[PATCH for-7.5 2/5] memcg: use high_work for kernel threads

[Shakeel Butt]

Kernel threads never return to userspace, so deferred memory.high
reclaim does not run. Reclaim and throttling during a charge can also
delay work they do for other tasks.

Treat kernel threads like interrupts. Queue high_work for the first
cgroup over memory.high and skip reclaim and throttling in the charging
thread.

User workers created by create_io_thread() and vhost_task_create() keep
their current behavior. Vhost kernel threads use the new path, even when
they borrow the owner's mm.

The worker requests a fixed batch of pages each time. A fast allocator
can keep usage well above memory.high. Kernel threads also skip
memory.swap.high throttling and its high-event reporting. The memory.max
and memory.swap.max checks stay the same.

Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
---
 mm/memcontrol.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 20c5dfe1a894..8116fce6ab96 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2705,6 +2705,7 @@ static void memcg_enforce_high(struct mem_cgroup *memcg, unsigned int nr_pages,
 			       gfp_t gfp_mask)
 {
 	bool allow_spinning = gfpflags_allow_spinning(gfp_mask);
+	bool use_worker = !in_task() || (current->flags & PF_KTHREAD);
 
 	/*
 	 * If the hierarchy is above the normal consumption range, schedule
@@ -2723,8 +2724,12 @@ static void memcg_enforce_high(struct mem_cgroup *memcg, unsigned int nr_pages,
 		swap_high = page_counter_read(&memcg->swap) >
 			READ_ONCE(memcg->swap.high);
 
-		/* Don't bother a random interrupted task */
-		if (!in_task()) {
+		/*
+		 * Don't make an unrelated interrupted task handle this charge.
+		 * Kernel threads may be working for other tasks, so let a worker
+		 * reclaim instead.
+		 */
+		if (use_worker) {
 			if (mem_high) {
 				if (allow_spinning)
 					schedule_work(&memcg->high_work);
@@ -2751,6 +2756,10 @@ static void memcg_enforce_high(struct mem_cgroup *memcg, unsigned int nr_pages,
 		}
 	} while ((memcg = parent_mem_cgroup(memcg)));
 
+	/* Interrupts and kernel threads stop here. */
+	if (use_worker)
+		return;
+
 	/*
 	 * Reclaim is set up above to be called from the userland
 	 * return path. But also attempt synchronous reclaim to avoid
-- 
2.53.0-Meta