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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 24/03/16 10:56, Chris Wilson wrote:
> On Wed, Mar 23, 2016 at 02:57:36PM +0000, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin@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
> performance.

Will do.

>> Also, gem_latency -n 100 shows 25% better throughput and CPU
>> usage, and 14% better latencies.
>
> Mention the benefits of parallelising dispatch.

Hm, actually this should be the same as before I think.

> 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.)

We can add all the numbers you get to the commit message as well.

>> 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).

Oh right, I've missed those.

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

Yeah it is nicer.

>> 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 */

Will do.

> 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
> implementation,
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

Okay I will respin with the above and we'll see.

Unfortunately my test platform just died so there will be a delay.

Regards,

Tvrtko

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