[igt-dev] [PATCH i-g-t] i915/gem_eio: Flush RCU before timing our own critical sections

[Chris Wilson <chris@chris-wilson.co.uk>]

We cannot control how long RCU takes to find a quiescent point as that
depends upon the background load and so may take an arbitrary time.
Instead, let's try to avoid that impacting our measurements by inserting
an rcu_barrier() before our critical timing sections and hope that hides
the issue, letting us always perform a fast reset. Fwiw, we do the
expedited RCU synchronize, but that is not always enough.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 tests/i915/gem_eio.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tests/i915/gem_eio.c b/tests/i915/gem_eio.c
index 8d6cb9760..49d2a99e9 100644
--- a/tests/i915/gem_eio.c
+++ b/tests/i915/gem_eio.c
@@ -71,6 +71,7 @@ static void trigger_reset(int fd)
 {
 	struct timespec ts = { };
 
+	rcu_barrier(fd); /* flush any excess work before we start timing */
 	igt_nsec_elapsed(&ts);
 
 	igt_kmsg(KMSG_DEBUG "Forcing GPU reset\n");
@@ -227,6 +228,10 @@ static void hang_handler(union sigval arg)
 	igt_debug("hang delay = %.2fus\n",
 		  igt_nsec_elapsed(&ctx->delay) / 1000.0);
 
+	/* flush any excess work before we start timing our reset */
+	igt_assert(igt_sysfs_printf(ctx->debugfs, "i915_drop_caches",
+				    "%d", DROP_RCU));
+
 	igt_nsec_elapsed(ctx->ts);
 	igt_assert(igt_sysfs_set(ctx->debugfs, "i915_wedged", "-1"));
 
-- 
2.24.0

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