Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

[KOSAKI Motohiro]

Hi

> On NUMA machines, the administrator can configure zone_reclaim_mode that is a
> more targetted form of direct reclaim. On machines with large NUMA distances,
> zone_reclaim_mode defaults to 1 meaning that clean unmapped pages will be
> reclaimed if the zone watermarks are not being met. The problem is that
> zone_reclaim() can be in a situation where it scans excessively without
> making progress.
> 
> One such situation is where a large tmpfs mount is occupying a large
> percentage of memory overall. The pages do not get cleaned or reclaimed by
> zone_reclaim(), but the lists are uselessly scanned frequencly making the
> CPU spin at 100%. The scanning occurs because zone_reclaim() cannot tell
> in advance the scan is pointless because the counters do not distinguish
> between pagecache pages backed by disk and by RAM.  The observation in
> the field is that malloc() stalls for a long time (minutes in some cases)
> when this situation occurs.
> 
> Accounting for ram-backed file pages was considered but not implemented on
> the grounds it would be introducing new branches and expensive checks into
> the page cache add/remove patches and increase the number of statistics
> needed in the zone. As zone_reclaim() failing is currently considered a
> corner case, this seemed like overkill. Note, if there are a large number
> of reports about CPU spinning at 100% on NUMA that is fixed by disabling
> zone_reclaim, then this assumption is false and zone_reclaim() scanning
> and failing is not a corner case but a common occurance
> 
> This patch reintroduces zone_reclaim_interval which was removed by commit
> 34aa1330f9b3c5783d269851d467326525207422 [zoned vm counters: zone_reclaim:
> remove /proc/sys/vm/zone_reclaim_interval] because the zone counters were
> considered sufficient to determine in advance if the scan would succeed.
> As unsuccessful scans can still occur, zone_reclaim_interval is still
> required.
> 
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx
> ---
>  Documentation/sysctl/vm.txt |   13 +++++++++++++
>  include/linux/mmzone.h      |    9 +++++++++
>  include/linux/swap.h        |    1 +
>  kernel/sysctl.c             |    9 +++++++++
>  mm/vmscan.c                 |   22 ++++++++++++++++++++++
>  5 files changed, 54 insertions(+), 0 deletions(-)
> 
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index c302ddf..f9b8db5 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -52,6 +52,7 @@ Currently, these files are in /proc/sys/vm:
>  - swappiness
>  - vfs_cache_pressure
>  - zone_reclaim_mode
> +- zone_reclaim_interval
>  
>  
>  ==============================================================
> @@ -620,4 +621,16 @@ Allowing regular swap effectively restricts allocations to the local
>  node unless explicitly overridden by memory policies or cpuset
>  configurations.
>  
> +================================================================
> +
> +zone_reclaim_interval:
> +
> +The time allowed for off node allocations after zone reclaim
> +has failed to reclaim enough pages to allow a local allocation.
> +
> +Time is set in seconds and set by default to 30 seconds.
> +
> +Reduce the interval if undesired off node allocations occur. However, too
> +frequent scans will have a negative impact on off-node allocation performance.
> +
>  ============ End of Document =================================
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index a47c879..f1f0fb2 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -337,6 +337,15 @@ struct zone {
>  	atomic_long_t		vm_stat[NR_VM_ZONE_STAT_ITEMS];
>  
>  	/*
> +	 * timestamp (in jiffies) of the last zone_reclaim that scanned
> +	 * but failed to free enough pages. This is used to avoid repeated
> +	 * scans when zone_reclaim() is unable to detect in advance that
> +	 * the scanning is useless. This can happen for example if a zone
> +	 * has large numbers of clean unmapped file pages on tmpfs
> +	 */
> +	unsigned long		zone_reclaim_failure;
> +
> +	/*
>  	 * prev_priority holds the scanning priority for this zone.  It is
>  	 * defined as the scanning priority at which we achieved our reclaim
>  	 * target at the previous try_to_free_pages() or balance_pgdat()
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index d476aad..6a71368 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -224,6 +224,7 @@ extern long vm_total_pages;
>  
>  #ifdef CONFIG_NUMA
>  extern int zone_reclaim_mode;
> +extern int zone_reclaim_interval;
>  extern int sysctl_min_unmapped_ratio;
>  extern int sysctl_min_slab_ratio;
>  extern int zone_reclaim(struct zone *, gfp_t, unsigned int);
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index b2970d5..cc0623c 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1192,6 +1192,15 @@ static struct ctl_table vm_table[] = {
>  		.extra1		= &zero,
>  	},
>  	{
> +		.ctl_name       = CTL_UNNUMBERED,
> +		.procname       = "zone_reclaim_interval",
> +		.data           = &zone_reclaim_interval,
> +		.maxlen         = sizeof(zone_reclaim_interval),
> +		.mode           = 0644,
> +		.proc_handler   = &proc_dointvec_jiffies,
> +		.strategy       = &sysctl_jiffies,
> +	},

hmmm, I think nobody can know proper interval settings on his own systems.
I agree with Wu. It can be hidden.


> +	{
>  		.ctl_name	= VM_MIN_UNMAPPED,
>  		.procname	= "min_unmapped_ratio",
>  		.data		= &sysctl_min_unmapped_ratio,
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d254306..ba211c1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2272,6 +2272,13 @@ int zone_reclaim_mode __read_mostly;
>  #define RECLAIM_SWAP (1<<2)	/* Swap pages out during reclaim */
>  
>  /*
> + * Minimum time between zone_reclaim() scans that failed. Ordinarily, a
> + * scan will not fail because it will be determined in advance if it can
> + * succeeed but this does not always work. See mmzone.h
> + */
> +int zone_reclaim_interval __read_mostly = 30*HZ;
> +
> +/*
>   * Priority for ZONE_RECLAIM. This determines the fraction of pages
>   * of a node considered for each zone_reclaim. 4 scans 1/16th of
>   * a zone.
> @@ -2390,6 +2397,11 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  			<= zone->min_slab_pages)
>  		return 0;
>  
> +	/* Do not attempt a scan if scanning failed recently */
> +	if (time_before(jiffies,
> +			zone->zone_reclaim_failure + zone_reclaim_interval))
> +		return 0;
> +
>  	if (zone_is_all_unreclaimable(zone))
>  		return 0;
>  
> @@ -2414,6 +2426,16 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	ret = __zone_reclaim(zone, gfp_mask, order);
>  	zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);
>  
> +	if (!ret) {
> +		/*
> +		 * We were unable to reclaim enough pages to stay on node and
> +		 * unable to detect in advance that the scan would fail. Allow
> +		 * off node accesses for zone_reclaim_inteval jiffies before
> +		 * trying zone_reclaim() again
> +		 */
> +		zone->zone_reclaim_failure = jiffies;

Oops, this simple assignment don't care jiffies round-trip.


> +	}
> +
>  	return ret;
>  }
>  #endif
> -- 
> 1.5.6.5
> 



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