[PATCH 2/2] drm/i915: Limit the busy wait on requests to 2us not 10ms!

[Chris Wilson]

When waiting for high frequency requests, the finite amount of time
required to set up the irq and wait upon it limits the response rate. By
busywaiting on the request completion for a short while we can service
the high frequency waits as quick as possible. However, if it is a slow
request, we want to sleep as quickly as possible. The tradeoff between
waiting and sleeping is roughly the time it takes to sleep on a request,
on the order of a microsecond. Based on measurements from big core, I
have set the limit for busywaiting as 2 microseconds.

The code currently uses the jiffie clock, but that is far too coarse (on
the order of 10 milliseconds) and results in poor interactivity as the
CPU ends up being hogged by slow requests. To get microsecond resolution
we need to use a high resolution timer. The cheapest of which is polling
local_clock(), but that is only valid on the same CPU. If we switch CPUs
because the task was preempted, we can also use that as an indicator that
 the system is too busy to waste cycles on spinning and we should sleep
instead.

__i915_spin_request was introduced in
commit 2def4ad99befa25775dd2f714fdd4d92faec6e34 [v4.2]
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date:   Tue Apr 7 16:20:41 2015 +0100

     drm/i915: Optimistically spin for the request completion

Reported-by: Jens Axboe <axboe@xxxxxxxxx>
Link: https://lkml.org/lkml/2015/11/12/621
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc; "Rogozhkin, Dmitry V" <dmitry.v.rogozhkin@xxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
Cc: Eero Tamminen <eero.t.tamminen@xxxxxxxxx>
Cc: "Rantala, Valtteri" <valtteri.rantala@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 drivers/gpu/drm/i915/i915_gem.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 740530c571d1..2a88158bd1f7 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1146,14 +1146,36 @@ static bool missed_irq(struct drm_i915_private *dev_priv,
 	return test_bit(ring->id, &dev_priv->gpu_error.missed_irq_rings);
 }
 
+static u64 local_clock_us(unsigned *cpu)
+{
+	u64 t;
+
+	*cpu = get_cpu();
+	t = local_clock() >> 10;
+	put_cpu();
+
+	return t;
+}
+
+static bool busywait_stop(u64 timeout, unsigned cpu)
+{
+	unsigned this_cpu;
+
+	if (time_after64(local_clock_us(&this_cpu), timeout))
+		return true;
+
+	return this_cpu != cpu;
+}
+
 static int __i915_spin_request(struct drm_i915_gem_request *req, int state)
 {
-	unsigned long timeout;
+	u64 timeout;
+	unsigned cpu;
 
 	if (i915_gem_request_get_ring(req)->irq_refcount)
 		return -EBUSY;
 
-	timeout = jiffies + 1;
+	timeout = local_clock_us(&cpu) + 2;
 	while (!need_resched()) {
 		if (i915_gem_request_completed(req, true))
 			return 0;
@@ -1161,7 +1183,7 @@ static int __i915_spin_request(struct drm_i915_gem_request *req, int state)
 		if (signal_pending_state(state, current))
 			break;
 
-		if (time_after_eq(jiffies, timeout))
+		if (busywait_stop(timeout, cpu))
 			break;
 
 		cpu_relax_lowlatency();
-- 
2.6.2

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