[RFC PATCH] ARM: mm: disable kmap_high_get() for SMP

[iamjoonsoo.kim@lge.com (Joonsoo Kim)]

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?

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
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