Re: [PATCH] cpumask: add cpumask_any_and_but()

[Rusty Russell]

Mark Rutland <mark.rutland@xxxxxxx> writes:
> In some cases, it's useful to be able to select a random cpu from the
> intersection of two masks, excluding a particular CPU.

Acked-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

Thanks,
Rusty.

> 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@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  include/linux/cpumask.h |  3 +++
>  lib/cpumask.c           | 23 +++++++++++++++++++++++
>  2 files changed, 26 insertions(+)
>
> This patch would help in cases like the Qualcomm L2 cache PMU driver [1]. If
> people are happy with this patch, I'd like to take it along with that patch
> (modified to use the new helper). I'm also happy to leave this as a subsequent
> cleanup.
>
> Thanks,
> Mark.
>
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2017-February/485762.html
>
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index c717f5e..99dc550 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -210,6 +210,9 @@ static inline unsigned int cpumask_next_zero(int n, const struct cpumask *srcp)
>  
>  int cpumask_next_and(int n, const struct cpumask *, const struct cpumask *);
>  int cpumask_any_but(const struct cpumask *mask, unsigned int cpu);
> +int cpumask_any_and_but(const struct cpumask *mask1,
> +			const struct cpumask *mask2,
> +			unsigned int cpu);
>  unsigned int cpumask_local_spread(unsigned int i, int node);
>  
>  /**
> diff --git a/lib/cpumask.c b/lib/cpumask.c
> index 81dedaa..641b526 100644
> --- a/lib/cpumask.c
> +++ b/lib/cpumask.c
> @@ -43,6 +43,29 @@ int cpumask_any_but(const struct cpumask *mask, unsigned int cpu)
>  }
>  EXPORT_SYMBOL(cpumask_any_but);
>  
> +/**
> + * 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.
> + */
> +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);
> +}
> +EXPORT_SYMBOL(cpumask_any_and_but);
> +
>  /* These are not inline because of header tangles. */
>  #ifdef CONFIG_CPUMASK_OFFSTACK
>  /**
> -- 
> 1.9.1