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Date: Fri, 2 Feb 2018 15:08:17 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Mel Gorman <mgorman@techsingularity.net>,
        Mike Galbraith <efault@gmx.de>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        srinivas.pandruvada@linux.intel.com
Subject: Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle
 candidate and the basis for SIS
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On Fri, Feb 02, 2018 at 01:46:47PM +0100, Peter Zijlstra wrote:

> +static void __intel_pstate_hwp_set_desired(int val)
> +{
> +	u64 value;
> +
> +	value = rdmsrl(MSR_HWP_REQUEST);
> +	value &= ~GENMASK_ULL(23, 16);
> +	value |= (val & 0xff) << 16;
> +	wrmsrl(MSR_HWP_REQUEST, val);
> +}

Also, if we keep a software shadow of that MSR, we can avoid the
rdmsr, which might also help.