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

From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	John Stultz <john.stultz@linaro.org>,
	Eric Dumazet <edumazet@google.com>,
	Anna-Maria Gleixner <anna-maria@linutronix.de>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	linux-pm@vger.kernel.org, Arjan van de Ven <arjan@infradead.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Rik van Riel <riel@redhat.com>
Subject: Re: [patch V2 08/10] timer: Implement the hierarchical pull model
Date: Wed, 19 Apr 2017 11:44:45 +0200	[thread overview]
Message-ID: <20170419094445.oj5zc624zanv2hfp@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170419090914.qdiamqwzro34b53d@hirez.programming.kicks-ass.net>

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.