linux-kernel - Re: [patch V2 08/10] timer: Implement the hierarchical pull model

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Date:   Wed, 19 Apr 2017 11:43:01 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-pm@...r.kernel.org, Arjan van de Ven <arjan@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [patch V2 08/10] timer: Implement the hierarchical pull model

On Wed, 19 Apr 2017, Peter Zijlstra wrote:
> > +done:
> > +	raw_spin_unlock(&group->lock);
> > +	return nextevt;
> > +}
> 
> Would it be very onerous to rewrite that into regular loops? That avoids
> us having to think (and worry) about blowing our stack.

The issue is that this is walking a hierarchy tree (limited depth) and not
a list of groups at a particular level.

In an earlier version we checked the stack usage and it was ~128 bytes per
nest level. With 32k CPUs this ends up with maximum 5 recursions, i.e. 640
bytes. I recheck with the current implementation.

Thanks,

	tglx