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

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Date:   Wed, 19 Apr 2017 11:44:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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, Apr 19, 2017 at 11:09:14AM +0200, Peter Zijlstra wrote:

> Would it be very onerous to rewrite that into regular loops? That avoids
> us having to think (and worry) about blowing our stack.

void walk_groups(bool (*up)(void *), void (*down)(void *), void *data)
{
	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
	struct group *stack[tmigr_hierarchy_levels];
	struct group *group = tmc->group;
	int i = 0;

	raw_spin_lock(&tmc->lock);

	do {
		stack[i++] = group;

		if (up(data))
			break;

	} while ((group = group->parent));

	do {
		group = stack[--i];

		down(data);

	} while (group != tmc->group);

	raw_spin_unlock(&tmc->lock);
}

Something like so, iterates the hierarchy for the current CPU and calls
@up and @down at each level in the proper order. And has obvious stack
usage.