linux-kernel - Re: acpi throttling: Use this_cpu

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Date:	Tue, 21 Dec 2010 16:43:24 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Tejun Heo <htejun@...il.com>
cc:	"H. Peter Anvin" <hpa@...or.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	rui.zhang@...el.com, lenb@...nel.org, linux-acpi@...r.kernel.org
Subject: Re: acpi throttling: Use this_cpu_has and simplify code

On Sat, 18 Dec 2010, Tejun Heo wrote:

> It's bothersome that these methods don't have any indication that
> they're bound to local CPU when they can't be called with @pr for
> another CPU as MSRs can only be accessed from local CPU.

Right.

> In the longer run, it would be nice if there's an indication that this
> is only for the local CPU and maybe a WARN_ON_ONCE().  Maybe dropping
> @pr and using this_cpu_*() is better for performance too?

That is precisely the difficulty with many other functions that take
pointers to cpu_info. If we can establish that they are called *only* with
pointers to the current cpu then we can improve the function by dropping
the parameter and using this_cpu ops to access the cpu_info fields.

Using this_cpu ops instead of access to a field of a struct pointed to by
an array avoids an addition of pointers and replacest with a operation
that essentially takes a global address with a segment prefix. It reduces
register pressure and avoids the additions of pointers but it will encode
the address in the instruction.

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