Re: [PATCH v4 2/5] drm/i915: Watchdog timeout: IRQ handler for gen8+

[Carlos Santa <carlos.santa@intel.com>]

On Mon, 2019-03-11 at 10:39 +0000, Tvrtko Ursulin wrote:
> On 08/03/2019 03:16, Carlos Santa wrote:
> > On Fri, 2019-03-01 at 09:36 +0000, Chris Wilson wrote:
> > > > 
> > > 
> > > Quoting Carlos Santa (2019-02-21 02:58:16)
> > > > +#define GEN8_WATCHDOG_1000US(dev_priv)
> > > > watchdog_to_clock_counts(dev_priv, 1000)
> > > > +static void gen8_watchdog_irq_handler(unsigned long data)
> > > > +{
> > > > +       struct intel_engine_cs *engine = (struct
> > > > intel_engine_cs
> > > > *)data;
> > > > +       struct drm_i915_private *dev_priv = engine->i915;
> > > > +       unsigned int hung = 0;
> > > > +       u32 current_seqno=0;
> > > > +       char msg[80];
> > > > +       unsigned int tmp;
> > > > +       int len;
> > > > +
> > > > +       /* Stop the counter to prevent further timeout
> > > > interrupts
> > > > */
> > > > +       I915_WRITE_FW(RING_CNTR(engine->mmio_base),
> > > > get_watchdog_disable(engine));
> > > > +
> > > > +       /* Read the heartbeat seqno once again to check if we
> > > > are
> > > > stuck? */
> > > > +       current_seqno =
> > > > intel_engine_get_hangcheck_seqno(engine);
> > > 
> > > I have said this before, but this doesn't exist either, it's just
> > > a
> > > temporary glitch in the matrix.
> > > 
> > 
> > Chris, Tvrtko, I need some guidance on how to find the quilty seqno
> > during a hang, can you please advice here what to do?
> 
> When an interrupt fires you need to ascertain whether the same
> request 
> which enabled the watchdog is running, correct?
> 
> So I think you would need this, with a disclaimer that I haven't
> thought 
> about the details really:
> 
> 1. Take a reference to timeline hwsp when setting up the watchdog for
> a 
> request.
> 
> 2. Store the initial seqno associated with this request.
> 
> 3. Force enable user interrupts.
> 
> 4. When timeout fires, inspect the HWSP seqno to see if the request 
> completed or not.
> 
> 5. Reset the engine if not completed.
> 
> 6. Put the timeline/hwsp reference.


static int gen8_emit_bb_start(struct i915_request *rq,
							u64 offset, u32
len,
							const unsigned
int flags)
{
	struct i915_timeline *tl;
	u32 seqno;

	if (enable_watchdog) {
		/* Start watchdog timer */
		cs = gen8_emit_start_watchdog(rq, cs);
		tl = ce->ring->timeline;
		i915_timeline_get_seqno(tl, rq, &seqno);
		/*Store initial hwsp seqno associated with this request 
		engine->watchdog_hwsp_seqno = tl->hwsp_seqno;
	}

}

static void gen8_watchdog_tasklet(unsigned long data)
{
		struct i915_request *rq;

		rq = intel_engine_find_active_request(engine);

		/* Inspect the watchdog seqno once again for
completion? */
		if (!i915_seqno_passed(engine->watchdog_hwsp_seqno, rq-
>fence.seqno)) {
			//Reset Engine
		}
}

Tvrtko, is the above acceptable to inspect whether the seqno has
completed?

I noticed there's a helper function i915_request_completed(struct
i915_request *rq) but it will require me to modify it in order to pass
2 different seqnos.

Regards,
Carlos

> 
> If the user interrupt fires with the request completed cancel the
> above 
> operations.
> 
> There could be an inherent race between inspecting the seqno and 
> deciding to reset. Not sure at the moment what to do. Maybe just call
> it 
> bad luck?
> 
> I also think for the software implementation you need to force no 
> request coalescing for contexts with timeout set. Because you want
> to 
> have 100% defined borders for request in and out - since the timeout
> is 
> defined per request.
> 
> In this case you don't need the user interrupt for the trailing edge 
> signal but can use context complete. Maybe putting hooks into 
> context_in/out in intel_lrc.c would work under these circumstances.
> 
> Also if preempted you need to cancel the timer setup and store
> elapsed 
> execution time.
> 
> Or it may make sense to just disable preemption for these contexts. 
> Otherwise there is no point in trying to mandate the timeout?
> 
> But it is also kind of bad since non-privileged contexts can make 
> themselves non-preemptable by setting the watchdog timeout.
> 
> Maybe as a compromise we need to automatically apply an elevated 
> priority level, but not as high to be completely non-preemptable.
> Sounds 
> like a hard question.
> 
> Regards,
> 
> Tvrtko

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