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

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

On 23/03/16 09:14, Chris Wilson wrote:
> On Wed, Mar 23, 2016 at 10:07:35AM +0100, Daniel Vetter wrote:
>> On Tue, Mar 22, 2016 at 05:30:04PM +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.
>>>
>>> Also, gem_latency -n 100 shows 25% better throughput and CPU
>>> usage, and 14% better latencies.
>
> Forgot gem_syslatency, since that does reflect UX under load really
> startlingly well.

gem_syslatency, before:

gem_syslatency: cycles=1532739, latency mean=416531.829us max=2499237us
gem_syslatency: cycles=1839434, latency mean=1458099.157us max=4998944us
gem_syslatency: cycles=1432570, latency mean=2688.451us max=1201185us
gem_syslatency: cycles=1533543, latency mean=416520.499us max=2498886us

with tasklet:

gem_syslatency: cycles=808907, latency mean=53.133us max=1640us
gem_syslatency: cycles=862154, latency mean=62.778us max=2117us
gem_syslatency: cycles=856039, latency mean=58.079us max=2123us
gem_syslatency: cycles=841683, latency mean=56.914us max=1667us

Is this smaller throughput and better latency?

gem_syslatency -n, before:

gem_syslatency: cycles=0, latency mean=2.446us max=18us
gem_syslatency: cycles=0, latency mean=7.220us max=37us
gem_syslatency: cycles=0, latency mean=6.949us max=36us
gem_syslatency: cycles=0, latency mean=5.931us max=39us

with tasklet:

gem_syslatency: cycles=0, latency mean=2.477us max=5us
gem_syslatency: cycles=0, latency mean=2.471us max=6us
gem_syslatency: cycles=0, latency mean=2.696us max=24us
gem_syslatency: cycles=0, latency mean=6.414us max=39us

This looks potentially the same or very similar. May need more runs to 
get a definitive picture.

gem_exec_nop/all has a huge improvement also, if we ignore the fact it 
locks up the system with the curret irq handler on full tilt, when I 
limit the max GPU frequency a bit it can avoid that problem but tasklets 
make it twice as fast here.

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

You think it is OK to continue sharing your one, 
https://bugs.freedesktop.org/show_bug.cgi?id=93467?

>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>>
>> I thought tasklets are considered unpopular nowadays? They still steal cpu

Did not know, last (and first) time I've used them was ~15 years ago. :) 
You got any links to read about it? Since (see below) I am not sure they 
"steal" CPU time.

>> time, just have the benefit of not also disabling hard interrupts. There
>> should be mitigation though to offload these softinterrupts to threads.
>> Have you tried to create a threaded interrupt thread just for these pins
>> instead? A bit of boilerplate, but not much using the genirq stuff iirc.
>
> Ah, you haven't been reading patches. Yes, there's been a patch to fix
> the hardlockup using kthreads for a few months. Tvrtko is trying to move
> this forward since he too has found a way of locking up his machine
> using execlist under load.

Correct.

> So far kthreads seems to have a slight edge in the benchmarks, or rather
> using tasklet I have some very wild results on Braswell. Using tasklets
> the CPU time is accounted to the process (i.e. whoever was running at
 > the time of the irq, typically the benchmark), using kthread we have

I thought they run from ksoftirqd so the CPU time is accounted against 
it. And looking at top, that even seems what actually happens.

> independent entries in the process table/top (which is quite nice to see
> just how much time is been eaten up by the context-switches).
>
> Benchmarks still progessing, haven't yet got on to the latency
> measureemnts....

My tasklets hack required surprisingly little code change, at least if 
there are not some missed corner cases to handle, but I don't mind your 
threads either.

Regards,

Tvrtko
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