On 23/02/2022 19:03, John Harrison wrote:
On 2/23/2022 04:13, Tvrtko Ursulin wrote:
On 23/02/2022 02:11, John Harrison wrote:
On 2/22/2022 01:52, Tvrtko Ursulin wrote:
On 18/02/2022 21:33, John.C.Harrison@xxxxxxxxx wrote:
From: John Harrison <John.C.Harrison@xxxxxxxxx>
GuC converts the pre-emption timeout and timeslice quantum values into
clock ticks internally. That significantly reduces the point of 32bit
overflow. On current platforms, worst case scenario is approximately
Where does 32-bit come from, the GuC side? We already use 64-bits so
that something to fix to start with. Yep...
Yes, the GuC API is defined as 32bits only and then does a straight
multiply by the clock speed with no range checking. We have requested
64bit support but there was push back on the grounds that it is not
something the GuC timer hardware supports and such long timeouts are
not real world usable anyway.
As long as compute are happy with 100 seconds, then it "should be
enough for everbody". :D
Compute disable all forms of reset and rely on manual kill. So yes.
But even if they aren't. That's all we can do at the moment. If there is
a genuine customer requirement for more then we can push for full 64bit
software implemented timers in the GuC but until that happens, we don't
have much choice.
Yeah.
./gt/uc/intel_guc_fwif.h: u32 execution_quantum;
./gt/uc/intel_guc_submission.c: desc->execution_quantum =
engine->props.timeslice_duration_ms * 1000;
./gt/intel_engine_types.h: unsigned long
timeslice_duration_ms;
timeslice_store/preempt_timeout_store:
err = kstrtoull(buf, 0, &duration);
So both kconfig and sysfs can already overflow GuC, not only because
of tick conversion internally but because at backend level nothing
was done for assigning 64-bit into 32-bit. Or I failed to find where
it is handled.
That's why I'm adding this range check to make sure we don't allow
overflows.
Yes and no, this fixes it, but the first bug was not only due GuC
internal tick conversion. It was present ever since the u64 from i915
was shoved into u32 sent to GuC. So even if GuC used the value without
additional multiplication, bug was be there. My point being when GuC
backend was added timeout_ms values should have been limited/clamped
to U32_MAX. The tick discovery is additional limit on top.
I'm not disagreeing. I'm just saying that the truncation wasn't noticed
until I actually tried using very long timeouts to debug a particular
problem. Now that it is noticed, we need some method of range checking
and this simple clamp solves all the truncation problems.
Agreed in principle, just please mention in the commit message all aspects of the problem.
I think we can get away without a Fixes: tag since it requires user fiddling to break things in unexpected ways.
I would though put in a code a clamping which expresses both, something like min(u32, ..GUC LIMIT..). So the full story is documented forever. Or "if > u32 || > ..GUC LIMIT..) return -EINVAL". Just in case GuC limit one day changes but u32 stays. Perhaps internal ticks go away or anything and we are left with plain 1:1 millisecond relationship.
110 seconds. Rather than allowing the user to set higher values and
then get confused by early timeouts, add limits when setting these
values.
Btw who is reviewing GuC patches these days - things have somehow
gotten pretty quiet in activity and I don't think that's due absence
of stuff to improve or fix? Asking since I think I noticed a few
already which you posted and then crickets on the mailing list.
Too much work to do and not enough engineers to do it all :(.
Signed-off-by: John Harrison <John.C.Harrison@xxxxxxxxx>
---
drivers/gpu/drm/i915/gt/intel_engine_cs.c | 15 +++++++++++++++
drivers/gpu/drm/i915/gt/sysfs_engines.c | 14 ++++++++++++++
drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h | 9 +++++++++
3 files changed, 38 insertions(+)
diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
index e53008b4dd05..2a1e9f36e6f5 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
@@ -389,6 +389,21 @@ static int intel_engine_setup(struct intel_gt
*gt, enum intel_engine_id id,
if (GRAPHICS_VER(i915) == 12 && engine->class == RENDER_CLASS)
engine->props.preempt_timeout_ms = 0;
+ /* Cap timeouts to prevent overflow inside GuC */
+ if (intel_guc_submission_is_wanted(>->uc.guc)) {
+ if (engine->props.timeslice_duration_ms >
GUC_POLICY_MAX_EXEC_QUANTUM_MS) {
Hm "wanted".. There's been too much back and forth on the GuC load
options over the years to keep track.. intel_engine_uses_guc work
sounds like would work and read nicer.
I'm not adding a new feature check here. I'm just using the existing
one. If we want to rename it yet again then that would be a different
patch set.
$ grep intel_engine_uses_guc . -rl
./i915_perf.c
./i915_request.c
./selftests/intel_scheduler_helpers.c
./gem/i915_gem_context.c
./gt/intel_context.c
./gt/intel_engine.h
./gt/intel_engine_cs.c
./gt/intel_engine_heartbeat.c
./gt/intel_engine_pm.c
./gt/intel_reset.c
./gt/intel_lrc.c
./gt/selftest_context.c
./gt/selftest_engine_pm.c
./gt/selftest_hangcheck.c
./gt/selftest_mocs.c
./gt/selftest_workarounds.c
Sounds better to me than intel_guc_submission_is_wanted. What does the
reader know whether "is wanted" translates to "is actually used".
Shrug on "is wanted".
Yes, but isn't '_uses' the one that hits a BUG_ON if you call it too
early in the boot up sequence? I never understood why that was necessary
or why we need so many different ways to ask the same question. But this
version already exists and definitely works without hitting any explosions.
No idea if it causes a bug on, doesn't in the helper itself so maybe you are saying it is called too early? Might be.. I think over time the nice idea we had that "setup" and "init" phases of engine setup clearly separated got destroyed a bit. There would always be an option to move this clamping in a later phase, once the submission method is known. One could argue that if the submission method is not yet known at this point, it is even wrong to clamp based on something which will only be decided later. Because:
int intel_engines_init(struct intel_gt *gt)
{
int (*setup)(struct intel_engine_cs *engine);
struct intel_engine_cs *engine;
enum intel_engine_id id;
int err;
if (intel_uc_uses_guc_submission(>->uc)) {
gt->submission_method = INTEL_SUBMISSION_GUC;
So this uses "uses", not "wanted". Presumably the point for having "wanted" and "uses" is that they can disagree, in which case if you clamp early based on "wanted" that suggests it could be wrong.
And limit to class instead of applying to all engines looks like a
miss.
As per follow up email, the class limit is not applied here.
+ drm_info(&engine->i915->drm, "Warning, clamping timeslice
duration to %d to prevent possibly overflow\n",
+ GUC_POLICY_MAX_EXEC_QUANTUM_MS);
+ engine->props.timeslice_duration_ms =
GUC_POLICY_MAX_EXEC_QUANTUM_MS;
I am not sure logging such message during driver load is useful.
Sounds more like a confused driver which starts with one value and
then overrides itself. I'd just silently set the value appropriate
for the active backend. Preemption timeout kconfig text already
documents the fact timeouts can get overriden at runtime depending
on platform+engine. So maybe just add same text to timeslice kconfig.
The point is to make people aware if they compile with unsupported
config options. As far as I know, there is no way to apply range
checking or other limits to config defines. Which means that a user
would silently get unwanted behaviour. That seems like a bad thing to
me. If the driver is confused because the user built it in a confused
manner then we should let them know.
Okay, but I think make it notice low level.
Also consider in patch 3/3 when you triple it, and then clamp back
down here. That's even more confused state since tripling gets nerfed.
I think that's also an argument to always account preempt timeout in
heartbeat interval calculation. Haven't got to your reply on 2/3 yet
though..
That sounds like even more reason to make sure the warning gets seen.
The more complex the system and the more chances there are to get it
wrong, the more important it is to have a nice easy to see and
understand notification that it did go wrong.
I did not disagree, just said make it notice, one level higher than info! :)
But also think how, if we agree to go with tripling, that you'd have to consider that in the sysfs store when hearbeat timeout is written, to consider whether or not to triple and error out if preemption timeout is over limit.
+ }
+
+ if (engine->props.preempt_timeout_ms >
GUC_POLICY_MAX_PREEMPT_TIMEOUT_MS) {
+ drm_info(&engine->i915->drm, "Warning, clamping
pre-emption timeout to %d to prevent possibly overflow\n",
+ GUC_POLICY_MAX_PREEMPT_TIMEOUT_MS);
+ engine->props.preempt_timeout_ms =
GUC_POLICY_MAX_PREEMPT_TIMEOUT_MS;
+ }
+ }
+
engine->defaults = engine->props; /* never to change again */
engine->context_size = intel_engine_context_size(gt,
engine->class);
diff --git a/drivers/gpu/drm/i915/gt/sysfs_engines.c
b/drivers/gpu/drm/i915/gt/sysfs_engines.c
index 967031056202..f57efe026474 100644
--- a/drivers/gpu/drm/i915/gt/sysfs_engines.c
+++ b/drivers/gpu/drm/i915/gt/sysfs_engines.c
@@ -221,6 +221,13 @@ timeslice_store(struct kobject *kobj, struct
kobj_attribute *attr,
if (duration > jiffies_to_msecs(MAX_SCHEDULE_TIMEOUT))
return -EINVAL;
+ if (intel_uc_uses_guc_submission(&engine->gt->uc) &&
+ duration > GUC_POLICY_MAX_EXEC_QUANTUM_MS) {
+ duration = GUC_POLICY_MAX_EXEC_QUANTUM_MS;
+ drm_info(&engine->i915->drm, "Warning, clamping timeslice
duration to %lld to prevent possibly overflow\n",
+ duration);
+ }
I would suggest to avoid duplicated clamping logic. Maybe hide the
all backend logic into the helpers then, like maybe:
d = intel_engine_validate_timeslice/preempt_timeout(engine,
duration);
if (d != duration)
return -EINVAL:
Returning -EINVAL would be equivalent to existing behaviour:
if (duration > jiffies_to_msecs(MAX_SCHEDULE_TIMEOUT))
return -EINVAL;
That way userspace has explicit notification and read-back is
identical to written in value. From engine setup you can just call
the helper silently.
Sure, EINVAL rather than clamping works as well. And can certainly
add helper wrappers. But as above, I don't like the idea of silently
disregarding a user specified config option.
Deal - with the open of heartbeat interval TBD.
+
WRITE_ONCE(engine->props.timeslice_duration_ms, duration);
if (execlists_active(&engine->execlists))
@@ -325,6 +332,13 @@ preempt_timeout_store(struct kobject *kobj,
struct kobj_attribute *attr,
if (timeout > jiffies_to_msecs(MAX_SCHEDULE_TIMEOUT))
return -EINVAL;
+ if (intel_uc_uses_guc_submission(&engine->gt->uc) &&
+ timeout > GUC_POLICY_MAX_PREEMPT_TIMEOUT_MS) {
+ timeout = GUC_POLICY_MAX_PREEMPT_TIMEOUT_MS;
+ drm_info(&engine->i915->drm, "Warning, clamping
pre-emption timeout to %lld to prevent possibly overflow\n",
+ timeout);
+ }
+
WRITE_ONCE(engine->props.preempt_timeout_ms, timeout);
if (READ_ONCE(engine->execlists.pending[0]))
diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h
b/drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h
index 6a4612a852e2..ad131092f8df 100644
--- a/drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h
+++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h
@@ -248,6 +248,15 @@ struct guc_lrc_desc {
#define GLOBAL_POLICY_DEFAULT_DPC_PROMOTE_TIME_US 500000
+/*
+ * GuC converts the timeout to clock ticks internally. Different
platforms have
+ * different GuC clocks. Thus, the maximum value before overflow
is platform
+ * dependent. Current worst case scenario is about 110s. So, limit
to 100s to be
+ * safe.
+ */
+#define GUC_POLICY_MAX_EXEC_QUANTUM_MS (100 * 1000)
+#define GUC_POLICY_MAX_PREEMPT_TIMEOUT_MS (100 * 1000)
Most important question -
how will we know/notice if/when new GuC arrives where these timeouts
would still overflow? Can this be queried somehow at runtime or
where does the limit comes from? How is GuC told about it? Set in
some field and it just allows too large values silently break things?
Currently, we don't notice except by debugging peculiar test failures
:(.
These limits are not in any GuC spec. Indeed, it took a while to
actually work out why increasing the value actually caused shorter
timeouts to occur! As above, there is no range checking inside GuC
itself. It does a truncated multiply which results in an effectively
random number and just happily uses it.
I will agree with what Daniele said - push on GuC fw folks to document
the max values they guarantee to support in the interface spec.
Otherwise it is too fragile.
I do agree. But that is going to take time. I would like to get
something merged now while we fight over spec updates.
Yeah that's okay, did not mean to imply I am against a quick fix. "Otherwise it is too fragile, *in the long run*" should have written or something like that.
Regards,
Tvrtko
