mm: Stop kswapd early when nothing's waiting for it to free pages

Keeping kswapd running when all the failed allocations that invoked it
are satisfied incurs a high overhead due to unnecessary page eviction
and writeback, as well as spurious VM pressure events to various
registered shrinkers. When kswapd doesn't need to work to make an
allocation succeed anymore, stop it prematurely to save resources.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>

include/linux/mmzone.h

@@ -664,6 +664,7 @@ typedef struct pglist_data {
 	unsigned long node_spanned_pages; /* total size of physical page
 					     range, including holes */
 	int node_id;
+	atomic_t kswapd_waiters;
 	wait_queue_head_t kswapd_wait;
 	wait_queue_head_t pfmemalloc_wait;
 	struct task_struct *kswapd;	/* Protected by

mm/page_alloc.c

@@ -4089,6 +4089,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	u64 utime, stime_s, stime_e, stime_d;
 
 	task_cputime(current, &utime, &stime_s);
+	pg_data_t *pgdat = ac->preferred_zoneref->zone->zone_pgdat;
 	bool woke_kswapd = false;
 
 	/*
@@ -6336,6 +6337,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat)
 	pgdat_page_ext_init(pgdat);
 	spin_lock_init(&pgdat->lru_lock);
 	lruvec_init(node_lruvec(pgdat));
+	pgdat->kswapd_waiters = (atomic_t)ATOMIC_INIT(0);
 
 	pgdat->per_cpu_nodestats = &boot_nodestats;