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

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Date:	Mon, 02 Jan 2012 15:30:47 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Gilad Ben-Yossef <gilad@...yossef.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 01/02/2012 01:59 PM, Gilad Ben-Yossef wrote:
> 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? :-)

It makes sense.  Depends on the number of call sites, really.  If there
are several, consolidation helps, also makes it easier to further refactor.

-- 
error compiling committee.c: too many arguments to function

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