Re: [PATCH 08/27] mm, vmscan: Simplify the logic deciding whether kswapd sleeps

[Vlastimil Babka <vbabka@suse.cz>]

On 06/09/2016 08:04 PM, Mel Gorman wrote:
> kswapd goes through some complex steps trying to figure out if it
> should stay awake based on the classzone_idx and the requested order.
> It is unnecessarily complex and passes in an invalid classzone_idx to
> balance_pgdat().  What matters most of all is whether a larger order has
> been requsted and whether kswapd successfully reclaimed at the previous
> order. This patch irons out the logic to check just that and the end result
> is less headache inducing.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  include/linux/mmzone.h |  5 ++--
>  mm/memory_hotplug.c    |  5 ++--
>  mm/page_alloc.c        |  2 +-
>  mm/vmscan.c            | 79 +++++++++++++++++++++++++-------------------------
>  4 files changed, 46 insertions(+), 45 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index aff621fcc6af..15ce454d0d59 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -663,8 +663,9 @@ typedef struct pglist_data {
>  	wait_queue_head_t pfmemalloc_wait;
>  	struct task_struct *kswapd;	/* Protected by
>  					   mem_hotplug_begin/end() */
> -	int kswapd_max_order;
> -	enum zone_type classzone_idx;
> +	int kswapd_order;
> +	enum zone_type kswapd_classzone_idx;
> +
>  #ifdef CONFIG_COMPACTION
>  	int kcompactd_max_order;
>  	enum zone_type kcompactd_classzone_idx;
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index c5278360ca66..42d758bffd0a 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1209,9 +1209,10 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
>
>  		arch_refresh_nodedata(nid, pgdat);
>  	} else {
> -		/* Reset the nr_zones and classzone_idx to 0 before reuse */
> +		/* Reset the nr_zones, order and classzone_idx before reuse */
>  		pgdat->nr_zones = 0;
> -		pgdat->classzone_idx = 0;
> +		pgdat->kswapd_order = 0;
> +		pgdat->kswapd_classzone_idx = -1;
>  	}
>
>  	/* we can use NODE_DATA(nid) from here */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4ce578b969da..d8cb483d5cad 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6036,7 +6036,7 @@ void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
>  	unsigned long end_pfn = 0;
>
>  	/* pg_data_t should be reset to zero when it's allocated */
> -	WARN_ON(pgdat->nr_zones || pgdat->classzone_idx);
> +	WARN_ON(pgdat->nr_zones || pgdat->kswapd_classzone_idx);

Above you changed the reset value of kswapd_classzone_idx from 0 to -1, 
so won't this trigger? Also should we check kswapd_order that's newly 
reset too?

>
>  	reset_deferred_meminit(pgdat);
>  	pgdat->node_id = nid;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 96bf841f9352..14b34eebedff 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2727,7 +2727,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
>
>  	/* kswapd must be awake if processes are being throttled */
>  	if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
> -		pgdat->classzone_idx = min(pgdat->classzone_idx,
> +		pgdat->kswapd_classzone_idx = min(pgdat->kswapd_classzone_idx,
>  						(enum zone_type)ZONE_NORMAL);
>  		wake_up_interruptible(&pgdat->kswapd_wait);
>  	}
> @@ -3211,6 +3211,12 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order,
>
>  	prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
>
> +	/* If kswapd has not been woken recently, then full sleep */
> +	if (classzone_idx == -1) {
> +		classzone_idx = balanced_classzone_idx = MAX_NR_ZONES - 1;
> +		goto full_sleep;

This will skip the wakeup_kcompactd() part.

> +	}
> +
>  	/* Try to sleep for a short interval */
>  	if (prepare_kswapd_sleep(pgdat, order, remaining,
>  						balanced_classzone_idx)) {
> @@ -3233,6 +3239,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order,
>  		prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
>  	}
>
> +full_sleep:
>  	/*
>  	 * After a short sleep, check if it was a premature sleep. If not, then
>  	 * go fully to sleep until explicitly woken up.
> @@ -3279,9 +3286,7 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order,
>   */
>  static int kswapd(void *p)
>  {
> -	unsigned long order, new_order;
> -	int classzone_idx, new_classzone_idx;
> -	int balanced_classzone_idx;
> +	unsigned int order, classzone_idx;
>  	pg_data_t *pgdat = (pg_data_t*)p;
>  	struct task_struct *tsk = current;
>
> @@ -3311,38 +3316,25 @@ static int kswapd(void *p)
>  	tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
>  	set_freezable();
>
> -	order = new_order = 0;
> -	classzone_idx = new_classzone_idx = pgdat->nr_zones - 1;
> -	balanced_classzone_idx = classzone_idx;
> +	pgdat->kswapd_order = order = 0;
> +	pgdat->kswapd_classzone_idx = classzone_idx = -1;
>  	for ( ; ; ) {
>  		bool ret;
>
> +kswapd_try_sleep:
> +		kswapd_try_to_sleep(pgdat, order, classzone_idx, classzone_idx);

The last two parameters are now the same, remove one?

> +
>  		/*
> -		 * While we were reclaiming, there might have been another
> -		 * wakeup, so check the values.
> +		 * Read the new order and classzone_idx which may be -1 if
> +		 * kswapd_try_to_sleep() woke up after a short timeout instead
> +		 * of being woken by the page allocator.
>  		 */
> -		new_order = pgdat->kswapd_max_order;
> -		new_classzone_idx = pgdat->classzone_idx;
> -		pgdat->kswapd_max_order =  0;
> -		pgdat->classzone_idx = pgdat->nr_zones - 1;
> -
> -		if (order < new_order || classzone_idx > new_classzone_idx) {
> -			/*
> -			 * Don't sleep if someone wants a larger 'order'
> -			 * allocation or has tigher zone constraints
> -			 */
> -			order = new_order;
> -			classzone_idx = new_classzone_idx;
> -		} else {
> -			kswapd_try_to_sleep(pgdat, order, classzone_idx,
> -						balanced_classzone_idx);
> -			order = pgdat->kswapd_max_order;
> -			classzone_idx = pgdat->classzone_idx;
> -			new_order = order;
> -			new_classzone_idx = classzone_idx;
> -			pgdat->kswapd_max_order = 0;
> -			pgdat->classzone_idx = pgdat->nr_zones - 1;
> -		}
> +		order = pgdat->kswapd_order;
> +		classzone_idx = pgdat->kswapd_classzone_idx;
> +		if (classzone_idx == -1)
> +			classzone_idx = MAX_NR_ZONES - 1;
> +		pgdat->kswapd_order = 0;
> +		pgdat->kswapd_classzone_idx = -1;
>
>  		ret = try_to_freeze();
>  		if (kthread_should_stop())
> @@ -3352,12 +3344,19 @@ static int kswapd(void *p)
>  		 * We can speed up thawing tasks if we don't call balance_pgdat
>  		 * after returning from the refrigerator
>  		 */
> -		if (!ret) {
> -			trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
> +		if (ret)
> +			continue;
>
> -			/* return value ignored until next patch */
> -			balance_pgdat(pgdat, order, classzone_idx);
> -		}
> +		/*
> +		 * Try reclaim the requested order but if that fails
> +		 * then try sleeping on the basis of the order reclaimed.

Is the last word really meant to be "reclaimed", or "requested"?

> +		 */
> +		trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
> +		if (balance_pgdat(pgdat, order, classzone_idx) < order)
> +			goto kswapd_try_sleep;

AFAICS now kswapd_try_to_sleep() will use the "requested" order. That's 
needed for proper wakeup_kcompactd(), but won't it prevent kswapd from 
actually going to sleep, because zone_balanced() in prepare-sleep will 
be false? So I think you need to give it both orders to do the right thing?

> +
> +		order = pgdat->kswapd_order;
> +		classzone_idx = pgdat->kswapd_classzone_idx;
>  	}
>
>  	tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
> @@ -3380,10 +3379,10 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
>  	if (!cpuset_zone_allowed(zone, GFP_KERNEL | __GFP_HARDWALL))
>  		return;
>  	pgdat = zone->zone_pgdat;
> -	if (pgdat->kswapd_max_order < order) {
> -		pgdat->kswapd_max_order = order;
> -		pgdat->classzone_idx = min(pgdat->classzone_idx, classzone_idx);
> -	}
> +	if (pgdat->kswapd_classzone_idx == -1)
> +		pgdat->kswapd_classzone_idx = classzone_idx;
> +	pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx, classzone_idx);
> +	pgdat->kswapd_order = max(pgdat->kswapd_order, order);
>  	if (!waitqueue_active(&pgdat->kswapd_wait))
>  		return;
>  	if (zone_balanced(zone, order, 0))
>