On Thu, Apr 21, 2011 at 12:08:41PM +0100, Mel Gorman wrote:
> On Thu, Apr 21, 2011 at 03:59:47PM +0900, Minchan Kim wrote:
> > Hi Mel,
> >
> > On Fri, Apr 15, 2011 at 7:12 PM, Mel Gorman <mgorman@xxxxxxx> wrote:
> > > With transparent hugepage support, handle_mm_fault() has to be careful
> > > that a normal PMD has been established before handling a PTE fault. To
> > > achieve this, it used __pte_alloc() directly instead of pte_alloc_map
> > > as pte_alloc_map is unsafe to run against a huge PMD. pte_offset_map()
> > > is called once it is known the PMD is safe.
> > >
> > > pte_alloc_map() is smart enough to check if a PTE is already present
> > > before calling __pte_alloc but this check was lost. As a consequence,
> > > PTEs may be allocated unnecessarily and the page table lock taken.
> > > Thi useless PTE does get cleaned up but it's a performance hit which
> > > is visible in page_test from aim9.
> > >
> > > This patch simply re-adds the check normally done by pte_alloc_map to
> > > check if the PTE needs to be allocated before taking the page table
> > > lock. The effect is noticable in page_test from aim9.
> > >
> > > AIM9
> > > 2.6.38-vanilla 2.6.38-checkptenone
> > > creat-clo 446.10 ( 0.00%) 424.47 (-5.10%)
> > > page_test 38.10 ( 0.00%) 42.04 ( 9.37%)
> > > brk_test 52.45 ( 0.00%) 51.57 (-1.71%)
> > > exec_test 382.00 ( 0.00%) 456.90 (16.39%)
> > > fork_test 60.11 ( 0.00%) 67.79 (11.34%)
> > > MMTests Statistics: duration
> > > Total Elapsed Time (seconds) 611.90 612.22
> > >
> > > (While this affects 2.6.38, it is a performance rather than a
> > > functional bug and normally outside the rules -stable. While the big
> > > performance differences are to a microbench, the difference in fork
> > > and exec performance may be significant enough that -stable wants to
> > > consider the patch)
> > >
> > > Reported-by: Raz Ben Yehuda <raziebe@xxxxxxxxx>
> > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> > > --
> > > mm/memory.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 5823698..1659574 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -3322,7 +3322,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > > * run pte_offset_map on the pmd, if an huge pmd could
> > > * materialize from under us from a different thread.
> > > */
> > > - if (unlikely(__pte_alloc(mm, vma, pmd, address)))
> > > + if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vma, pmd, address))
> > > return VM_FAULT_OOM;
> > > /* if an huge pmd materialized from under us just retry later */
> > > if (unlikely(pmd_trans_huge(*pmd)))
> > >
> >
> > Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
> >
> > Sorry for jumping in too late. I have a just nitpick.
> >
>
> Better late than never :)
>
> > We have another place, do_huge_pmd_anonymous_page.
> > Although it isn't workload of page_test, is it valuable to expand your
> > patch to cover it?
> > If there is workload there are many thread and share one shared anon
> > vma in ALWAYS THP mode, same problem would happen.
>
> We already checked pmd_none() in handle_mm_fault() before calling
> into do_huge_pmd_anonymous_page(). We could race for the fault while
> attempting to allocate a huge page but it wouldn't be as severe a
> problem particularly as it is encountered after failing a 2M allocation.
Right you are. Fail ot 2M allocation would affect as throttle.
Thanks.
As I failed let you add the check, I have to reveal my mind. :)
Actually, what I want is consistency of the code.
The code have been same in two places but you find the problem in page_test of aim9,
you changed one of them slightly. I think in future someone will
have a question about that and he will start grep git log but it will take
a long time as the log is buried other code piled up.
I hope adding the comment in this case.
/*
* PTEs may be allocated unnecessarily and the page table lock taken.
* The useless PTE does get cleaned up but it's a performance hit in
* some micro-benchmark. Let's check pmd_none before __pte_alloc to
* reduce the overhead.
*/
- if (unlikely(__pte_alloc(mm, vma, pmd, address)))
+ if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vma, pmd, address))
If you mind it as someone who have a question can find the log at last
although he need some time, I wouldn't care of the nitpick any more. :)
It's up to you.
Thanks, Mel.
>
> --
> Mel Gorman
> SUSE Labs
--
Kind regards,
Minchan Kim
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