linux-kernel - Re: [PATCH v5 16/18] timer: Implement the hierarchical pull model

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Date:   Tue, 21 Mar 2023 16:29:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v5 16/18] timer: Implement the hierarchical pull model

On Wed, Mar 01, 2023 at 03:17:42PM +0100, Anna-Maria Behnsen wrote:
> +static bool tmigr_active_up(struct tmigr_group *group,
> +			    struct tmigr_group *child,
> +			    void *ptr)
> +{
> +	union tmigr_state curstate, newstate;
> +	struct tmigr_walk *data = ptr;
> +	bool walk_done;
> +	u32 childmask;
> +
> +	childmask = data->childmask;
> +	newstate = curstate = data->groupstate;
> +
> +retry:
> +	walk_done = true;
> +
> +	if (newstate.migrator == TMIGR_NONE) {
> +		newstate.migrator = (u8)childmask;
> +
> +		/* Changes need to be propagated */
> +		walk_done = false;
> +	}
> +
> +	newstate.active |= (u8)childmask;
> +
> +	newstate.seq++;
> +
> +	if (atomic_cmpxchg(group->migr_state, curstate.state, newstate.state) != curstate.state) {
> +		newstate.state = curstate.state = atomic_read(group->migr_state);
> +		goto retry;
> +	}

	if (!atomic_try_cmpxchg(group->migr_state, &curstate.state, newstate.state)) {
		newstate.state = curstate.state;
		goto retry;
	}

> +
> +	if (group->parent && (walk_done == false)) {
> +		data->groupstate.state = atomic_read(group->parent->migr_state);
> +		data->childmask = group->childmask;
> +	}
> +
> +	/*
> +	 * Group is active, event will be ignored - bit is updated without
> +	 * holding the lock. In case bit is set while another CPU already
> +	 * handles remote events, nothing happens, because it is clear that
> +	 * CPU became active just in this moment, or in worst case event is
> +	 * handled remote. Nothing to worry about.
> +	 */
> +	group->groupevt.ignore = 1;
> +
> +	return walk_done;
> +}