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

Wed Mar 23 09:14:27 UTC 2016

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 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.
> > 
> > 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.

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

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

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
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....
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre