Quoting Tvrtko Ursulin (2018-06-27 14:15:22)
>
> On 27/06/2018 11:58, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2018-06-27 11:40:32)
> >>
> >> On 25/06/2018 10:48, Chris Wilson wrote:
> >>> Back in commit 27af5eea54d1 ("drm/i915: Move execlists irq handler to a
> >>> bottom half"), we came to the conclusion that running our CSB processing
> >>> and ELSP submission from inside the irq handler was a bad idea. A really
> >>> bad idea as we could impose nearly 1s latency on other users of the
> >>> system, on average! Deferring our work to a tasklet allowed us to do the
> >>> processing with irqs enabled, reducing the impact to an average of about
> >>> 50us.
> >>>
> >>> We have since eradicated the use of forcewaked mmio from inside the CSB
> >>> processing and ELSP submission, bringing the impact down to around 5us
> >>> (on Kabylake); an order of magnitude better than our measurements 2
> >>> years ago on Broadwell and only about 2x worse on average than the
> >>> gem_syslatency on an unladen system.
> >>>
> >>> In this iteration of the tasklet-vs-direct submission debate, we seek a
> >>> compromise where by we submit new requests immediately to the HW but
> >>> defer processing the CS interrupt onto a tasklet. We gain the advantage
> >>> of low-latency and ksoftirqd avoidance when waking up the HW, while
> >>> avoiding the system-wide starvation of our CS irq-storms.
> >>>
> >>> Comparing the impact on the maximum latency observed (that is the time
> >>> stolen from an RT process) over a 120s interval, repeated several times
> >>> (using gem_syslatency, similar to RT's cyclictest) while the system is
> >>> fully laden with i915 nops, we see that direct submission an actually
> >>> improve the worse case.
> >>>
> >>> Maximum latency in microseconds of a third party RT thread
> >>> (gem_syslatency -t 120 -f 2)
> >>> x Always using tasklets (a couple of >1000us outliers removed)
> >>> + Only using tasklets from CS irq, direct submission of requests
> >>> +------------------------------------------------------------------------+
> >>> | + |
> >>> | + |
> >>> | + |
> >>> | + + |
> >>> | + + + |
> >>> | + + + + x x x |
> >>> | +++ + + + x x x x x x |
> >>> | +++ + ++ + + *x x x x x x |
> >>> | +++ + ++ + * *x x * x x x |
> >>> | + +++ + ++ * * +*xxx * x x xx |
> >>> | * +++ + ++++* *x+**xx+ * x x xxxx x |
> >>> | **x++++*++**+*x*x****x+ * +x xx xxxx x x |
> >>> |x* ******+***************++*+***xxxxxx* xx*x xxx + x+|
> >>> | |__________MA___________| |
> >>> | |______M__A________| |
> >>> +------------------------------------------------------------------------+
> >>> N Min Max Median Avg Stddev
> >>> x 118 91 186 124 125.28814 16.279137
> >>> + 120 92 187 109 112.00833 13.458617
> >>> Difference at 95.0% confidence
> >>> -13.2798 +/- 3.79219
> >>> -10.5994% +/- 3.02677%
> >>> (Student's t, pooled s = 14.9237)
> >>>
> >>> However the mean latency is adversely affected:
> >>>
> >>> Mean latency in microseconds of a third party RT thread
> >>> (gem_syslatency -t 120 -f 1)
> >>> x Always using tasklets
> >>> + Only using tasklets from CS irq, direct submission of requests
> >>> +------------------------------------------------------------------------+
> >>> | xxxxxx + ++ |
> >>> | xxxxxx + ++ |
> >>> | xxxxxx + +++ ++ |
> >>> | xxxxxxx +++++ ++ |
> >>> | xxxxxxx +++++ ++ |
> >>> | xxxxxxx +++++ +++ |
> >>> | xxxxxxx + ++++++++++ |
> >>> | xxxxxxxx ++ ++++++++++ |
> >>> | xxxxxxxx ++ ++++++++++ |
> >>> | xxxxxxxxxx +++++++++++++++ |
> >>> | xxxxxxxxxxx x +++++++++++++++ |
> >>> |x xxxxxxxxxxxxx x + + ++++++++++++++++++ +|
> >>> | |__A__| |
> >>> | |____A___| |
> >>> +------------------------------------------------------------------------+
> >>> N Min Max Median Avg Stddev
> >>> x 120 3.506 3.727 3.631 3.6321417 0.02773109
> >>> + 120 3.834 4.149 4.039 4.0375167 0.041221676
> >>> Difference at 95.0% confidence
> >>> 0.405375 +/- 0.00888913
> >>> 11.1608% +/- 0.244735%
> >>> (Student's t, pooled s = 0.03513)
> >>>
> >>> However, since the mean latency corresponds to the amount of irqsoff
> >>> processing we have to do for a CS interrupt, we only need to speed that
> >>> up to benefit not just system latency but our own throughput.
> >>>
> >>> v2: Remember to defer submissions when under reset.
> >>> v4: Only use direct submission for new requests
> >>> v5: Be aware that with mixing direct tasklet evaluation and deferred
> >>> tasklets, we may end up idling before running the deferred tasklet.
> >>>
> >>> Testcase: igt/gem_exec_latency/*rthog*
> >>> References: 27af5eea54d1 ("drm/i915: Move execlists irq handler to a bottom half")
> >>> 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_gem.h | 5 +
> >>> drivers/gpu/drm/i915/i915_irq.c | 11 +-
> >>> drivers/gpu/drm/i915/intel_engine_cs.c | 8 +-
> >>> drivers/gpu/drm/i915/intel_lrc.c | 147 ++++++++++++++----------
> >>> drivers/gpu/drm/i915/intel_ringbuffer.h | 1 -
> >>> 5 files changed, 98 insertions(+), 74 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/i915_gem.h b/drivers/gpu/drm/i915/i915_gem.h
> >>> index 261da577829a..7892ac773916 100644
> >>> --- a/drivers/gpu/drm/i915/i915_gem.h
> >>> +++ b/drivers/gpu/drm/i915/i915_gem.h
> >>> @@ -88,4 +88,9 @@ static inline void __tasklet_enable_sync_once(struct tasklet_struct *t)
> >>> tasklet_kill(t);
> >>> }
> >>>
> >>> +static inline bool __tasklet_is_enabled(const struct tasklet_struct *t)
> >>> +{
> >>> + return likely(!atomic_read(&t->count));
> >>> +}
> >>> +
> >>
> >> For the unlikely-likely chain from
> >> __submit_queue->reset_in_progress->__tasklet_is_enabled I think it would
> >> be better to drop the likely/unlikely from low-level helpers and put the
> >> one unlikely into the __submit_queue.
> >
> > Tasklets are rarely disabled, I think that's quite important to stress.
> > Tasklets do not function very well (heavy spinning) while disabled.
>
> I think we shouldn't be concerned by that. Purpose of this is to wrap
> internal implementation we even shouldn't be touching if we could help
> it, and I feel correct thing is to express the branching hint higher up
> the stack. Caller wants to optimize certain scenarios, while the helper
> doesn't know who is calling it and why. On top we have this
> likely-unlikley chain which I mentioned. Even just one unlikely in
> reset_in_progress would probably be enough for what you wanted to ensure.
I already acquiesced and did extra that.
> >>> #endif /* __I915_GEM_H__ */
> >>> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> >>> index 46aaef5c1851..316d0b08d40f 100644
> >>> --- a/drivers/gpu/drm/i915/i915_irq.c
> >>> +++ b/drivers/gpu/drm/i915/i915_irq.c
> >>> @@ -1469,14 +1469,10 @@ static void snb_gt_irq_handler(struct drm_i915_private *dev_priv,
> >>> static void
> >>> gen8_cs_irq_handler(struct intel_engine_cs *engine, u32 iir)
> >>> {
> >>> - struct intel_engine_execlists * const execlists = &engine->execlists;
> >>> bool tasklet = false;
> >>>
> >>> - if (iir & GT_CONTEXT_SWITCH_INTERRUPT) {
> >>> - if (READ_ONCE(engine->execlists.active))
> >>
> >> What is the thinking behind this change? It used to be that we scheduled
> >> the tasklet only when we knew we are expecting interrupts and now we
> >> don't care any more for some reason?
> >
> > The filtering is done inside process_csb(). We filtered on active
> > previously as some interrupts were seemingly going astray, now I am much
> > more confident that all are accounted for.
>
> Hm how? We filter extra interrupts, we can't filter to get what's missing?
Not quite, since we process the CSB more frequently than interrupts, we
may also get an interrupt after having already processed the CSB.
> >>> - tasklet = !test_and_set_bit(ENGINE_IRQ_EXECLIST,
> >>> - &engine->irq_posted);
> >>
> >> And this is gone as well. Can you put a paragraph in the commit message
> >> explaining the change? It doesn't seem immediately connected with direct
> >> submission.
> >
> > Removing one heavyweight atomic operation in the latency sensitive
> > interrupt.
>
> But on the higher level - why we don't need this any more.
Because we are using the CSB, ok I think that can be a separate step.
> >>> + if (iir & GT_CONTEXT_SWITCH_INTERRUPT)
> >>> + tasklet = true;
> >>>
> >>> if (iir & GT_RENDER_USER_INTERRUPT) {
> >>> notify_ring(engine);
> >>> @@ -1484,7 +1480,7 @@ gen8_cs_irq_handler(struct intel_engine_cs *engine, u32 iir)
> >>> }
> >>>
> >>> if (tasklet)
> >>> - tasklet_hi_schedule(&execlists->tasklet);
> >>> + tasklet_hi_schedule(&engine->execlists.tasklet);
> >>> }
> >>>
> >>> static void gen8_gt_irq_ack(struct drm_i915_private *i915,
> >>> @@ -2216,7 +2212,6 @@ static irqreturn_t cherryview_irq_handler(int irq, void *arg)
> >>>
> >>> I915_WRITE(VLV_IER, ier);
> >>> I915_WRITE(GEN8_MASTER_IRQ, GEN8_MASTER_IRQ_CONTROL);
> >>> - POSTING_READ(GEN8_MASTER_IRQ);
> >>
> >> What is this?
> >
> > Something that I haven't managed to kill yet.
>
> No sneaking in this patch then either! :D
Just close your eyes. Nothing to see here.
> >>> diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
> >>> index 7209c22798e6..ace93958689e 100644
> >>> --- a/drivers/gpu/drm/i915/intel_engine_cs.c
> >>> +++ b/drivers/gpu/drm/i915/intel_engine_cs.c
> >>> @@ -1353,12 +1353,10 @@ static void intel_engine_print_registers(const struct intel_engine_cs *engine,
> >>> ptr = I915_READ(RING_CONTEXT_STATUS_PTR(engine));
> >>> read = GEN8_CSB_READ_PTR(ptr);
> >>> write = GEN8_CSB_WRITE_PTR(ptr);
> >>> - drm_printf(m, " Execlist CSB read %d [%d cached], write %d [%d from hws], interrupt posted? %s, tasklet queued? %s (%s)\n",
> >>> + drm_printf(m, " Execlist CSB read %d [%d cached], write %d [%d from hws], tasklet queued? %s (%s)\n",
> >>> read, execlists->csb_head,
> >>> write,
> >>> intel_read_status_page(engine, intel_hws_csb_write_index(engine->i915)),
> >>> - yesno(test_bit(ENGINE_IRQ_EXECLIST,
> >>> - &engine->irq_posted)),
> >>> yesno(test_bit(TASKLET_STATE_SCHED,
> >>> &engine->execlists.tasklet.state)),
> >>> enableddisabled(!atomic_read(&engine->execlists.tasklet.count)));
> >>> @@ -1570,11 +1568,9 @@ void intel_engine_dump(struct intel_engine_cs *engine,
> >>> spin_unlock(&b->rb_lock);
> >>> local_irq_restore(flags);
> >>>
> >>> - drm_printf(m, "IRQ? 0x%lx (breadcrumbs? %s) (execlists? %s)\n",
> >>> + drm_printf(m, "IRQ? 0x%lx (breadcrumbs? %s)\n",
> >>> engine->irq_posted,
> >>> yesno(test_bit(ENGINE_IRQ_BREADCRUMB,
> >>> - &engine->irq_posted)),
> >>> - yesno(test_bit(ENGINE_IRQ_EXECLIST,
> >>> &engine->irq_posted)));
> >>>
> >>> drm_printf(m, "HWSP:\n");
> >>> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> >>> index 5a12b8fc9d8f..c82efa3ac105 100644
> >>> --- a/drivers/gpu/drm/i915/intel_lrc.c
> >>> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> >>> @@ -562,13 +562,15 @@ static void complete_preempt_context(struct intel_engine_execlists *execlists)
> >>> {
> >>> GEM_BUG_ON(!execlists_is_active(execlists, EXECLISTS_ACTIVE_PREEMPT));
> >>>
> >>> + __unwind_incomplete_requests(container_of(execlists,
> >>> + typeof(struct intel_engine_cs),
> >>> + execlists));
> >>> execlists_cancel_port_requests(execlists);
> >>> - execlists_unwind_incomplete_requests(execlists);
> >>
> >> Is the ordering change significant and why?
> >
> > Mostly for consistency and reasoning about request reference lifetimes.
> > (Unwind => we retain the request reference, as it is moved back to the
> > protected execution lists.)
>
> Remove from this patch then?
Move to an earlier patch then.
> >>> +static void execlists_submission_tasklet(unsigned long data)
> >>> +{
> >>> + struct intel_engine_cs * const engine = (struct intel_engine_cs *)data;
> >>> + unsigned long flags;
> >>> +
> >>> + GEM_TRACE("%s awake?=%d, active=%x\n",
> >>> + engine->name,
> >>> + engine->i915->gt.awake,
> >>> + engine->execlists.active);
> >>> +
> >>> + spin_lock_irqsave(&engine->timeline.lock, flags);
> >>> +
> >>> + if (engine->i915->gt.awake) /* we may be delayed until after we idle! */
> >>> + __execlists_submission_tasklet(engine);
> >>
> >> Sounds quite bad! this means we fail to process pending CSB. And going
> >> idle syncs the tasklets so what am I missing?
> >
> > That tasklets get kicked randomly, I think was the culprit.
>
> What do you mean? I hope we have busy-idle quite controlled and we know
> when we should and should expect a tasklet. If we synced them when
> transitioning to idle they cannot happen. Otherwise we better be active!
> GEM_BUG_ON(!engine->i915->gt.awake) instead? Does that trigger?!
tasklet_schedule() is called off the main path, without locking, so
unsynchronized to parking. Just because.
-Chris
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