Hello Mel,
On 01/30/2012 05:52 PM, Maxime Coquelin wrote:

On 01/30/2012 04:22 PM, Mel Gorman wrote:

quoted

You may be able to use the existing arch_alloc_page() hook and
call PASR on architectures that support it if and only if PASR is
present and enabled by the administrator but even this is likely to be
unpopular as it'll have a measurable performance impact on platforms
with PASR (not to mention the PASR lock will be even heavier as it'll
now be also used for per-cpu page allocations). To get the hook you
want, you'd need to show significant benefit before they were happy with
the hook.

Your proposal sounds good.
AFAIK, per-cpu allocation maximum size is 32KB. Please correct me if 
I'm wrong.
Since pasr_kget/kput() calls the PASR framework only on MAX_ORDER 
allocations, we wouldn't add any locking risks nor contention compared 
to current patch.
I will update the patch set using  arch_alloc/free_page().

I just had a deeper look at when arch_alloc_page() is called. I think it 
does not fit with PASR framework needs.
pasr_kget() calls pasr_get() only for max order pages (same for 
pasr_kput()) to avoid overhead.

In current patch set, pasr_kget() is called when pages are removed from 
the free lists, and pasr_kput() when pages are inserted in the free lists.
So, pasr_get() is called in case of :
     - allocation of a max order page
     - split of a max order page into lower order pages to fulfill 
allocation of pages smaller than max order
And pasr_put() is called in case of:
     - release of a max order page
     - coalescence of two "max order -1" pages when smaller pages are 
released

If we call the PASR framework in arch_alloc_page(), we have two 
possibilities:
     1) using pasr_kget(): the PASR framework will only be notified of 
max order allocations, so the coalesce/split of free pages case will not 
be taken into account.
     2) using pasr_get(): the PASR framework will be called for every 
orders of page allocation/release. The induced overhead is not acceptable.

To avoid calling pasr_kget/kput() directly in page_alloc.c, do you think 
adding some arch specific hooks when a page is inserted or removed from 
the free lists could be acceptable?
Something like arch_insert_freepage(struct page *page, int order) and 
arch_remove_freepage(struct page *page, int order).

Regards,
Maxime

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