[PATCH] memcg: Optimize exit to user space

[Thomas Gleixner]

memcg uses TIF_NOTIFY_RESUME to handle reclaiming on exit to user
space. TIF_NOTIFY_RESUME is a multiplexing TIF bit, which is utilized by
other entities as well.

This results in a unconditional mem_cgroup_handle_over_high() call for
every invocation of resume_user_mode_work(), which is a pointless
exercise as most of the time there is no reclaim work to do.

Especially since RSEQ is used by glibc, TIF_NOTIFY_RESUME is raised
quite frequently and the empty calls show up in exit path profiling.

Optimize this by doing a quick check of the reclaim condition before
invoking it.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 include/linux/memcontrol.h |    8 +++++++-
 mm/memcontrol.c            |    4 ++--
 2 files changed, 9 insertions(+), 3 deletions(-)

--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -900,7 +900,13 @@ unsigned long mem_cgroup_get_zone_lru_si
 	return READ_ONCE(mz->lru_zone_size[zone_idx][lru]);
 }
 
-void mem_cgroup_handle_over_high(gfp_t gfp_mask);
+void __mem_cgroup_handle_over_high(gfp_t gfp_mask);
+
+static inline void mem_cgroup_handle_over_high(gfp_t gfp_mask)
+{
+	if (unlikely(current->memcg_nr_pages_over_high))
+		__mem_cgroup_handle_over_high(gfp_mask);
+}
 
 unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg);
 
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2203,7 +2203,7 @@ static unsigned long calculate_high_dela
  * try_charge() (context permitting), as well as from the userland
  * return path where reclaim is always able to block.
  */
-void mem_cgroup_handle_over_high(gfp_t gfp_mask)
+void __mem_cgroup_handle_over_high(gfp_t gfp_mask)
 {
 	unsigned long penalty_jiffies;
 	unsigned long pflags;
@@ -2486,7 +2486,7 @@ static int try_charge_memcg(struct mem_c
 	if (current->memcg_nr_pages_over_high > MEMCG_CHARGE_BATCH &&
 	    !(current->flags & PF_MEMALLOC) &&
 	    gfpflags_allow_blocking(gfp_mask))
-		mem_cgroup_handle_over_high(gfp_mask);
+		__mem_cgroup_handle_over_high(gfp_mask);
 	return 0;
 }