Re: [PATCH]vmscan: handle underflow for get_scan_ratio

[Wu Fengguang]

On Fri, Apr 02, 2010 at 02:50:52PM +0800, Li, Shaohua wrote:
> On Wed, Mar 31, 2010 at 01:53:27PM +0800, KOSAKI Motohiro wrote:
> > > > On Tue, Mar 30, 2010 at 02:08:53PM +0800, KOSAKI Motohiro wrote:
> > > > > Hi
> > > > > 
> > > > > > Commit 84b18490d1f1bc7ed5095c929f78bc002eb70f26 introduces a regression.
> > > > > > With it, our tmpfs test always oom. The test has a lot of rotated anon
> > > > > > pages and cause percent[0] zero. Actually the percent[0] is a very small
> > > > > > value, but our calculation round it to zero. The commit makes vmscan
> > > > > > completely skip anon pages and cause oops.
> > > > > > An option is if percent[x] is zero in get_scan_ratio(), forces it
> > > > > > to 1. See below patch.
> > > > > > But the offending commit still changes behavior. Without the commit, we scan
> > > > > > all pages if priority is zero, below patch doesn't fix this. Don't know if
> > > > > > It's required to fix this too.
> > > > > 
> > > > > Can you please post your /proc/meminfo and reproduce program? I'll digg it.
> > > > > 
> > > > > Very unfortunately, this patch isn't acceptable. In past time, vmscan 
> > > > > had similar logic, but 1% swap-out made lots bug reports. 
> > > > if 1% is still big, how about below patch?
> > > 
> > > This patch makes a lot of sense than previous. however I think <1% anon ratio
> > > shouldn't happen anyway because file lru doesn't have reclaimable pages.
> > > <1% seems no good reclaim rate.
> > 
> > Oops, the above mention is wrong. sorry. only 1 page is still too big.
> > because under streaming io workload, the number of scanning anon pages should
> > be zero. this is very strong requirement. if not, backup operation will makes
> > a lot of swapping out.
> Sounds there is no big impact for the workload which you mentioned with the patch.
> please see below descriptions.
> I updated the description of the patch as fengguang suggested.
> 
> 
> 
> Commit 84b18490d introduces a regression. With it, our tmpfs test always oom.
> The test uses a 6G tmpfs in a system with 3G memory. In the tmpfs, there are
> 6 copies of kernel source and the test does kbuild for each copy. My
> investigation shows the test has a lot of rotated anon pages and quite few
> file pages, so get_scan_ratio calculates percent[0] to be zero. Actually
> the percent[0] shoule be a very small value, but our calculation round it
> to zero. The commit makes vmscan completely skip anon pages and cause oops.
> 
> To avoid underflow, we don't use percentage, instead we directly calculate
> how many pages should be scaned. In this way, we should get several scan pages
> for < 1% percent. With this fix, my test doesn't oom any more.
> 
> Note, this patch doesn't really change logics, but just increase precise. For
> system with a lot of memory, this might slightly changes behavior. For example,
> in a sequential file read workload, without the patch, we don't swap any anon
> pages. With it, if anon memory size is bigger than 16G, we will say one anon page

                                                                  see?

> swapped. The 16G is calculated as PAGE_SIZE * priority(4096) * (fp/ap). fp/ap
> is assumed to be 1024 which is common in this workload. So the impact sounds not
> a big deal.
> 
> Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx>
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 79c8098..80a7ed5 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1519,27 +1519,50 @@ static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
>  }
>  
>  /*
> + * Smallish @nr_to_scan's are deposited in @nr_saved_scan,
> + * until we collected @swap_cluster_max pages to scan.
> + */
> +static unsigned long nr_scan_try_batch(unsigned long nr_to_scan,
> +				       unsigned long *nr_saved_scan)
> +{
> +	unsigned long nr;
> +
> +	*nr_saved_scan += nr_to_scan;
> +	nr = *nr_saved_scan;
> +
> +	if (nr >= SWAP_CLUSTER_MAX)
> +		*nr_saved_scan = 0;
> +	else
> +		nr = 0;
> +
> +	return nr;
> +}
> +
> +/*
>   * Determine how aggressively the anon and file LRU lists should be
>   * scanned.  The relative value of each set of LRU lists is determined
>   * by looking at the fraction of the pages scanned we did rotate back
>   * onto the active list instead of evict.
>   *
> - * percent[0] specifies how much pressure to put on ram/swap backed
> - * memory, while percent[1] determines pressure on the file LRUs.
> + * nr[x] specifies how many pages should be scaned

The new comment loses information..

>   */
> -static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
> -					unsigned long *percent)
> +static void get_scan_count(struct zone *zone, struct scan_control *sc,
> +				unsigned long *nr, int priority)
>  {
>  	unsigned long anon, file, free;
>  	unsigned long anon_prio, file_prio;
>  	unsigned long ap, fp;
>  	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
> +	unsigned long fraction[2], denominator[2];

denominator[2] can be reduced to denominator.
because denominator[0] == denominator[1] always holds.

> +	enum lru_list l;
>  
>  	/* If we have no swap space, do not bother scanning anon pages. */
>  	if (!sc->may_swap || (nr_swap_pages <= 0)) {
> -		percent[0] = 0;
> -		percent[1] = 100;
> -		return;
> +		fraction[0] = 0;
> +		denominator[0] = 1;
> +		fraction[1] = 1;
> +		denominator[1] = 1;
> +		goto out;
>  	}
>  
>  	anon  = zone_nr_lru_pages(zone, sc, LRU_ACTIVE_ANON) +
> @@ -1552,9 +1575,11 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
>  		/* If we have very few page cache pages,
>  		   force-scan anon pages. */
>  		if (unlikely(file + free <= high_wmark_pages(zone))) {
> -			percent[0] = 100;
> -			percent[1] = 0;
> -			return;
> +			fraction[0] = 1;
> +			denominator[0] = 1;
> +			fraction[1] = 0;
> +			denominator[1] = 1;
> +			goto out;
>  		}
>  	}
>  
> @@ -1601,29 +1626,29 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
>  	fp = (file_prio + 1) * (reclaim_stat->recent_scanned[1] + 1);
>  	fp /= reclaim_stat->recent_rotated[1] + 1;
>  
> -	/* Normalize to percentages */
> -	percent[0] = 100 * ap / (ap + fp + 1);
> -	percent[1] = 100 - percent[0];
> -}
> -
> -/*
> - * Smallish @nr_to_scan's are deposited in @nr_saved_scan,
> - * until we collected @swap_cluster_max pages to scan.
> - */
> -static unsigned long nr_scan_try_batch(unsigned long nr_to_scan,
> -				       unsigned long *nr_saved_scan)
> -{
> -	unsigned long nr;
> +	fraction[0] = ap;
> +	denominator[0] = ap + fp + 1;
> +	fraction[1] = fp;
> +	denominator[1] = ap + fp + 1;
>  
> -	*nr_saved_scan += nr_to_scan;
> -	nr = *nr_saved_scan;
> +out:
> +	for_each_evictable_lru(l) {
> +		int file = is_file_lru(l);
> +		unsigned long scan;
>  
> -	if (nr >= SWAP_CLUSTER_MAX)
> -		*nr_saved_scan = 0;
> -	else
> -		nr = 0;
> +		if (fraction[file] == 0) {
> +			nr[l] = 0;
> +			continue;
> +		}
>  
> -	return nr;
> +		scan = zone_nr_lru_pages(zone, sc, l);
> +		if (priority) {
> +			scan >>= priority;
> +			scan = (scan * fraction[file] / denominator[file]);

The "()" is not necessary here, or better end it here^

Thanks,
Fengguang

> +		}
> +		nr[l] = nr_scan_try_batch(scan,
> +					  &reclaim_stat->nr_saved_scan[l]);
> +	}
>  }
>  
>  /*
> @@ -1634,31 +1659,11 @@ static void shrink_zone(int priority, struct zone *zone,
>  {
>  	unsigned long nr[NR_LRU_LISTS];
>  	unsigned long nr_to_scan;
> -	unsigned long percent[2];	/* anon @ 0; file @ 1 */
>  	enum lru_list l;
>  	unsigned long nr_reclaimed = sc->nr_reclaimed;
>  	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
> -	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
> -
> -	get_scan_ratio(zone, sc, percent);
>  
> -	for_each_evictable_lru(l) {
> -		int file = is_file_lru(l);
> -		unsigned long scan;
> -
> -		if (percent[file] == 0) {
> -			nr[l] = 0;
> -			continue;
> -		}
> -
> -		scan = zone_nr_lru_pages(zone, sc, l);
> -		if (priority) {
> -			scan >>= priority;
> -			scan = (scan * percent[file]) / 100;
> -		}
> -		nr[l] = nr_scan_try_batch(scan,
> -					  &reclaim_stat->nr_saved_scan[l]);
> -	}
> +	get_scan_count(zone, sc, nr, priority);
>  
>  	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
>  					nr[LRU_INACTIVE_FILE]) {
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