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

[Chris Wilson]

On Thu, Mar 24, 2016 at 05:56:40PM +0200, Imre Deak wrote:
> On ke, 2016-03-23 at 14:57 +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.
> > 
> > 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.
> > 
> > Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> 
> You also have to synchronize against the tasklet now whenever we
> synchronize against the IRQ, see gen6_disable_rps_interrupts(),
> gen8_irq_power_well_pre_disable() and
> intel_runtime_pm_disable_interrupts(). Not saying you should use a
> threaded IRQ instead, but it does provide for this automatically.

But we don't synchronize against the irq for execlists since this
tasklet is guarded by the rpm wakeref (though mark_busy / mark_idle)
and we stop it before we finally release the irq. Or have I missed
something?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre