linux-kernel - Re: [thisops uV2 02/10] vmstat: Optimize zone counter modifications through the use of this cpu operations

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Date:	Mon, 29 Nov 2010 10:16:24 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [thisops uV2 02/10] vmstat: Optimize zone counter modifications
 through the use of this cpu operations

On Sat, 27 Nov 2010, Mathieu Desnoyers wrote:

> * Christoph Lameter (cl@...ux.com) wrote:
> > this cpu operations can be used to slightly optimize the function. The
> > changes will avoid some address calculations and replace them with the
> > use of the percpu segment register.
> >
> > If one would have this_cpu_inc_return and this_cpu_dec_return then it
> > would be possible to optimize inc_zone_page_state and dec_zone_page_state even
> > more.
>
> Then we might want to directly target the implementation with
> this_cpu_add_return/this_cpu_sub_return (you implement these in patch 03), which
> would not need to disable preemption on the fast path. I think we already
> discussed this in the past. The reason eludes me at the moment, but I remember
> discussing that changing the increment/decrement delta to the nearest powers of
> two would let us deal with overflow cleanly. But it's probably too early in the
> morning for me to wrap my head around the issue at the moment.

We still would need to disable preemption because multiple this_cpu_ops
are needed. The only way to avoid preemption would be if the modifications
to the differential and the adding to the per zone counters would be
per cpu atomic.


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