[RFC] drm/i915: Move execlists irq handler to a bottom half

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From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Doing a lot of work in the interrupt handler introduces huge
latencies to the system as a whole.

Most dramatic effect can be seen by running an all engine
stress test like igt/gem_exec_nop/all where, when the kernel
config is lean enough, the whole system can be brought into
multi-second periods of complete non-interactivty. That can
look for example like this:

 NMI watchdog: BUG: soft lockup - CPU#0 stuck for 23s! [kworker/u8:3:143]
 Modules linked in: [redacted for brevity]
 CPU: 0 PID: 143 Comm: kworker/u8:3 Tainted: G     U       L  4.5.0-160321+ #183
 Hardware name: Intel Corporation Broadwell Client platform/WhiteTip Mountain 1
 Workqueue: i915 gen6_pm_rps_work [i915]
 task: ffff8800aae88000 ti: ffff8800aae90000 task.ti: ffff8800aae90000
 RIP: 0010:[<ffffffff8104a3c2>]  [<ffffffff8104a3c2>] __do_softirq+0x72/0x1d0
 RSP: 0000:ffff88014f403f38  EFLAGS: 00000206
 RAX: ffff8800aae94000 RBX: 0000000000000000 RCX: 00000000000006e0
 RDX: 0000000000000020 RSI: 0000000004208060 RDI: 0000000000215d80
 RBP: ffff88014f403f80 R08: 0000000b1b42c180 R09: 0000000000000022
 R10: 0000000000000004 R11: 00000000ffffffff R12: 000000000000a030
 R13: 0000000000000082 R14: ffff8800aa4d0080 R15: 0000000000000082
 FS:  0000000000000000(0000) GS:ffff88014f400000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007fa53b90c000 CR3: 0000000001a0a000 CR4: 00000000001406f0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Stack:
  042080601b33869f ffff8800aae94000 00000000fffc2678 ffff88010000000a
  0000000000000000 000000000000a030 0000000000005302 ffff8800aa4d0080
  0000000000000206 ffff88014f403f90 ffffffff8104a716 ffff88014f403fa8
 Call Trace:
  <IRQ>
  [<ffffffff8104a716>] irq_exit+0x86/0x90
  [<ffffffff81031e7d>] smp_apic_timer_interrupt+0x3d/0x50
  [<ffffffff814f3eac>] apic_timer_interrupt+0x7c/0x90
  <EOI>
  [<ffffffffa01c5b40>] ? gen8_write64+0x1a0/0x1a0 [i915]
  [<ffffffff814f2b39>] ? _raw_spin_unlock_irqrestore+0x9/0x20
  [<ffffffffa01c5c44>] gen8_write32+0x104/0x1a0 [i915]
  [<ffffffff8132c6a2>] ? n_tty_receive_buf_common+0x372/0xae0
  [<ffffffffa017cc9e>] gen6_set_rps_thresholds+0x1be/0x330 [i915]
  [<ffffffffa017eaf0>] gen6_set_rps+0x70/0x200 [i915]
  [<ffffffffa0185375>] intel_set_rps+0x25/0x30 [i915]
  [<ffffffffa01768fd>] gen6_pm_rps_work+0x10d/0x2e0 [i915]
  [<ffffffff81063852>] ? finish_task_switch+0x72/0x1c0
  [<ffffffff8105ab29>] process_one_work+0x139/0x350
  [<ffffffff8105b186>] worker_thread+0x126/0x490
  [<ffffffff8105b060>] ? rescuer_thread+0x320/0x320
  [<ffffffff8105fa64>] kthread+0xc4/0xe0
  [<ffffffff8105f9a0>] ? kthread_create_on_node+0x170/0x170
  [<ffffffff814f351f>] ret_from_fork+0x3f/0x70
  [<ffffffff8105f9a0>] ? kthread_create_on_node+0x170/0x170

I could not explain, or find a code path, which would explain
a +20 second lockup, but from some instrumentation it was
apparent the interrupts off proportion of time was between
10-25% under heavy load which is quite bad.

By moving the GT interrupt handling to a tasklet in a most
simple way, the problem above disappears completely.

Also, gem_latency -n 100 shows 25% better throughput and CPU
usage, and 14% better latencies.

I did not find any gains or regressions with Synmark2 or
GLbench under light testing. More benchmarking is certainly
required.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_irq.c         |  2 +-
 drivers/gpu/drm/i915/intel_lrc.c        | 19 +++++++++++++------
 drivers/gpu/drm/i915/intel_lrc.h        |  1 -
 drivers/gpu/drm/i915/intel_ringbuffer.h |  1 +
 4 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 8f3e3309c3ab..e68134347007 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -1324,7 +1324,7 @@ gen8_cs_irq_handler(struct intel_engine_cs *engine, u32 iir, int test_shift)
 	if (iir & (GT_RENDER_USER_INTERRUPT << test_shift))
 		notify_ring(engine);
 	if (iir & (GT_CONTEXT_SWITCH_INTERRUPT << test_shift))
-		intel_lrc_irq_handler(engine);
+		tasklet_schedule(&engine->irq_tasklet);
 }
 
 static irqreturn_t gen8_gt_irq_handler(struct drm_i915_private *dev_priv,
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 6916991bdceb..283426c02f8b 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -538,21 +538,23 @@ get_context_status(struct intel_engine_cs *engine, unsigned int read_pointer,
 
 /**
  * intel_lrc_irq_handler() - handle Context Switch interrupts
- * @ring: Engine Command Streamer to handle.
+ * @engine: Engine Command Streamer to handle.
  *
  * Check the unread Context Status Buffers and manage the submission of new
  * contexts to the ELSP accordingly.
  */
-void intel_lrc_irq_handler(struct intel_engine_cs *engine)
+void intel_lrc_irq_handler(unsigned long data)
 {
+	struct intel_engine_cs *engine = (struct intel_engine_cs *)data;
 	struct drm_i915_private *dev_priv = engine->dev->dev_private;
 	u32 status_pointer;
 	unsigned int read_pointer, write_pointer;
 	u32 csb[GEN8_CSB_ENTRIES][2];
 	unsigned int csb_read = 0, i;
 	unsigned int submit_contexts = 0;
+	unsigned long flags;
 
-	spin_lock(&dev_priv->uncore.lock);
+	spin_lock_irqsave(&dev_priv->uncore.lock, flags);
 	intel_uncore_forcewake_get__locked(dev_priv, FORCEWAKE_ALL);
 
 	status_pointer = I915_READ_FW(RING_CONTEXT_STATUS_PTR(engine));
@@ -579,9 +581,9 @@ void intel_lrc_irq_handler(struct intel_engine_cs *engine)
 				    engine->next_context_status_buffer << 8));
 
 	intel_uncore_forcewake_put__locked(dev_priv, FORCEWAKE_ALL);
-	spin_unlock(&dev_priv->uncore.lock);
+	spin_unlock_irqrestore(&dev_priv->uncore.lock, flags);
 
-	spin_lock(&engine->execlist_lock);
+	spin_lock_irqsave(&engine->execlist_lock, flags);
 
 	for (i = 0; i < csb_read; i++) {
 		if (unlikely(csb[i][0] & GEN8_CTX_STATUS_PREEMPTED)) {
@@ -604,7 +606,7 @@ void intel_lrc_irq_handler(struct intel_engine_cs *engine)
 			execlists_context_unqueue(engine);
 	}
 
-	spin_unlock(&engine->execlist_lock);
+	spin_unlock_irqrestore(&engine->execlist_lock, flags);
 
 	if (unlikely(submit_contexts > 2))
 		DRM_ERROR("More than two context complete events?\n");
@@ -2020,6 +2022,8 @@ void intel_logical_ring_cleanup(struct intel_engine_cs *engine)
 	if (!intel_engine_initialized(engine))
 		return;
 
+	tasklet_kill(&engine->irq_tasklet);
+
 	dev_priv = engine->dev->dev_private;
 
 	if (engine->buffer) {
@@ -2093,6 +2097,9 @@ logical_ring_init(struct drm_device *dev, struct intel_engine_cs *engine)
 	INIT_LIST_HEAD(&engine->execlist_retired_req_list);
 	spin_lock_init(&engine->execlist_lock);
 
+	tasklet_init(&engine->irq_tasklet, intel_lrc_irq_handler,
+		     (unsigned long)engine);
+
 	logical_ring_init_platform_invariants(engine);
 
 	ret = i915_cmd_parser_init_ring(engine);
diff --git a/drivers/gpu/drm/i915/intel_lrc.h b/drivers/gpu/drm/i915/intel_lrc.h
index a17cb12221ba..0b0853eee91e 100644
--- a/drivers/gpu/drm/i915/intel_lrc.h
+++ b/drivers/gpu/drm/i915/intel_lrc.h
@@ -118,7 +118,6 @@ int intel_execlists_submission(struct i915_execbuffer_params *params,
 			       struct drm_i915_gem_execbuffer2 *args,
 			       struct list_head *vmas);
 
-void intel_lrc_irq_handler(struct intel_engine_cs *engine);
 void intel_execlists_retire_requests(struct intel_engine_cs *engine);
 
 #endif /* _INTEL_LRC_H_ */
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 221a94627aab..29810cba8a8c 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -266,6 +266,7 @@ struct  intel_engine_cs {
 	} semaphore;
 
 	/* Execlists */
+	struct tasklet_struct irq_tasklet;
 	spinlock_t execlist_lock;
 	struct list_head execlist_queue;
 	struct list_head execlist_retired_req_list;
-- 
1.9.1

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