Re: [PATCH v6 2/2] nohz: set isolcpus when nohz_full is set

[Frederic Weisbecker]

On Thu, Apr 09, 2015 at 02:01:46PM -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@xxxxxxxxxx>
> ---
> I've come full circle on this issue.  For v5 I liked Frederic's
> idea of adding an API, but the more I thought about it seemed
> like an overly-generic mechanism for what is really a very specific
> issue, namely the interaction between the scheduler and nohz_full.
> We already have plenty of nohz_full-specific code in sched/core.c,
> and I think we now have a cpumask function name that is OK, so
> now I'm proposing we just stick with directly modifying the
> cpu_isolated_map using the new name.  However, I've moved the
> update to the map right to the top of sched_init_smp(), which seems
> like a cleaner place to make this kind of a change.
> 
> 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..041845c568d2 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_remove_cpus_from(cpu_isolated_map);

This should be the other one I guess.

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