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SubjectRe: [patch v2 3/3] mm: page_alloc: fair zone allocator policy
Hello Hannes,

On Fri, Aug 02, 2013 at 11:37:26AM -0400, Johannes Weiner wrote:
> Each zone that holds userspace pages of one workload must be aged at a
> speed proportional to the zone size. Otherwise, the time an
> individual page gets to stay in memory depends on the zone it happened
> to be allocated in. Asymmetry in the zone aging creates rather
> unpredictable aging behavior and results in the wrong pages being
> reclaimed, activated etc.
>
> But exactly this happens right now because of the way the page
> allocator and kswapd interact. The page allocator uses per-node lists
> of all zones in the system, ordered by preference, when allocating a
> new page. When the first iteration does not yield any results, kswapd
> is woken up and the allocator retries. Due to the way kswapd reclaims
> zones below the high watermark while a zone can be allocated from when
> it is above the low watermark, the allocator may keep kswapd running
> while kswapd reclaim ensures that the page allocator can keep
> allocating from the first zone in the zonelist for extended periods of
> time. Meanwhile the other zones rarely see new allocations and thus
> get aged much slower in comparison.
>
> The result is that the occasional page placed in lower zones gets
> relatively more time in memory, even gets promoted to the active list
> after its peers have long been evicted. Meanwhile, the bulk of the
> working set may be thrashing on the preferred zone even though there
> may be significant amounts of memory available in the lower zones.
>
> Even the most basic test -- repeatedly reading a file slightly bigger
> than memory -- shows how broken the zone aging is. In this scenario,
> no single page should be able stay in memory long enough to get
> referenced twice and activated, but activation happens in spades:
>
> $ grep active_file /proc/zoneinfo
> nr_inactive_file 0
> nr_active_file 0
> nr_inactive_file 0
> nr_active_file 8
> nr_inactive_file 1582
> nr_active_file 11994
> $ cat data data data data >/dev/null
> $ grep active_file /proc/zoneinfo
> nr_inactive_file 0
> nr_active_file 70
> nr_inactive_file 258753
> nr_active_file 443214
> nr_inactive_file 149793
> nr_active_file 12021
>
> Fix this with a very simple round robin allocator. Each zone is
> allowed a batch of allocations that is proportional to the zone's
> size, after which it is treated as full. The batch counters are reset
> when all zones have been tried and the allocator enters the slowpath
> and kicks off kswapd reclaim. Allocation and reclaim is now fairly
> spread out to all available/allowable zones:
>
> $ grep active_file /proc/zoneinfo
> nr_inactive_file 0
> nr_active_file 0
> nr_inactive_file 174
> nr_active_file 4865
> nr_inactive_file 53
> nr_active_file 860
> $ cat data data data data >/dev/null
> $ grep active_file /proc/zoneinfo
> nr_inactive_file 0
> nr_active_file 0
> nr_inactive_file 666622
> nr_active_file 4988
> nr_inactive_file 190969
> nr_active_file 937
>
> When zone_reclaim_mode is enabled, allocations will now spread out to
> all zones on the local node, not just the first preferred zone (which
> on a 4G node might be a tiny Normal zone).

I really want to give Reviewed-by but before that, I'd like to clear out
my concern which didn't handle enoughly in previous iteration.

Let's assume system has normal zone : 800M High zone : 800M
and there are two parallel workloads.

1. alloc_pages(GFP_KERNEL) : 800M
2. alloc_pages(GFP_MOVABLE) + mlocked : 800M

With old behavior, allocation from both workloads is fulfilled happily
because most of allocation from GFP_KERNEL would be done in normal zone
while most of allocation from GFP_MOVABLE would be done in high zone.
There is no OOM kill in this scenario.

With you change, normal zone would be fullfilled with GFP_KERNEL:400M
and GFP_MOVABLE:400M while high zone will have GFP_MOVABLE:400 + free 400M.
Then, someone would be OOM killed.

Of course, you can argue that if there is such workloads, he should make
sure it via lowmem_reseve but it's rather overkill if we consider more examples
because any movable pages couldn't be allocated from normal zone so memory
efficieny would be very bad.

As I said, I like your approach because I have no idea to handle unbalanced
aging problem better and we can get more benefits rather than lost by above
corner case but at least, I'd like to confirm what you think about
above problem before further steps. Maybe we can introduce "mlock with
newly-allocation or already-mapped page could be migrated to high memory zone"
when someone reported out? (we thougt mlocked page migration would be problem
RT latency POV but Peter confirmed it's no problem.)


>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Tested-by: Zlatko Calusic <zcalusic@bitsync.net>
> ---
> include/linux/mmzone.h | 1 +
> mm/page_alloc.c | 69 ++++++++++++++++++++++++++++++++++++++++++--------
> 2 files changed, 60 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index af4a3b7..dcad2ab 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -352,6 +352,7 @@ struct zone {
> * free areas of different sizes
> */
> spinlock_t lock;
> + int alloc_batch;
> int all_unreclaimable; /* All pages pinned */
> #if defined CONFIG_COMPACTION || defined CONFIG_CMA
> /* Set to true when the PG_migrate_skip bits should be cleared */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3b27d3e..b2cdfd0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1817,6 +1817,11 @@ static void zlc_clear_zones_full(struct zonelist *zonelist)
> bitmap_zero(zlc->fullzones, MAX_ZONES_PER_ZONELIST);
> }
>
> +static bool zone_local(struct zone *local_zone, struct zone *zone)
> +{
> + return node_distance(local_zone->node, zone->node) == LOCAL_DISTANCE;
> +}
> +
> static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
> {
> return node_isset(local_zone->node, zone->zone_pgdat->reclaim_nodes);
> @@ -1854,6 +1859,11 @@ static void zlc_clear_zones_full(struct zonelist *zonelist)
> {
> }
>
> +static bool zone_local(struct zone *local_zone, struct zone *zone)
> +{
> + return true;
> +}
> +
> static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
> {
> return true;
> @@ -1901,6 +1911,26 @@ zonelist_scan:
> if (alloc_flags & ALLOC_NO_WATERMARKS)
> goto try_this_zone;
> /*
> + * Distribute pages in proportion to the individual
> + * zone size to ensure fair page aging. The zone a
> + * page was allocated in should have no effect on the
> + * time the page has in memory before being reclaimed.
> + *
> + * When zone_reclaim_mode is enabled, try to stay in
> + * local zones in the fastpath. If that fails, the
> + * slowpath is entered, which will do another pass
> + * starting with the local zones, but ultimately fall
> + * back to remote zones that do not partake in the
> + * fairness round-robin cycle of this zonelist.
> + */
> + if (alloc_flags & ALLOC_WMARK_LOW) {
> + if (zone->alloc_batch <= 0)
> + continue;
> + if (zone_reclaim_mode &&
> + !zone_local(preferred_zone, zone))
> + continue;
> + }
> + /*
> * When allocating a page cache page for writing, we
> * want to get it from a zone that is within its dirty
> * limit, such that no single zone holds more than its
> @@ -2006,7 +2036,8 @@ this_zone_full:
> goto zonelist_scan;
> }
>
> - if (page)
> + if (page) {
> + zone->alloc_batch -= 1U << order;
> /*
> * page->pfmemalloc is set when ALLOC_NO_WATERMARKS was
> * necessary to allocate the page. The expectation is
> @@ -2015,6 +2046,7 @@ this_zone_full:
> * for !PFMEMALLOC purposes.
> */
> page->pfmemalloc = !!(alloc_flags & ALLOC_NO_WATERMARKS);
> + }
>
> return page;
> }
> @@ -2346,16 +2378,28 @@ __alloc_pages_high_priority(gfp_t gfp_mask, unsigned int order,
> return page;
> }
>
> -static inline
> -void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
> - enum zone_type high_zoneidx,
> - enum zone_type classzone_idx)
> +static void prepare_slowpath(gfp_t gfp_mask, unsigned int order,
> + struct zonelist *zonelist,
> + enum zone_type high_zoneidx,
> + struct zone *preferred_zone)
> {
> struct zoneref *z;
> struct zone *zone;
>
> - for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
> - wakeup_kswapd(zone, order, classzone_idx);
> + for_each_zone_zonelist(zone, z, zonelist, high_zoneidx) {
> + if (!(gfp_mask & __GFP_NO_KSWAPD))
> + wakeup_kswapd(zone, order, zone_idx(preferred_zone));
> + /*
> + * Only reset the batches of zones that were actually
> + * considered in the fast path, we don't want to
> + * thrash fairness information for zones that are not
> + * actually part of this zonelist's round-robin cycle.
> + */
> + if (zone_reclaim_mode && !zone_local(preferred_zone, zone))
> + continue;
> + zone->alloc_batch = high_wmark_pages(zone) -
> + low_wmark_pages(zone);
> + }
> }
>
> static inline int
> @@ -2451,9 +2495,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> goto nopage;
>
> restart:
> - if (!(gfp_mask & __GFP_NO_KSWAPD))
> - wake_all_kswapd(order, zonelist, high_zoneidx,
> - zone_idx(preferred_zone));
> + prepare_slowpath(gfp_mask, order, zonelist,
> + high_zoneidx, preferred_zone);
>
> /*
> * OK, we're below the kswapd watermark and have kicked background
> @@ -4754,6 +4797,9 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
> zone_seqlock_init(zone);
> zone->zone_pgdat = pgdat;
>
> + /* For bootup, initialized properly in watermark setup */
> + zone->alloc_batch = zone->managed_pages;
> +
> zone_pcp_init(zone);
> lruvec_init(&zone->lruvec);
> if (!size)
> @@ -5525,6 +5571,9 @@ static void __setup_per_zone_wmarks(void)
> zone->watermark[WMARK_LOW] = min_wmark_pages(zone) + (tmp >> 2);
> zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp >> 1);
>
> + zone->alloc_batch = high_wmark_pages(zone) -
> + low_wmark_pages(zone);
> +
> setup_zone_migrate_reserve(zone);
> spin_unlock_irqrestore(&zone->lock, flags);
> }
> --
> 1.8.3.2
>
> --
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--
Kind regards,
Minchan Kim


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