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SubjectRE: [PATCH 1/2] sched/wait: Break up long wake list walk
Date
> > Because that code sequence doesn't actually depend on
> > "wait_on_page_lock()" for _correctness_ anyway, afaik. Anybody who
> > does "migration_entry_wait()" _has_ to retry anyway, since the page
> > table contents may have changed by waiting.
> >
> > So I'm not proud of the attached patch, and I don't think it's really
> > acceptable as-is, but maybe it's worth testing? And maybe it's
> > arguably no worse than what we have now?
> >
> > Comments?
> >
>
> The transhuge migration path for numa balancing doesn't go through the
> migration_entry_wait patch despite similarly named functions that suggest
> it does so this may only has the most effect when THP is disabled. It's
> worth trying anyway.

I just finished the test of yield patch (only functionality not performance).
Yes, it works well with THP disabled.
With THP enabled, I observed one LOCKUP caused by long queue wait.

Here is the call stack with THP enabled.
#
100.00% (ffffffff9e1aefca)
|
---wait_on_page_bit
do_huge_pmd_numa_page
__handle_mm_fault
handle_mm_fault
__do_page_fault
do_page_fault
page_fault
|
|--60.39%--0x2b7b7
| |
| |--34.26%--0x127d8
| | start_thread
| |
| --25.95%--0x127a2
| start_thread
|
--39.25%--0x2b788
|
--38.81%--0x127a2
start_thread


>
> Covering both paths would be something like the patch below which spins
> until the page is unlocked or it should reschedule. It's not even boot
> tested as I spent what time I had on the test case that I hoped would be
> able to prove it really works.

I will give it a try.

Thanks,
Kan

>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 79b36f57c3ba..31cda1288176 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -517,6 +517,13 @@ static inline void wait_on_page_locked(struct page
> *page)
> wait_on_page_bit(compound_head(page), PG_locked);
> }
>
> +void __spinwait_on_page_locked(struct page *page);
> +static inline void spinwait_on_page_locked(struct page *page)
> +{
> + if (PageLocked(page))
> + __spinwait_on_page_locked(page);
> +}
> +
> static inline int wait_on_page_locked_killable(struct page *page)
> {
> if (!PageLocked(page))
> diff --git a/mm/filemap.c b/mm/filemap.c
> index a49702445ce0..c9d6f49614bc 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1210,6 +1210,15 @@ int __lock_page_or_retry(struct page *page,
> struct mm_struct *mm,
> }
> }
>
> +void __spinwait_on_page_locked(struct page *page)
> +{
> + do {
> + cpu_relax();
> + } while (PageLocked(page) && !cond_resched());
> +
> + wait_on_page_locked(page);
> +}
> +
> /**
> * page_cache_next_hole - find the next hole (not-present entry)
> * @mapping: mapping
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 90731e3b7e58..c7025c806420 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1443,7 +1443,7 @@ int do_huge_pmd_numa_page(struct vm_fault
> *vmf, pmd_t pmd)
> if (!get_page_unless_zero(page))
> goto out_unlock;
> spin_unlock(vmf->ptl);
> - wait_on_page_locked(page);
> + spinwait_on_page_locked(page);
> put_page(page);
> goto out;
> }
> @@ -1480,7 +1480,7 @@ int do_huge_pmd_numa_page(struct vm_fault
> *vmf, pmd_t pmd)
> if (!get_page_unless_zero(page))
> goto out_unlock;
> spin_unlock(vmf->ptl);
> - wait_on_page_locked(page);
> + spinwait_on_page_locked(page);
> put_page(page);
> goto out;
> }
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e84eeb4e4356..9b6c3fc5beac 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -308,7 +308,7 @@ void __migration_entry_wait(struct mm_struct *mm,
> pte_t *ptep,
> if (!get_page_unless_zero(page))
> goto out;
> pte_unmap_unlock(ptep, ptl);
> - wait_on_page_locked(page);
> + spinwait_on_page_locked(page);
> put_page(page);
> return;
> out:
>

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