On 2021/7/30 14:50, Michal Hocko wrote:

On Thu 29-07-21 20:06:45, Roman Gushchin wrote:

quoted

On Thu, Jul 29, 2021 at 08:57:52PM +0800, Miaohe Lin wrote:

quoted

Since percpu_charge_mutex is only used inside drain_all_stock(), we can
narrow the scope of percpu_charge_mutex by moving it here.

Signed-off-by: Miaohe Lin <redacted>
---
 mm/memcontrol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6580c2381a3e..a03e24e57cd9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2050,7 +2050,6 @@ struct memcg_stock_pcp {
 #define FLUSHING_CACHED_CHARGE	0
 };
 static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
-static DEFINE_MUTEX(percpu_charge_mutex);
 
 #ifdef CONFIG_MEMCG_KMEM
 static void drain_obj_stock(struct obj_stock *stock);

@@ -2209,6 +2208,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
  */
 static void drain_all_stock(struct mem_cgroup *root_memcg)
 {
+	static DEFINE_MUTEX(percpu_charge_mutex);
 	int cpu, curcpu;

It's considered a good practice to protect data instead of code paths. After
the proposed change it becomes obvious that the opposite is done here: the mutex
is used to prevent a simultaneous execution of the code of the drain_all_stock()
function.

The purpose of the lock was indeed to orchestrate callers more than any
data structure consistency.
 

quoted

Actually we don't need a mutex here: nobody ever sleeps on it. So I'd replace
it with a simple atomic variable or even a single bitfield. Then the change will
be better justified, IMO.

Yes, mutex can be replaced by an atomic in a follow up patch.

Thanks for both of you. It's a really good suggestion. What do you mean is something like below？

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 616d1a72ece3..508a96e80980 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -2208,11 +2208,11 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
  */
 static void drain_all_stock(struct mem_cgroup *root_memcg)
 {
-       static DEFINE_MUTEX(percpu_charge_mutex);
        int cpu, curcpu;
+       static atomic_t drain_all_stocks = ATOMIC_INIT(-1);

        /* If someone's already draining, avoid adding running more workers. */
-       if (!mutex_trylock(&percpu_charge_mutex))
+       if (!atomic_inc_not_zero(&drain_all_stocks))
                return;
        /*
         * Notify other cpus that system-wide "drain" is running

@@ -2244,7 +2244,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
                }
        }
        put_cpu();
-       mutex_unlock(&percpu_charge_mutex);
+       atomic_dec(&drain_all_stocks);
 }

 static int memcg_hotplug_cpu_dead(unsigned int cpu)