2013/3/7 Nicolas Pitre <nicolas.pi...@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. > - lock the kmap array against concurrent changes > > - if the given page is not highmem, unlock and return NULL > > - otherwise increment that page reference count, unlock, and return the > mapped address for that page. > > There is almost zero cost to this function, independently of the number > of kmap entries, whereas it does save much bigger costs elsewhere when > it is successful. > > > Nicolas > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/