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[patch] fs, proc: unconditional cond_resched when reading smaps

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  [patch] fs, proc: unconditional cond_resched when reading smaps David Rientjes <rientjes@google.com> - 2017-08-21 23:10 +0200
    Re: [patch] fs, proc: unconditional cond_resched when reading smaps "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-08-21 23:50 +0200

#1716885 — [patch] fs, proc: unconditional cond_resched when reading smaps

If there are large numbers of hugepages to iterate while reading
/proc/pid/smaps, the page walk never does cond_resched().  On archs
without split pmd locks, there can be significant and observable
contention on mm->page_table_lock which cause lengthy delays without
rescheduling.

Always reschedule in smaps_pte_range() if necessary since the pagewalk
iteration can be expensive.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 fs/proc/task_mmu.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -599,11 +599,11 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	if (ptl) {
 		smaps_pmd_entry(pmd, addr, walk);
 		spin_unlock(ptl);
-		return 0;
+		goto out;
 	}
 
 	if (pmd_trans_unstable(pmd))
-		return 0;
+		goto out;
 	/*
 	 * The mmap_sem held all the way back in m_start() is what
 	 * keeps khugepaged out of here and from collapsing things
@@ -613,6 +613,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	for (; addr != end; pte++, addr += PAGE_SIZE)
 		smaps_pte_entry(pte, addr, walk);
 	pte_unmap_unlock(pte - 1, ptl);
+out:
 	cond_resched();
 	return 0;
 }

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#1716900

On Mon, Aug 21, 2017 at 02:06:45PM -0700, David Rientjes wrote:
> If there are large numbers of hugepages to iterate while reading
> /proc/pid/smaps, the page walk never does cond_resched().  On archs
> without split pmd locks, there can be significant and observable
> contention on mm->page_table_lock which cause lengthy delays without
> rescheduling.
> 
> Always reschedule in smaps_pte_range() if necessary since the pagewalk
> iteration can be expensive.
> 
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  fs/proc/task_mmu.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -599,11 +599,11 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  	if (ptl) {
>  		smaps_pmd_entry(pmd, addr, walk);
>  		spin_unlock(ptl);
> -		return 0;
> +		goto out;
>  	}
>  
>  	if (pmd_trans_unstable(pmd))
> -		return 0;
> +		goto out;
>  	/*
>  	 * The mmap_sem held all the way back in m_start() is what
>  	 * keeps khugepaged out of here and from collapsing things
> @@ -613,6 +613,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  	for (; addr != end; pte++, addr += PAGE_SIZE)
>  		smaps_pte_entry(pte, addr, walk);
>  	pte_unmap_unlock(pte - 1, ptl);
> +out:
>  	cond_resched();
>  	return 0;
>  }

Maybe just call cond_resched() at the beginning of the function and don't
bother with gotos?

-- 
 Kirill A. Shutemov

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