linux-kernel - Re: [PATCH v7 2/2] nohz: set isolcpus when nohz

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Message-ID: <20150414003711.GD6121@lerouge>
Date:	Tue, 14 Apr 2015 02:37:11 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	"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 v7 2/2] nohz: set isolcpus when nohz_full is set

On Fri, Apr 10, 2015 at 04:53:52PM -0400, Chris Metcalf wrote:
> nohz_full is only useful with isolcpus also set, since otherwise the
> scheduler has to run periodically to try to determine whether to steal
> work from other cores.
> 
> Accordingly, when booting with nohz_full=xxx on the command line, we
> should act as if isolcpus=xxx was also set, and set (or extend) the
> isolcpus set to include the nohz_full cpus.
> 
> Signed-off-by: Chris Metcalf <cmetcalf@...hip.com>
> ---
> v7: oops, use add_cpus_to, not remove_cpus_from [Frederic]
> 
> v6: back to just using a direct update, with the new names
> 
> v5: test run of Frederic's proposal to use sched_isolated_map_add()
> 
> v4: update accessor names to make them clearer [PeterZ]
> 
> v3: update changelog language to avoid using unclear "implies" [PeterZ]
> 
>  kernel/sched/core.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f0f831e8a345..4a5681b5a4fd 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7031,6 +7031,9 @@ void __init sched_init_smp(void)
>  	alloc_cpumask_var(&non_isolated_cpus, GFP_KERNEL);
>  	alloc_cpumask_var(&fallback_doms, GFP_KERNEL);
>  
> +	/* nohz_full won't take effect without isolating the cpus. */
> +	tick_nohz_full_add_cpus_to(cpu_isolated_map);
> +

I can't say I like this, but I can not come up with a better name.
So that patch is ok to me (if Peterz acks it) :-)

>  	sched_init_numa();
>  
>  	/*
> -- 
> 2.1.2
> 
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