linux-kernel - Re: [PATCH] thp: reduce khugepaged freezing latency

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Date:	Wed, 09 Nov 2011 01:31:07 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	"Rafael J. Wysocki" <rjw@...e.com>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Jiri Slaby <jirislaby@...il.com>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] thp: reduce khugepaged freezing latency

On 11/08/2011 08:59 PM, Andrea Arcangeli wrote:
> Lack of set_freezable_with_signal() prevented khugepaged to be waken
> up (and prevented to sleep again) across the
> schedule_timeout_interruptible() calls after freezing() becomes
> true. The tight loop in khugepaged_alloc_hugepage() also missed one
> try_to_freeze() call in case alloc_hugepage() would repeatedly fail in
> turn preventing the loop to break and to reach the try_to_freeze() in
> the khugepaged main loop.
> 
> khugepaged would still freeze just fine by trying again the next
> minute but it's better if it freezes immediately.
> 
> Reported-by: Jiri Slaby <jslaby@...e.cz>
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> ---
>  mm/huge_memory.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 4298aba..67311d1 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2277,6 +2277,7 @@ static struct page *khugepaged_alloc_hugepage(void)
>  		if (!hpage) {
>  			count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
>  			khugepaged_alloc_sleep();
> +			try_to_freeze();
>  		} else
>  			count_vm_event(THP_COLLAPSE_ALLOC);
>  	} while (unlikely(!hpage) &&
> @@ -2331,7 +2332,7 @@ static int khugepaged(void *none)
>  {
>  	struct mm_slot *mm_slot;
> 
> -	set_freezable();
> +	set_freezable_with_signal();
>  	set_user_nice(current, 19);
> 
>  	/* serialize with start_khugepaged() */
> 

Why do we need to use both set_freezable_with_signal() and an additional
try_to_freeze()? Won't just using either one of them be good enough?
Or am I missing something here?

Thanks,
Srivatsa S. Bhat

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