[Intel-gfx] [RFC v2] drm/i915: Move execlists irq handler to a bottom half

Chris Wilson chris at chris-wilson.co.uk
Thu Mar 24 10:56:14 UTC 2016

On Wed, Mar 23, 2016 at 02:57:36PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 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.

Perfect segue into gem_syslatency. I think gem_syslatency is the better
tool to correlate disruptive system behaviour. And then continue on with
gem_latency to demonstrate that is doesn't adversely affect our

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

Mention the benefits of parallelising dispatch.

As fairly hit-and-miss as perf testing is on these machines, it is
looking in favour of using tasklets vs the rt kthread. The numbers swing
between 2-10%, but consistently improves in the nop sync latencies.
There's still several hours to go in this run before we cover the
dispatch latenies, but so far reasonable.

(Hmm, looks like there may be a possible degredation on the single nop
dispatch but an improvement on the continuous nop dispatch.)

> I did not find any gains or regressions with Synmark2 or
> GLbench under light testing. More benchmarking is certainly
> required.
> v2:
>    * execlists_lock should be taken as spin_lock_bh when
>      queuing work from userspace now. (Chris Wilson)
>    * uncore.lock must be taken with spin_lock_irq when
>      submitting requests since that now runs from either
>      softirq or process context.

There are a couple of execlist_lock usage outside of intel_lrc that may
or may not be useful to convert (low frequency reset / debug paths, so
way off the critical paths, but consistency in locking is invaluable).

> +	tasklet_init(&engine->irq_tasklet, intel_lrc_irq_handler,
> +		     (unsigned long)engine);

I like trying to split lines to cluster arguments if possible. Here I
think intel_lrc_irq_handler pairs with engine,

		     intel_lrc_irq_handler, (unsigned long)engine);


> 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;

spinlock_t execlist_lock; /* used inside tasklet, use spin_lock_bh */

It's looking good, but once this run completes, I'm going to repeat it
just to confirm how stable my numbers are.

Critical bugfix, improvements, simpler patch than my kthread
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

Chris Wilson, Intel Open Source Technology Centre

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