Messages in this thread | | | Date | Tue, 19 Mar 2013 08:40:26 -0400 (EDT) | From | Nicolas Pitre <> | Subject | Re: [RFC PATCH] ARM: mm: disable kmap_high_get() for SMP |
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On Tue, 19 Mar 2013, Joonsoo Kim wrote:
> On Thu, Mar 07, 2013 at 07:35:51PM +0900, JoonSoo Kim wrote: > > 2013/3/7 Nicolas Pitre <nicolas.pitre@linaro.org>: > > > On Thu, 7 Mar 2013, Joonsoo Kim wrote: > > > > > >> Hello, Nicolas. > > >> > > >> On Tue, Mar 05, 2013 at 05:36:12PM +0800, Nicolas Pitre wrote: > > >> > On Mon, 4 Mar 2013, Joonsoo Kim wrote: > > >> > > > >> > > With SMP and enabling kmap_high_get(), it makes users of kmap_atomic() > > >> > > sequential ordered, because kmap_high_get() use global kmap_lock(). > > >> > > It is not welcome situation, so turn off this optimization for SMP. > > >> > > > >> > I'm not sure I understand the problem. > > >> > > > >> > The lock taken by kmap_high_get() is released right away before that > > >> > function returns and therefore this is not actually serializing > > >> > anything. > > >> > > >> Yes, you understand what I want to say correctly. > > >> Sorry for bad explanation. > > >> > > >> Following is reasons why I send this patch with RFC tag. > > >> > > >> If we have more cpus, performance degration is possible although > > >> it is very short time to holding the lock in kmap_high_get(). > > >> > > >> And kmap has maximum 512 entries(512 * 4K = 2M) and some mobile devices > > >> has 2G memory(highmem 1G>), so probability for finding matched entry > > >> is approximately < 1/512. This probability can be more decreasing > > >> for device which have more memory. So I think that waste time to find > > >> matched entry is more than saved time. > > >> > > >> Above is my humble opinion, so please let me know what I am missing. > > > > > > Please look at the kmap_high_get() code again. It performs no > > > searching at all. What it does is: > > > > If page is not highmem, it may be already filtered in kmap_atomic(). > > So we only consider highmem page. > > > > For highmem page, it perform searching. > > In kmap_high_get(), page_address() is called. > > In page_address(), it hash PA and iterate a list for this hashed value. > > > > And another advantage of disabling ARCH_NEEDS_KMAP_HIGH_GET is > > that kmap(), kunmap() works without irq disabled. > > > > Thanks. > > Hello, Nicolas. > > For just confirm, you don't agree with this, right?
Right, I don't agree. I don't believe the saving you claim are bigger than the advantages from this functionality.
Nicolas
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