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

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Date:   Wed, 19 Jan 2022 09:59:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Oskolkov <posk@...gle.com>
Cc:     Peter Oskolkov <posk@...k.io>, 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, avagin@...gle.com, jannh@...gle.com,
        tdelisle@...terloo.ca
Subject: Re: [RFC][PATCH 3/3] sched: User Mode Concurency Groups

On Tue, Jan 18, 2022 at 10:19:21AM -0800, Peter Oskolkov wrote:

> =========== signals and the general approach
> 
> My version of the patchset has all of these things working. What it
> does not have,
> compared to the new approach we are discussing here, is runqueues per server
> and proper signal handling (and potential integration with proxy execution).
> 
> Runqueues per server, in the LAZY mode, are easy to emulate in my patchset:
> nothing prevents the userspace to partition workers among servers, and have
> servers that "own" their workers to be pointed at by idle_server_tid_ptr.
> 
> The only thing that is missing is proper treating of signals. But my patchset
> does ensure a single running worker per server, had pagefaults and preemptions
> sorted out, etc. Basically, everything works except signals. This patchet
> has issues with pagefaults, 

Already fixed pagefaults per:

  YeGvovSckivQnKX8@...ez.programming.kicks-ass.net

> worker timeouts

I still have no clear answer as to what you actually want there.

> , worker-to-worker context
> switches (do workers move runqueues when they context switch?), etc.

Not in kernel, if they need to be migrated, userspace needs to do that.

> And my patchset now actually looks smaller and simpler, on the kernel side,
> that what this patchset is shaping up to be.
> 
> What if I fix signals in my patchset? I think the way you deal with signals
> will work in my approach equally well; I'll also use umcg_kick() to preempt
> workers instead of sending them a signal.
> 
> What do you think? 

I still absolutely hate how long you do page pinning, it *will* wreck
things like CMA which are somewhat latency critical for silly things
like Android camera apps and who knows what else.

You've also forgotten about this:

  YcWutpu7BDeG+dQ2@...ez.programming.kicks-ass.net

That's not optional given how you're using page-pinning. Also, I think
we need at least one direct access to the page after getting the pin in
order to make it work.

That also very much limits it to Anon pages.