cpumask: add cpumask_any_and_but()

In some cases, it's useful to be able to select a random cpu from the
intersection of two masks, excluding a particular CPU.

For example, in some systems an uncore PMU is shared by a subset of
CPUs, and management of this PMU is assigned to some arbitrary CPU in
this set. Whenever the management CPU is hotplugged out, we wish to
migrate responsibility to another arbitrary CPU which is both in this
set and online.

Today we can use cpumask_any_and() to select an arbitrary CPU in the
intersection of two masks. We can also use cpumask_any_but() to select
any arbitrary cpu in a mask excluding, a particular CPU.

To do both, we either need to use a temporary cpumask, which is
wasteful, or use some lower-level cpumask helpers, which can be unclear.

This patch adds a new cpumask_any_and_but() to cater for these cases.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Dawei Li <dawei.li@shingroup.cn>
Acked-by: Yury Norov <yury.norov@gmail.com>
Link: https://lore.kernel.org/r/20240403155950.2068109-2-dawei.li@shingroup.cn
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 1c29947db848..121f3ac757ff 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -389,6 +389,29 @@ unsigned int cpumask_any_but(const struct cpumask *mask, unsigned int cpu)
 }
 
 /**
+ * cpumask_any_and_but - pick a "random" cpu from *mask1 & *mask2, but not this one.
+ * @mask1: the first input cpumask
+ * @mask2: the second input cpumask
+ * @cpu: the cpu to ignore
+ *
+ * Returns >= nr_cpu_ids if no cpus set.
+ */
+static inline
+unsigned int cpumask_any_and_but(const struct cpumask *mask1,
+				 const struct cpumask *mask2,
+				 unsigned int cpu)
+{
+	unsigned int i;
+
+	cpumask_check(cpu);
+	i = cpumask_first_and(mask1, mask2);
+	if (i != cpu)
+		return i;
+
+	return cpumask_next_and(cpu, mask1, mask2);
+}
+
+/**
  * cpumask_nth - get the Nth cpu in a cpumask
  * @srcp: the cpumask pointer
  * @cpu: the Nth cpu to find, starting from 0