On 18/12/2019 09:40, Chris Wilson wrote:
When doing our global park, we like to be a good citizen and shrink our
slab caches (of which we have quite a few now), but each
kmem_cache_shrink() incurs a stop_machine() and so ends up being quite
expensive, causing machine-wide stalls. While ideally we would like to
throw away unused pages in our slab caches whenever it appears that we
are idling, doing so will require a much cheaper mechanism. In the
meantime use a delayed worked to impose a rate-limit that means we have
to have been idle for more than 2 seconds before we start shrinking.
I was thinking about this yesterday, while looking at rapid park-unpark
cycles!
References: https://gitlab.freedesktop.org/drm/intel/issues/848
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_globals.c | 53 ++++++++++++++++++++++++-----
1 file changed, 44 insertions(+), 9 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_globals.c b/drivers/gpu/drm/i915/i915_globals.c
index be127cd28931..3aa213684293 100644
--- a/drivers/gpu/drm/i915/i915_globals.c
+++ b/drivers/gpu/drm/i915/i915_globals.c
@@ -20,7 +20,10 @@ static LIST_HEAD(globals);
static atomic_t active;
static atomic_t epoch;
static struct park_work {
- struct rcu_work work;
+ struct delayed_work work;
+ struct rcu_head rcu;
+ unsigned long flags;
+#define PENDING 0
int epoch;
} park;
@@ -37,11 +40,33 @@ static void i915_globals_shrink(void)
global->shrink();
}
+static void __i915_globals_grace(struct rcu_head *rcu)
+{
+ /* Ratelimit parking as shrinking is quite slow */
+ schedule_delayed_work(&park.work, round_jiffies_up_relative(2 * HZ));
+}
+
+static void __i915_globals_queue_rcu(void)
+{
+ park.epoch = atomic_inc_return(&epoch);
+ if (!atomic_read(&active)) {
+ init_rcu_head(&park.rcu);
Do we need to do init/destroy more than once? I think once on driver
load/exit would be more correct since the head is statically allocated.
Otherwise looks correct to me.
Regards,
Tvrtko
+ call_rcu(&park.rcu, __i915_globals_grace);
+ }
+}
+
static void __i915_globals_park(struct work_struct *work)
{
+ destroy_rcu_head(&park.rcu);
+
/* Confirm nothing woke up in the last grace period */
- if (park.epoch == atomic_read(&epoch))
- i915_globals_shrink();
+ if (park.epoch != atomic_read(&epoch)) {
+ __i915_globals_queue_rcu();
+ return;
+ }
+
+ clear_bit(PENDING, &park.flags);
+ i915_globals_shrink();
}
void __init i915_global_register(struct i915_global *global)
@@ -85,7 +110,7 @@ int __init i915_globals_init(void)
}
}
- INIT_RCU_WORK(&park.work, __i915_globals_park);
+ INIT_DELAYED_WORK(&park.work, __i915_globals_park);
return 0;
}
@@ -103,8 +128,9 @@ void i915_globals_park(void)
if (!atomic_dec_and_test(&active))
return;
- park.epoch = atomic_inc_return(&epoch);
- queue_rcu_work(system_wq, &park.work);
+ /* Queue cleanup after the next RCU grace period has freed slabs */
+ if (!test_and_set_bit(PENDING, &park.flags))
+ __i915_globals_queue_rcu();
}
void i915_globals_unpark(void)
@@ -113,12 +139,21 @@ void i915_globals_unpark(void)
atomic_inc(&active);
}
+static void __exit __i915_globals_flush(void)
+{
+ atomic_inc(&active); /* skip shrinking */
+
+ rcu_barrier(); /* wait for the work to be queued */
+ flush_delayed_work(&park.work);
+
+ atomic_dec(&active);
+}
+
void __exit i915_globals_exit(void)
{
- /* Flush any residual park_work */
- atomic_inc(&epoch);
- flush_rcu_work(&park.work);
+ GEM_BUG_ON(atomic_read(&active));
+ __i915_globals_flush();
__i915_globals_cleanup();
/* And ensure that our DESTROY_BY_RCU slabs are truly destroyed */
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