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SubjectRe: [PATCH v6 2/2] nohz: set isolcpus when nohz_full is set
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@ezchip.com>
> ---
> 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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