Re: [PATCH v7 1/2] nohz: add tick_nohz_full_add_cpus_to() and _remove_cpus_from() APIs

From: Chris Metcalf
Date: Mon Apr 13 2015 - 20:49:48 EST

On 4/13/2015 8:33 PM, Frederic Weisbecker wrote:
On Fri, Apr 10, 2015 at 04:53:51PM -0400, Chris Metcalf wrote:
The "removes_cpus_from" API is useful, for example, to modify a cpumask
to avoid the nohz cores so that interrupts aren't sent to them.

Likewise the "add_cpus_to" API is useful to modify a cpumask indicating
some special nohz-type functionality so that the nohz cores are
automatically added to that set.

Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 9c085dc12ae9..8d1754c0f694 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -186,6 +186,18 @@ static inline bool tick_nohz_full_cpu(int cpu)
return cpumask_test_cpu(cpu, tick_nohz_full_mask);
+static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
+ if (tick_nohz_full_enabled())
+ cpumask_or(mask, mask, tick_nohz_full_mask);
+static inline void tick_nohz_full_remove_cpus_from(struct cpumask *mask)
+ if (tick_nohz_full_enabled())
+ cpumask_andnot(mask, mask, tick_nohz_full_mask);
I would prefer that you don't introduce new APIs if they aren't used in your
patchset. It seems that's the case for tick_nohz_full_remove_cpus_from().

Yes, it's used in a tile-tree patch to remove nohz_full cpus from the set that
take interrupts from the tilegx network driver.

I can certainly make this patchset have just the _add_cpus_to() variant,
and the other patchset have just the _remove_cpus_from() variant.
It seemed to make sense to include them together as a matched set,
but doing it the way you suggest makes an equal if different amount of sense.

Also we have housekeeping_affine() that affines a task to CPUs outside the
range of nohz full. In case you still need tick_nohz_full_remove_cpus_from()
in a further patchset, housekeeping_affine_cpumask() would extend the existing

I like housekeeping_affine(), but it overwrites the affinity mask of
the task. So I would expect housekeeping_affine_mask() to overwrite
the specified argument cpumask, which it doesn't in my definition.

I don't know that I can think of a good name in the "housekeeping_xxx"
namespace that feels better than the one I proposed. In context of the
proposed client of the API so far, it's:

if (!network_cpus_init()) {
network_cpus_map = *cpu_online_mask;

If housekeeping_mask were defined for non-nohz_full I could just use
it unconditionally here. Alternately I could just put in an #ifdef for
CONFIG_NO_HZ_FULL and either use cpu_only_mask or housekeeping_mask
to initialize network_cpus_map, which dodges the bullet of creating
an acceptable API name here for the moment.

Frederic, what's your thought/preference?

Chris Metcalf, EZChip Semiconductor

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