Re: [PATCH] drm/i915: Exit cherryview_irq_handler() after one pass

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Mar 10, 2016 at 12:10:46PM +0000, Chris Wilson wrote:
> On Thu, Mar 10, 2016 at 02:01:27PM +0200, Ville Syrjälä wrote:
> > On Thu, Mar 10, 2016 at 11:44:28AM +0000, Chris Wilson wrote:
> > > This effectively reverts
> > > 
> > > commit 8e5fd599eb219f1054e39b40d18b217af669eea9
> > > Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > Date:   Wed Apr 9 13:28:50 2014 +0300
> > > 
> > >     drm/i915/chv: Make CHV irq handler loop until all interrupts are consumed
> > > 
> > > as under continuous execlists load we can saturate the IRQ handler,
> > > destablising the tsc clock and triggering the NMI watchdog to declare a hung
> > > CPU.
> > > 
> > > [  552.756051] clocksource: timekeeping watchdog on CPU0: Marking clocksource 'tsc' as unstable because the skew is too large:
> > > [  552.756080] clocksource:                       'refined-jiffies' wd_now: 10003b480 wd_last: 10003b28c mask: ffffffff
> > > [  552.756091] clocksource:                       'tsc' cs_now: d55d31aa50 cs_last: d17446166c mask: ffffffffffffffff
> > > [  552.756210] clocksource: Switched to clocksource refined-jiffies
> > > [  575.217870] NMI watchdog: Watchdog detected hard LOCKUP on cpu 1
> > > [  575.217893] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.5.0-rc7+ #18
> > > [  575.217905] Hardware name:                  /NUC5CPYB, BIOS PYBSWCEL.86A.0027.2015.0507.1758 05/07/2015
> > > [  575.217915]  0000000000000000 ffff88027fd05bc0 ffffffff81288c6d 0000000000000000
> > > [  575.217935]  0000000000000001 ffff88027fd05be0 ffffffff810e72d1 0000000000000000
> > > [  575.217951]  ffff88027fd05c80 ffff88027fd05c20 ffffffff81114b60 0000000181015f1e
> > > [  575.217967] Call Trace:
> > > [  575.217973]  <NMI>  [<ffffffff81288c6d>] dump_stack+0x4f/0x72
> > > [  575.217994]  [<ffffffff810e72d1>] watchdog_overflow_callback+0x151/0x160
> > > [  575.218003]  [<ffffffff81114b60>] __perf_event_overflow+0xa0/0x1e0
> > > [  575.218016]  [<ffffffff811154c4>] perf_event_overflow+0x14/0x20
> > > [  575.218028]  [<ffffffff8101d2ca>] intel_pmu_handle_irq+0x1da/0x460
> > > [  575.218042]  [<ffffffff814a8aae>] ? poll_idle+0x3e/0x70
> > > [  575.218052]  [<ffffffff814a8aae>] ? poll_idle+0x3e/0x70
> > > [  575.218064]  [<ffffffff81014ae8>] perf_event_nmi_handler+0x28/0x50
> > > [  575.218075]  [<ffffffff81007540>] nmi_handle+0x60/0x130
> > > [  575.218086]  [<ffffffff814a8aae>] ? poll_idle+0x3e/0x70
> > > [  575.218096]  [<ffffffff810079c0>] do_nmi+0x140/0x470
> > > [  575.218108]  [<ffffffff81559ec7>] end_repeat_nmi+0x1a/0x1e
> > > [  575.218119]  [<ffffffff814a8aae>] ? poll_idle+0x3e/0x70
> > > [  575.218129]  [<ffffffff814a8aae>] ? poll_idle+0x3e/0x70
> > > [  575.218139]  [<ffffffff814a8aae>] ? poll_idle+0x3e/0x70
> > > [  575.218148]  <<EOE>>  [<ffffffff814a8353>] cpuidle_enter_state+0xf3/0x2f0
> > > [  575.218164]  [<ffffffff814a8587>] cpuidle_enter+0x17/0x20
> > > [  575.218175]  [<ffffffff810aaa3a>] call_cpuidle+0x2a/0x40
> > > [  575.218185]  [<ffffffff810aade3>] cpu_startup_entry+0x273/0x330
> > > [  575.218196]  [<ffffffff81033a1e>] start_secondary+0x10e/0x130
> > > 
> > > However, not servicing all available IIR within the handler does hurt the
> > > throughput of pathological nop execbuf by about 20%, with a similar effect
> > > upon the dispatch latency of a series of execbuf.
> > > 
> > > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93467
> > > Testcase: igt/gem_exec_nop/basic # requires NMI watchdog
> > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > Cc: Antti Koskipää <antti.koskipaa@linux.intel.com
> > > Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > > Cc: stable@vger.kernel.org
> > > ---
> > >  drivers/gpu/drm/i915/i915_irq.c | 40 +++++++++++++++++++---------------------
> > >  1 file changed, 19 insertions(+), 21 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> > > index 53e5104964b3..8a3230427884 100644
> > > --- a/drivers/gpu/drm/i915/i915_irq.c
> > > +++ b/drivers/gpu/drm/i915/i915_irq.c
> > > @@ -1829,35 +1829,33 @@ static irqreturn_t cherryview_irq_handler(int irq, void *arg)
> > >  	/* IRQs are synced during runtime_suspend, we don't require a wakeref */
> > >  	disable_rpm_wakeref_asserts(dev_priv);
> > >  
> > > -	for (;;) {
> > > -		master_ctl = I915_READ(GEN8_MASTER_IRQ) & ~GEN8_MASTER_IRQ_CONTROL;
> > > -		iir = I915_READ(VLV_IIR);
> > > +	master_ctl = I915_READ(GEN8_MASTER_IRQ) & ~GEN8_MASTER_IRQ_CONTROL;
> > > +	iir = I915_READ(VLV_IIR);
> > >  
> > > -		if (master_ctl == 0 && iir == 0)
> > > -			break;
> > > +	if (master_ctl == 0 && iir == 0)
> > > +		break;
> > 
> > goto something?
> 
> Sigh. The problem of rewriting the "obvious" patch against -nightly. I
> just changed the for(;;) into do {} while(0) for testing. Perhaps I
> should stick with that in case we need to flip flop agin.
> 
> > Apart from that I have no objections if it doesn't cause problems
> > with interrupts getting lost and whatnot. That was the original reason
> > for it I think, but at least I myself never really looked into it. IIRC
> > Rafael just told me they needed to do it to get the thing working, so
> > I just put the patch in. And that was before I had even seen any silicon.
> 
> My testing only looks at the GT side, and we do stress that pretty hard
> because of execlists and have reasonable methods of detection if we stop
> processing execbuf. I'm more worried about the display and pipe interrupts.

IIRC GT was where the problem was originally.

And just as a side note, I do have a branch somewhere that rewrites all
the gmch irq handlers to not loop. Just never actually found the time to
really run it on anything :) So I like moving towards that direction in
any case.

-- 
Ville Syrjälä
Intel OTC
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