF_IDLE set, or something like that, but that lookup is bad for
performance.

So I'm really not sure what way to go yet.