Re: [PULL] cpumask: finally make them variable size w/ CPUMASK_OFFSTACK.

From: KOSAKI Motohiro
Date: Wed May 09 2012 - 22:44:14 EST

(5/9/12 10:16 PM), Rusty Russell wrote:
On Wed, 09 May 2012 21:32:57 -0400, KOSAKI Motohiro<kosaki.motohiro@xxxxxxxxx> 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.


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));
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?

1) your code always use GFP_KERNEL. it is trouble maker when alloc_pages w/ GFP_ATOMIC.
2) When CONFIG_CPUMASK_OFFSTACK=n and NR_CPUS is relatively large, cpumask on stack may
cause stack overflow. because of, alloc_pages() can be called from very deep call stack.


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