On 09/30/2013 12:11 PM, Luis Henriques wrote:
> 3.5.7.22 -stable review patch. If anyone has any objections, please let me
> know.
>
> ------------------
>
> From: Khalid Aziz <[email protected]>
>
> commit 7cb2ef56e6a8b7b368b2e883a0a47d02fed66911 upstream.
>
> I am working with a tool that simulates oracle database I/O workload.
> This tool (orion to be specific -
> <http://docs.oracle.com/cd/E11882_01/server.112/e16638/iodesign.htm#autoId24>)
> allocates hugetlbfs pages using shmget() with SHM_HUGETLB flag. It then
> does aio into these pages from flash disks using various common block
> sizes used by database. I am looking at performance with two of the most
> common block sizes - 1M and 64K. aio performance with these two block
> sizes plunged after Transparent HugePages was introduced in the kernel.
> Here are performance numbers:
>
> pre-THP 2.6.39 3.11-rc5
> 1M read 8384 MB/s 5629 MB/s 6501 MB/s
> 64K read 7867 MB/s 4576 MB/s 4251 MB/s
>
> I have narrowed the performance impact down to the overheads introduced by
> THP in __get_page_tail() and put_compound_page() routines. perf top shows
>> 40% of cycles being spent in these two routines. Every time direct I/O
> to hugetlbfs pages starts, kernel calls get_page() to grab a reference to
> the pages and calls put_page() when I/O completes to put the reference
> away. THP introduced significant amount of locking overhead to get_page()
> and put_page() when dealing with compound pages because hugepages can be
> split underneath get_page() and put_page(). It added this overhead
> irrespective of whether it is dealing with hugetlbfs pages or transparent
> hugepages. This resulted in 20%-45% drop in aio performance when using
> hugetlbfs pages.
>
> Since hugetlbfs pages can not be split, there is no reason to go through
> all the locking overhead for these pages from what I can see. I added
> code to __get_page_tail() and put_compound_page() to bypass all the
> locking code when working with hugetlbfs pages. This improved performance
> significantly. Performance numbers with this patch:
>
> pre-THP 3.11-rc5 3.11-rc5 + Patch
> 1M read 8384 MB/s 6501 MB/s 8371 MB/s
> 64K read 7867 MB/s 4251 MB/s 6510 MB/s
>
> Performance with 64K read is still lower than what it was before THP, but
> still a 53% improvement. It does mean there is more work to be done but I
> will take a 53% improvement for now.
>
> Please take a look at the following patch and let me know if it looks
> reasonable.
>
> [[email protected]: tweak comments]
> Signed-off-by: Khalid Aziz <[email protected]>
> Cc: Pravin B Shelar <[email protected]>
> Cc: Christoph Lameter <[email protected]>
> Cc: Andrea Arcangeli <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Cc: Mel Gorman <[email protected]>
> Cc: Rik van Riel <[email protected]>
> Cc: Minchan Kim <[email protected]>
> Cc: Andi Kleen <[email protected]>
> Signed-off-by: Andrew Morton <[email protected]>
> Signed-off-by: Linus Torvalds <[email protected]>
> [ luis: backported to 3.5: adjusted context ]
> Signed-off-by: Luis Henriques <[email protected]>
Hi Greg,
I suppose this patch also needed for 3.4, right?
Regards,
Jack
> ---
> mm/swap.c | 77
> ++++++++++++++++++++++++++++++++++++++++++---------------------
> 1 file changed, 52 insertions(+), 25 deletions(-)
>
> diff --git a/mm/swap.c b/mm/swap.c
> index 4e7e2ec..0c833e8 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -30,6 +30,7 @@
> #include <linux/backing-dev.h>
> #include <linux/memcontrol.h>
> #include <linux/gfp.h>
> +#include <linux/hugetlb.h>
>
> #include "internal.h"
>
> @@ -77,6 +78,19 @@ static void __put_compound_page(struct page *page)
>
> static void put_compound_page(struct page *page)
> {
> + /*
> + * hugetlbfs pages cannot be split from under us. If this is a
> + * hugetlbfs page, check refcount on head page and release the page if
> + * the refcount becomes zero.
> + */
> + if (PageHuge(page)) {
> + page = compound_head(page);
> + if (put_page_testzero(page))
> + __put_compound_page(page);
> +
> + return;
> + }
> +
> if (unlikely(PageTail(page))) {
> /* __split_huge_page_refcount can run under us */
> struct page *page_head = compound_trans_head(page);
> @@ -180,38 +194,51 @@ bool __get_page_tail(struct page *page)
> * proper PT lock that already serializes against
> * split_huge_page().
> */
> - unsigned long flags;
> bool got = false;
> - struct page *page_head = compound_trans_head(page);
> + struct page *page_head;
>
> - if (likely(page != page_head && get_page_unless_zero(page_head))) {
> + /*
> + * If this is a hugetlbfs page it cannot be split under us. Simply
> + * increment refcount for the head page.
> + */
> + if (PageHuge(page)) {
> + page_head = compound_head(page);
> + atomic_inc(&page_head->_count);
> + got = true;
> + } else {
> + unsigned long flags;
> +
> + page_head = compound_trans_head(page);
> + if (likely(page != page_head &&
> + get_page_unless_zero(page_head))) {
> +
> + /* Ref to put_compound_page() comment. */
> + if (PageSlab(page_head)) {
> + if (likely(PageTail(page))) {
> + __get_page_tail_foll(page, false);
> + return true;
> + } else {
> + put_page(page_head);
> + return false;
> + }
> + }
>
> - /* Ref to put_compound_page() comment. */
> - if (PageSlab(page_head)) {
> + /*
> + * page_head wasn't a dangling pointer but it
> + * may not be a head page anymore by the time
> + * we obtain the lock. That is ok as long as it
> + * can't be freed from under us.
> + */
> + flags = compound_lock_irqsave(page_head);
> + /* here __split_huge_page_refcount won't run anymore */
> if (likely(PageTail(page))) {
> __get_page_tail_foll(page, false);
> - return true;
> - } else {
> - put_page(page_head);
> - return false;
> + got = true;
> }
> + compound_unlock_irqrestore(page_head, flags);
> + if (unlikely(!got))
> + put_page(page_head);
> }
> -
> - /*
> - * page_head wasn't a dangling pointer but it
> - * may not be a head page anymore by the time
> - * we obtain the lock. That is ok as long as it
> - * can't be freed from under us.
> - */
> - flags = compound_lock_irqsave(page_head);
> - /* here __split_huge_page_refcount won't run anymore */
> - if (likely(PageTail(page))) {
> - __get_page_tail_foll(page, false);
> - got = true;
> - }
> - compound_unlock_irqrestore(page_head, flags);
> - if (unlikely(!got))
> - put_page(page_head);
> }
> return got;
> }
>
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