linux-kernel - Re: [RFC][PATCH 3/3] sched: User Mode Concurency Groups

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Message-ID: <YeVdSYrkmMW2i8+N@hirez.programming.kicks-ass.net>
Date:   Mon, 17 Jan 2022 13:12:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Oskolkov <posk@...k.io>
Cc:     mingo@...hat.com, tglx@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-api@...r.kernel.org, x86@...nel.org,
        pjt@...gle.com, posk@...gle.com, avagin@...gle.com,
        jannh@...gle.com, tdelisle@...terloo.ca
Subject: Re: [RFC][PATCH 3/3] sched: User Mode Concurency Groups

On Fri, Jan 14, 2022 at 03:09:55PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 21, 2021 at 05:19:00PM +0000, Peter Oskolkov wrote:

> > > +	/*
> > > +	 * Workers will still block in umcg_notify_resume() before they can
> > > +	 * consume their error, servers however need to get the error asap.
> > > +	 *
> > > +	 * Still, things might be unrecoverably screwy after this. Not our
> > > +	 * problem.
> > 
> > I think we should explicitly document the unrecoverable screwiness
> > of errors here, so that the userspace proactively kills itself
> > to avoid badness. The only reason that returning an error here is
> > mildly preferable to just killing the task (we already do that
> > in other places) is to give the userspace an opportunity to
> > log an error, with more state/info than we can do here.
> 
> Bah, I should've written a better comment, because I can't quite
> remember the case I had in mind. Also, again from the LAZY patch, I
> think we can actually do better in some of the cases here.
> 
> Specifically, currently we'll enqueue on ::runnable_workers_ptr and fail
> waking ::next_tid and leave it at that. While I think waking
> ::server_tid in that case makes sense.
> 
> I'll go prod at this.

Is anybody actually planning to use ::next_tid for workers?

My current thinking is that much of the problems here stem from that.

Let me ponder more..