linux-kernel - Re: [PATCH v2 2/2] nohz: make nohz

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Message-ID: <20150408094114.GX23123@twins.programming.kicks-ass.net>
Date:	Wed, 8 Apr 2015 11:41:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	cmetcalf@...hip.com
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] nohz: make nohz_full imply isolcpus

On Mon, Apr 06, 2015 at 02:16:45PM -0400, cmetcalf@...hip.com wrote:
> From: Chris Metcalf <cmetcalf@...hip.com>
> 
> It's not clear that nohz_full is useful without isolcpus also
> set, since otherwise the scheduler has to run periodically to
> try to determine whether to steal work from other cores.

So the Changelog and the patch don't seem to agree with one another.

The Changelog states that nohz_full should depend on isolcpus.
The patch implies nohz_full for isolcpus.

These are not the same; and I don't see the argument for the former make
sense for the latter.

In specific isolcpus without nohz_full does make sense.

> Signed-off-by: Chris Metcalf <cmetcalf@...hip.com>
> Acked-by: Mike Galbraith <umgwanakikbuti@...il.com> ["thumbs up!"]
> Acked-by: Rik van Riel <riel@...hat.com>
> ---
>  kernel/sched/core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f0f831e8a345..275f12c608f2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6836,6 +6836,7 @@ static int init_sched_domains(const struct cpumask *cpu_map)
>  	doms_cur = alloc_sched_domains(ndoms_cur);
>  	if (!doms_cur)
>  		doms_cur = &fallback_doms;
> +	tick_nohz_full_set_cpus(cpu_isolated_map);
>  	cpumask_andnot(doms_cur[0], cpu_map, cpu_isolated_map);
>  	err = build_sched_domains(doms_cur[0], NULL);
>  	register_sched_domain_sysctl();
--
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