On 04/04/2017 13:05, Chris Wilson wrote:
If the engine is continually completing nops, we can saturate the
signaler and keep it working indefinitely. This angers the NMI watchdog!
A good example is to disable semaphores on snb and run igt/gem_exec_nop -
the parallel, multi-engine workloads are more than sufficient to hog the
CPU, preventing the system from even processing ICMP echo replies.
v2: Tvrtko dug into cond_resched() on x86 and found that it only
depended upon preempt_cound and not tif_need_resched() - which means
that we would always call schedule() at that point.
Fixes: c81d46138da6 ("drm/i915: Convert trace-irq to the breadcrumb waiter")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/intel_breadcrumbs.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 308c56a021ab..9ccbf26124c6 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -580,6 +580,8 @@ static int intel_breadcrumbs_signaler(void *arg)
signaler_set_rtpriority();
do {
+ bool do_schedule = true;
+
set_current_state(TASK_INTERRUPTIBLE);
/* We are either woken up by the interrupt bottom-half,
@@ -626,7 +628,18 @@ static int intel_breadcrumbs_signaler(void *arg)
spin_unlock_irq(&b->rb_lock);
i915_gem_request_put(request);
- } else {
+
+ /* If the engine is saturated we may be continually
+ * processing completed requests. This angers the
+ * NMI watchdog if we never let anything else
+ * have access to the CPU. Let's pretend to be nice
+ * and relinquish the CPU if we burn through the
+ * entire RT timeslice!
+ */
+ do_schedule = need_resched();
+ }
+
+ if (unlikely(do_schedule)) {
DEFINE_WAIT(exec);
if (kthread_should_park())
My thinking was to add a check before the request assignment like:
rcu_read_lock();
request = ...;
if (request && !need_resched())
request = ...;
else
request = NULL;
rcu_read_unlock();
But this looks correct as well, maybe it is just my preference on what
would have been easier to understand.
I trust that you have tested it both for solving the NMI lockup detector
issue and that it doesn't affect the signaller latency a lot.
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Regards,
Tvrtko
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