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Date: Wed, 09 May 2012 22:43:39 -0400 From: KOSAKI Motohiro <kosaki.motohiro@...il.com> To: Rusty Russell <rusty@...tcorp.com.au> CC: KOSAKI Motohiro <kosaki.motohiro@...il.com>, 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> Subject: Re: [PULL] cpumask: finally make them variable size w/ CPUMASK_OFFSTACK. (5/9/12 10:16 PM), Rusty Russell wrote: > 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? 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. Thought? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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