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Date: Tue, 14 Jun 2016 13:47:09 +0300
From: Ville =?iso-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>
To: Ben Hutchings <ben@decadent.org.uk>
Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        akpm@linux-foundation.org, Chris Wilson <chris@chris-wilson.co.uk>,
        Jani Nikula <jani.nikula@intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
        Antti =?iso-8859-1?Q?Koskip=E4=E4?= 
	<antti.koskipaa@linux.intel.com>
Subject: Re: [PATCH 3.16 012/114] drm/i915: Exit cherryview_irq_handler()
 after one pass
Message-ID: <20160614104709.GT4329@intel.com>
References: <lsq.1465842997.838358341@decadent.org.uk>
 <lsq.1465842997.558331598@decadent.org.uk>
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On Mon, Jun 13, 2016 at 07:36:37PM +0100, Ben Hutchings wrote:
> 3.16.36-rc1 review patch.  If anyone has any objections, please let me know.

Do not backport this one. It'll break things.

> 
> ------------------
> 
> From: Chris Wilson <chris@chris-wilson.co.uk>
> 
> commit 9dbaab56ac09f07a73fe83bf69bec3e31060080a upstream.
> 
> 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.
> 
> v2: use do {} while(0) for a smaller patch, and easier to revert again
> 
> I have reasonable confidence that we do not miss GT interrupts (as
> execlists provides a stress case with a failure mechanism easily
> detected by igt), however I have less confidence about all the other
> sources of interrupts and worry that may lose a display hotplug
> interrupt, for example.
> 
> 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>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Link: http://patchwork.freedesktop.org/patch/msgid/1457946117-6714-1-git-send-email-chris@chris-wilson.co.uk
> (cherry picked from commit 579de73b048a0a4c66c25a033ac76a2836e0cf73)
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> ---
>  drivers/gpu/drm/i915/i915_irq.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -1875,7 +1875,7 @@ static irqreturn_t cherryview_irq_handle
>  	u32 master_ctl, iir;
>  	irqreturn_t ret = IRQ_NONE;
>  
> -	for (;;) {
> +	do {
>  		master_ctl = I915_READ(GEN8_MASTER_IRQ) & ~GEN8_MASTER_IRQ_CONTROL;
>  		iir = I915_READ(VLV_IIR);
>  
> @@ -1897,7 +1897,7 @@ static irqreturn_t cherryview_irq_handle
>  		POSTING_READ(GEN8_MASTER_IRQ);
>  
>  		ret = IRQ_HANDLED;
> -	}
> +	} while (0);
>  
>  	return ret;
>  }

-- 
Ville Syrjälä
Intel OTC