linux-kernel - Re: [PATCH 05/13] rcu: Fix unraised IPI to timekeeping CPU

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Message-ID: <20131218121204.GS13532@twins.programming.kicks-ass.net>
Date:	Wed, 18 Dec 2013 13:12:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	John Stultz <john.stultz@...aro.org>,
	Alex Shi <alex.shi@...aro.org>,
	Kevin Hilman <khilman@...aro.org>
Subject: Re: [PATCH 05/13] rcu: Fix unraised IPI to timekeeping CPU

On Tue, Dec 17, 2013 at 11:51:24PM +0100, Frederic Weisbecker wrote:
> -			rcu_kick_nohz_cpu(tick_do_timer_cpu);
> +			smp_send_reschedule(tick_do_timer_cpu);

Hurm.. so I don't really like this (same for the other patches doing the
similar change).

Why not have a function called: wake_time_keeper_cpu(), and have that do
the right thing?

Also, afaict, all you want is that remote cpu to run
tick_nohz_full_check(), right?

So why are you (ab)using the scheduler_ipi for this at all? Why not have
something like:

static DEFINE_PER_CPU(struct call_single_data, nohz_full_csd);

int init_nohz_full(void)
{
	int cpu;

	for_each_possible_cpu(cpu) {
		struct call_single_data *csd = &per_cpu(nohz_full_csd, cpu);

		csd->flags = 0;
		csd->func = &tick_nohz_full_check;
		csd->info = NULL;
	}

	return 0;
}

void wake_time_keeper_cpu(void)
{
	int cpu = pick_timekeeper_cpu();
	struct single_call_data *csd = &per_cpu(nohz_full_csd, cpu);

	__smp_call_function_single(cpu, csd, 0);
}
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