linux-kernel - Re: [PATCH v4 4/5] slub: Only IPI CPUs that have per cpu obj to flush

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Date:	Mon, 2 Jan 2012 13:59:30 +0200
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Pekka Enberg <penberg@...nel.org>, linux-kernel@...r.kernel.org,
	Chris Metcalf <cmetcalf@...era.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Russell King <linux@....linux.org.uk>, linux-mm@...ck.org,
	Matt Mackall <mpm@...enic.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, apkm@...ux-foundation.org
Subject: Re: [PATCH v4 4/5] slub: Only IPI CPUs that have per cpu obj to flush

On Sun, Jan 1, 2012 at 6:50 PM, Avi Kivity <avi@...hat.com> wrote:
> On 01/01/2012 06:12 PM, Gilad Ben-Yossef wrote:
>> >
>> > Since this seems to be a common pattern, how about:
>> >
>> >   zalloc_cpumask_var_or_all_online_cpus(&cpus, GFTP_ATOMIC);
>> >   ...
>> >   free_cpumask_var(cpus);
>> >
>> > The long-named function at the top of the block either returns a newly
>> > allocated zeroed cpumask, or a static cpumask with all online cpus set.
>> > The code in the middle is only allowed to set bits in the cpumask
>> > (should be the common usage).  free_cpumask_var() needs to check whether
>> > the freed object is the static variable.
>>
>> Thanks for the feedback and advice! I totally agree the repeating
>> pattern needs abstracting.
>>
>> I ended up chosing to try a different abstraction though - basically a wrapper
>> on_each_cpu_cond that gets a predicate function to run per CPU to
>> build the mask
>> to send the IPI to. It seems cleaner to me not having to mess with
>> free_cpumask_var
>> and it abstracts more of the general pattern.
>>
>
> This converts the algorithm to O(NR_CPUS) from a potentially lower
> complexity algorithm.  Also, the existing algorithm may not like to be
> driven by cpu number.  Both are true for kvm.
>

Right, I was only thinking on my own uses, which are O(NR_CPUS) by nature.

I wonder if it would be better to create a safe_cpumask_var type with
its own alloc function
free and and sset_cpu function but no clear_cpu function so that the
compiler will catch
cases of trying to clear bits off of such a cpumask?

It seems safer and also makes handling the free function easier.

Does that makes sense or am I over engineering it? :-)

Gilad


-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
http://benyossef.com

"Unfortunately, cache misses are an equal opportunity pain provider."
-- Mike Galbraith, LKML
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