[Intel-gfx] [PATCH v3 12/13] drm/i915/execlists: Preemption!

[Chris Wilson]

Quoting Joonas Lahtinen (2017-09-29 08:29:57)
> On Thu, 2017-09-28 at 20:39 +0100, Chris Wilson wrote:
> > When we write to ELSP, it triggers a context preemption at the earliest
> > arbitration point (3DPRIMITIVE, some PIPECONTROLs, a few other
> > operations and the explicit MI_ARB_CHECK). If this is to the same
> > context, it triggers a LITE_RESTORE where the RING_TAIL is merely
> > updated (used currently to chain requests from the same context
> > together, avoiding bubbles). However, if it is to a different context, a
> > full context-switch is performed and it will start to execute the new
> > context saving the image of the old for later execution.
> > 
> > Previously we avoided preemption by only submitting a new context when
> > the old was idle. But now we wish embrace it, and if the new request has
> > a higher priority than the currently executing request, we write to the
> > ELSP regardless, thus triggering preemption, but we tell the GPU to
> > switch to our special preemption context (not the target). In the
> > context-switch interrupt handler, we know that the previous contexts
> > have finished execution and so can unwind all the incomplete requests
> > and compute the new highest priority request to execute.
> > 
> > It would be feasible to avoid the switch-to-idle intermediate by
> > programming the ELSP with the target context. The difficulty is in
> > tracking which request that should be whilst maintaining the dependency
> > change, the error comes in with coalesced requests. As we only track the
> > most recent request and its priority, we may run into the issue of being
> > tricked in preempting a high priority request that was followed by a
> > low priority request from the same context (e.g. for PI); worse still
> > that earlier request may be our own dependency and the order then broken
> > by preemption. By injecting the switch-to-idle and then recomputing the
> > priority queue, we avoid the issue with tracking in-flight coalesced
> > requests. Having tried the preempt-to-busy approach, and failed to find
> > a way around the coalesced priority issue, Michal's original proposal to
> > inject an idle context (based on handling GuC preemption) succeeds.
> > 
> > The current heuristic for deciding when to preempt are only if the new
> > request is of higher priority, and has the privileged priority of
> > greater than 0. Note that the scheduler remains unfair!
> > 
> > v2: Disable for gen8 (bdw/bsw) as we need additional w/a for GPGPU.
> > Since, the feature is now conditional and not always available when we
> > have a scheduler, make it known via the HAS_SCHEDULER GETPARAM (now a
> > capability mask).
> > v3: Stylistic tweaks.
> > 
> > Suggested-by: Michal Winiarski <michal.winiarski at intel.com>
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Michal Winiarski <michal.winiarski at intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Arkadiusz Hiler <arkadiusz.hiler at intel.com>
> > Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> > Cc: Ben Widawsky <benjamin.widawsky at intel.com>
> > Cc: Zhenyu Wang <zhenyuw at linux.intel.com>
> > Cc: Zhi Wang <zhi.a.wang at intel.com>
> 
> I'm still voting for "preempting" variable name + kerneldoc.

I'm open to suggestions, or even a list of questions that you would like
answered for @preempt[ing].

That goes for anything. If at any time anyone sees something odd or not
clear from the context while reading the code, just send a patch adding a
question. Knowing at what point the confusion arose gives us the perfect
place to address that for the next reader.
-Chris