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Date: Mon, 6 Feb 2017 14:48:13 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH] cpumask: add cpumask_any_and_but()
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On Mon, Feb 06, 2017 at 11:38:52AM +0000, Mark Rutland wrote:
> 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
> ---
>  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.

Have at; looks ok I suppose.