linux-kernel - Re: [PULL] cpumask: finally make them variable size w/ CPUMASK

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Date:	Thu, 10 May 2012 11:46:23 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>, anton@...ba.org,
	Arnd Bergmann <arnd@...db.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mike Travis <travis@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>, kosaki.motohiro@...il.com
Subject: Re: [PULL] cpumask: finally make them variable size w/ CPUMASK_OFFSTACK.

On Wed, 09 May 2012 21:32:57 -0400, KOSAKI Motohiro <kosaki.motohiro@...il.com> wrote:
> (5/9/12 2:10 AM), Rusty Russell wrote:
> > Hi Ingo,
> >
> >          I finally rebased this on top of your tip tree, and tested it
> > locally.  Some more old-style cpumask usages have crept in, but it's a
> > fairly simple series.
> >
> > The final result is that if you enable CONFIG_CPUMASK_OFFSTACK, then
> > 'struct cpumask' becomes an undefined type.  You can't accidentally take
> > the size of it, assign it, or pass it by value.  And thus it's safe for
> > us to make it smaller if nr_cpu_ids<  NR_CPUS, as the final patch does.
> >
> > It unfortunately requires the lglock cleanup patch, which Al already has
> > queued, so I've included it here.
> 
> Hi
> 
> Thanks this effort. This is very cleaner than I expected.
> However I should NAK following one patch. sorry. because of, lru-drain is
> called from memory reclaim context. It mean, additional allocation may not
> work. Please just use bare NR_CPUS bitmap instead. space wasting is minor
> issue than that.

But if it fails the allocation, that's fine: we just send a few more
IPIs to every CPU:

+	if (!zalloc_cpumask_var(&cpus_with_pcps, GFP_KERNEL)) {
+		on_each_cpu(drain_local_pages, NULL, 1);
+		return;
+	}

We can do it the other way, but it sets a bad example, and after we get
rid of cpumask, it becomes:

        static DECLARE_BITMAP(cpus_with_pcps, NR_CPUS);

        ......

 		if (has_pcps)
			cpumask_set_cpu(cpu, to_cpumask(cpus_with_pcps));
		else
			cpumask_clear_cpu(cpu, to_cpumask(cpus_with_pcps));
 	}
	on_each_cpu_mask(to_cpumask(cpus_with_pcps), drain_local_pages, NULL, 1);

Or is there a reason we shouldn't even try to allocate here?

Thanks,
Rusty.
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