linux-kernel - Re: [PATCH 2/2] mm: warn about allocations which stall for too long

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Date:   Thu, 29 Sep 2016 18:02:44 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org, akpm@...ux-foundation.org
Cc:     hannes@...xchg.org, mgorman@...e.de, dave.hansen@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, mhocko@...e.com
Subject: Re: [PATCH 2/2] mm: warn about allocations which stall for too long

Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> Currently we do warn only about allocation failures but small
> allocations are basically nofail and they might loop in the page
> allocator for a long time.  Especially when the reclaim cannot make
> any progress - e.g. GFP_NOFS cannot invoke the oom killer and rely on
> a different context to make a forward progress in case there is a lot
> memory used by filesystems.
> 
> Give us at least a clue when something like this happens and warn about
> allocations which take more than 10s. Print the basic allocation context
> information along with the cumulative time spent in the allocation as
> well as the allocation stack. Repeat the warning after every 10 seconds so
> that we know that the problem is permanent rather than ephemeral.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>  mm/page_alloc.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 969ffc97045b..73f60ad6315f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3495,6 +3495,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	enum compact_result compact_result;
>  	int compaction_retries = 0;
>  	int no_progress_loops = 0;
> +	unsigned long alloc_start = jiffies;
> +	unsigned int stall_timeout = 10 * HZ;
>  
>  	/*
>  	 * In the slowpath, we sanity check order to avoid ever trying to
> @@ -3650,6 +3652,14 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & __GFP_REPEAT))
>  		goto nopage;
>  
> +	/* Make sure we know about allocations which stall for too long */
> +	if (time_after(jiffies, alloc_start + stall_timeout)) {
> +		warn_alloc(gfp_mask,

I expect "gfp_mask & ~__GFP_NOWARN" rather than "gfp_mask" here.
Otherwise, we can't get a clue for __GFP_NOWARN allocations.

> +			"page alloction stalls for %ums, order:%u\n",
> +			jiffies_to_msecs(jiffies-alloc_start), order);
> +		stall_timeout += 10 * HZ;
> +	}
> +
>  	if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
>  				 did_some_progress > 0, &no_progress_loops))
>  		goto retry;
> -- 
> 2.9.3