On 11/05/2018 09:31, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2018-05-11 09:25:00)
On 10/05/2018 18:40, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2018-05-10 18:26:31)
On 10/05/2018 17:25, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2018-05-10 17:09:14)
On 09/05/2018 15:27, Chris Wilson wrote:
Bypass using the tasklet to submit the first request to HW, as the
tasklet may be deferred unto ksoftirqd and at a minimum will add in
excess of 10us (and maybe tens of milliseconds) to our execution
latency. This latency reduction is most notable when execution flows
between engines.
v2: Beware handling preemption completion from the direct submit path as
well.
v3: Make the abuse clear and track our extra state inside i915_tasklet.
Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_tasklet.h | 24 +++++++
drivers/gpu/drm/i915/intel_guc_submission.c | 10 ++-
drivers/gpu/drm/i915/intel_lrc.c | 71 +++++++++++++++++----
3 files changed, 89 insertions(+), 16 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_tasklet.h b/drivers/gpu/drm/i915/i915_tasklet.h
index 42b002b88edb..99e2fa2241ba 100644
--- a/drivers/gpu/drm/i915/i915_tasklet.h
+++ b/drivers/gpu/drm/i915/i915_tasklet.h
@@ -8,8 +8,11 @@
#define _I915_TASKLET_H_
#include <linux/atomic.h>
+#include <linux/bitops.h>
#include <linux/interrupt.h>
+#include "i915_gem.h"
+
/**
* struct i915_tasklet - wrapper around tasklet_struct
*
@@ -19,6 +22,8 @@
*/
struct i915_tasklet {
struct tasklet_struct base;
+ unsigned long flags;
+#define I915_TASKLET_DIRECT_SUBMIT BIT(0)
I would suggest a more generic name for the bit since i915_tasklet is
generic-ish. For instance simply I915_TASKLET_DIRECT would signify the
callback has been invoked directly and not (necessarily) from softirq
context. Then it is for each user to know what that means for them
specifically.
Problem is we have two direct invocations, only one is special. It
really wants to be something like I915_TASKLET_ENGINE_IS_LOCKED - you can
see why I didn't propose that.
TBC...
-static void __submit_queue(struct intel_engine_cs *engine, int prio)
+static void __wakeup_queue(struct intel_engine_cs *engine, int prio)
{
engine->execlists.queue_priority = prio;
+}
Why is this called wakeup? Plans to add something in it later?
Yes. It's called wakeup because it's setting the value that the dequeue
wakes up at. First name was kick_queue, but it doesn't kick either.
The later side-effect involves controlling timers.
__restart_queue()?
__update_queue_priority? :)
It doesn't just update the priority...
Now a choice between restart_queue and update_queue.
Update sounds better match to me.
+static void __schedule_queue(struct intel_engine_cs *engine)
+{
i915_tasklet_schedule(&engine->execlists.tasklet);
}
+static bool __direct_submit(struct intel_engine_execlists *const execlists)
+{
+ struct i915_tasklet * const t = &execlists->tasklet;
+
+ if (!tasklet_trylock(&t->base))
+ return false;
+
+ t->flags |= I915_TASKLET_DIRECT_SUBMIT;
+ i915_tasklet_run(t);
+ t->flags &= ~I915_TASKLET_DIRECT_SUBMIT;
+
+ tasklet_unlock(&t->base);
Feels like this whole sequence belongs to i915_tasklet since it touches
the internals. Maybe i915_tasklet_try_run, or i915_tasklet_run_or_schedule?
Keep reading the series and you'll see just why this is so special and
confined to execlists.
... TBC here.
Having peeked ahead, it feels a bit not generic enough as it is, a bit
too hacky.
Would it work to pass context together with the invocation. Like:
i915_tasklet_try(..., I915_TASKLET_SUBMIT_IDLE);
i915_tasklet_try(..., I915_TASKLET_SUBMIT_IRQ);
i915_tasklet.flags field namespace would then be owned by the caller
completely. And the tasklet func itself would have more context on what
to do.
That doesn't apply very well to the use case either. It's not the
tasklet being called from irq/process that's significant but whether we
are calling it with the engine/data locked.
That's why I am proposing to allow a generic mechanism to pass in a
"token" to the API, which API will pass down to the user if invoking the
tasklet directly.
The user then decides how to interpret the token.
I915_TASKLET_SUBMIT_IDLE would mean "I know this is the path with
timeline lock already taken".
I915_TASKLET_SUBMIT_IRQ token would mean "I need to take the lock and I
need an early abort if tasklet is disabled".
I don't see a reason to extend it to a generic mechanism yet.
direct-submit-onto-idle is the special case. I've 3 users for the normal
case (just calling into the tasklet directly), one for wanting to pass
along private state after claiming the tasklet for itself, and one that
does unspeakable things that doesn't match any of the above ;)
I don't know what is this last one, but, talking about the three call
sites form this series.. not moving more of the logic to the
i915_tasklet IMO diminishes the reason for it to exists. If it is only
wrapping a single token, which the callers have to fiddle with manually
for majority of (special) cases, for me it is a harder sell.
We could just as well just use a data field in struct intel_execlists,
as I suggested earlier, and write our own i915_execlists_tasklet_try or
something wrappers. It would be the same thing and much less code.
So for me either making it more generic, or not having it at all, seems
that it would result in less code and the functional design would be the
same.
Regards,
Tvrtko
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