On 12/10/21 08:01, Sebastian Andrzej Siewior wrote:
On 2021-12-09 21:52:28 [-0500], Waiman Long wrote:
…
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
…
@@ -2210,7 +2211,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
struct memcg_stock_pcp *stock;
unsigned long flags;
- local_irq_save(flags);
+ local_lock_irqsave(&memcg_stock.lock, flags);
Why is this one using the lock? It isn't accessing irq_obj, right?
Well, the lock isn't just for irq_obj. It protects the whole memcg_stock
structure which include irq_obj. Sometimes, data in irq_obj (or
task_obj) will get transfer to nr_pages and vice versa. So it is easier
to use one single lock for the whole thing.
stock = this_cpu_ptr(&memcg_stock);
if (stock->cached != memcg) { /* reset if necessary */
@@ -2779,29 +2780,28 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
* which is cheap in non-preempt kernel. The interrupt context object stock
* can only be accessed after disabling interrupt. User context code can
* access interrupt object stock, but not vice versa.
+ *
+ * This task and interrupt context optimization is disabled for PREEMPT_RT
+ * as there is no performance gain in this case.
*/
static inline struct obj_stock *get_obj_stock(unsigned long *pflags)
{
- struct memcg_stock_pcp *stock;
-
- if (likely(in_task())) {
+ if (likely(in_task()) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
*pflags = 0UL;
preempt_disable();
- stock = this_cpu_ptr(&memcg_stock);
- return &stock->task_obj;
+ return this_cpu_ptr(&memcg_stock.task_obj);
}
We usually add the local_lock_t to the object it protects, struct
obj_stock it this case.
That would give you two different locks (instead of one) so you wouldn't
have to use preempt_disable() to avoid lockdep's complains. Also it
would warn you if you happen to use that obj_stock in !in_task() which
is isn't possible now.
The only downside would be that drain_local_stock() needs to acquire two
locks.
As said above, having separate locks will complicate the interaction
between irq_obj and the broader memcg_stock fields. Besides throughput
is a less important matrix for PREEMPT_RT, so I am not trying to
optimize throughput performance for PREEMPT_RT here.
Cheers,
Longman
