[PATCH v3] drm/i915: Use exponential backoff for wait_for()

Chris Wilson
2017-11-21T21:00:49Z

Instead of sleeping for a fixed 1ms (roughly, depending on timer slack),
start with a small sleep and exponentially increase the sleep on each
cycle.

A good example of a beneficiary is the guc mmio communication channel.
Typically we expect (and so spin) for 10us for a quick response, but this
doesn't cover everything and so sometimes we fallback to the millisecond+
sleep. This incurs a significant delay in time-critical operations like
preemption (igt/gem_exec_latency), which can be improved significantly by
using a small sleep after the spin fails.

We've made this suggestion many times, but had little experimental data
to support adding the complexity.

v2: Bump the minimum usleep to 10us on advice of
Documentation/timers/timers-howto.txt (Tvrko)
v3: Specify min, max range for usleep intervals -- some code may
crucially depend upon and so want to specify the sleep pattern.

References: 1758b90e38f5 ("drm/i915: Use a hybrid scheme for fast register waits")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: John Harrison <John.C.Harrison@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
---
drivers/gpu/drm/i915/intel_drv.h | 11 +++++++----
drivers/gpu/drm/i915/intel_pm.c | 2 +-
2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 635a96fcd788..c00441a3d649 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -48,8 +48,9 @@
* having timed out, since the timeout could be due to preemption or similar and
* we've never had a chance to check the condition before the timeout.
*/
-#define _wait_for(COND, US, W) ({ \
+#define _wait_for(COND, US, Wmin, Wmax) ({ \
unsigned long timeout__ = jiffies + usecs_to_jiffies(US) + 1; \
+ long wait__ = (Wmin); /* recommended min for usleep is 10 us */ \
int ret__; \
might_sleep(); \
for (;;) { \
@@ -62,12 +63,14 @@
ret__ = -ETIMEDOUT; \
break; \
} \
- usleep_range((W), (W) * 2); \
+ usleep_range(wait__, wait__ * 2); \
+ if (wait__ < (Wmax)) \
+ wait__ <<= 1; \
} \
ret__; \
})

-#define wait_for(COND, MS) _wait_for((COND), (MS) * 1000, 1000)
+#define wait_for(COND, MS) _wait_for((COND), (MS) * 1000, 10, 1000)

/* If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false. */
#if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT)
@@ -116,7 +119,7 @@
int ret__; \
BUILD_BUG_ON(!__builtin_constant_p(US)); \
if ((US) > 10) \
- ret__ = _wait_for((COND), (US), 10); \
+ ret__ = _wait_for((COND), (US), 10, 10); \
else \
ret__ = _wait_for_atomic((COND), (US), 0); \
ret__; \
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index e445ec174831..f07f14ae198d 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -9294,7 +9294,7 @@ int skl_pcode_request(struct drm_i915_private *dev_priv, u32 mbox, u32 request,
ret = 0;
goto out;
}
- ret = _wait_for(COND, timeout_base_ms * 1000, 10);
+ ret = _wait_for(COND, timeout_base_ms * 1000, 10, 10);
if (!ret)
goto out;

--
2.15.0