linux-kernel - Re: [PATCH 19/25] sched/vite: Handle nice updates under vtime

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Date:   Mon, 26 Nov 2018 19:41:51 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 19/25] sched/vite: Handle nice updates under vtime

On Mon, Nov 26, 2018 at 05:11:03PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 26, 2018 at 04:53:54PM +0100, Frederic Weisbecker wrote:
> > > > +	irq_work_queue_on(&per_cpu(vtime_set_nice_work, cpu), cpu);
> > > 
> > > What happens if you already had one pending? Do we loose updates?
> > 
> > No, if irq_work is already pending, it doesn't requeue iff the work hasn't
> > been executed yet and it's guaranteed it will see the freshest update.
> > (you should trust more the code you wrote ;-)
> 
> Yeah, I do remember hoq irq_work works. What I was asking was about how
> this specific handler deals with 'missing' updates.
> 
> Suppose we start with state A, set it to B and raise the IPI, then set
> it to C before the interrupt happens.
> 
> That means the irq_work handler will see C and never observe B.

Ah right, but that's ok, we can miss a few intermediate very short states. In this
case the most recent update is the relevant one.