On 2/24/2022 03:41, Tvrtko Ursulin wrote:
On 23/02/2022 20:00, John Harrison wrote:
On 2/23/2022 05:58, Tvrtko Ursulin wrote:
On 23/02/2022 02:45, John Harrison wrote:
On 2/22/2022 03:19, Tvrtko Ursulin wrote:
On 18/02/2022 21:33, John.C.Harrison@xxxxxxxxx wrote:
From: John Harrison <John.C.Harrison@xxxxxxxxx>
Compute workloads are inherantly not pre-emptible for long
periods on
current hardware. As a workaround for this, the pre-emption timeout
for compute capable engines was disabled. This is undesirable
with GuC
submission as it prevents per engine reset of hung contexts.
Hence the
next patch will re-enable the timeout but bumped up by an order of
magnititude.
(Some typos above.)
I'm spotting 'inherently' but not anything else.
Magnititude! O;)
Doh!
[snip]
Whereas, bumping all heartbeat periods to be greater than the
pre-emption timeout is wasteful and unnecessary. That leads to a
total heartbeat time of about a minute. Which is a very long time
to wait for a hang to be detected and recovered. Especially when
the official limit on a context responding to an 'are you dead'
query is only 7.5 seconds.
Not sure how did you get one minute?
7.5 * 2 (to be safe) = 15. 15 * 5 (number of heartbeat periods) = 75
=> 1 minute 15 seconds
Even ignoring any safety factor and just going with 7.5 * 5 still
gets you to 37.5 seconds which is over a half a minute and likely to
race.
Ah because my starting point is there should be no preempt timeout =
heartbeat * 3, I just think that's too ugly.
Then complain at the hardware designers to give us mid-thread
pre-emption back. The heartbeat is only one source of pre-emption
events. For example, a user can be running multiple contexts in parallel
and expecting them to time slice on a single engine. Or maybe a user is
just running one compute task in the background but is doing render work
in the foreground. Etc.
There was a reason the original hack was to disable pre-emption rather
than increase the heartbeat. This is simply a slightly less ugly version
of the same hack. And unfortunately, the basic idea of the hack is
non-negotiable.
As per other comments, 'tP(RCS) = tH *3' or 'tP(RCS) = tP(default) * 12'
or 'tP(RCS) = 7500' are the available options. Given that the heartbeat
is the ever present hard limit, it seems most plausible to base the hack
on that. Any of the others works, though. Although I think a explicit
hardcoded value is the most ugly. I guess the other option is to add
CONFIG_DRM_I915_PREEMPT_TIMEOUT_COMPUTE and default that to 7500.
Take your pick. But 640ms is not allowed.
Regardless, crux of argument was to avoid GuC engine reset and
heartbeat reset racing with each other, and to do that by
considering the preempt timeout with the heartbeat interval. I was
thinking about this scenario in this series:
[Please use fixed width font and no line wrap to view.]
A)
tP = preempt timeout
tH = hearbeat interval
tP = 3 * tH
1) Background load = I915_PRIORITY_DISPLAY
<-- [tH] --> Pulse1 <-- [tH] --> Pulse2 <-- [tH] --> Pulse3 <---- [2
* tH] ----> FULL RESET
|
\- preemption
triggered, tP = 3 * tH ------\
\-> preempt timeout would hit here
Here we have collateral damage due full reset, since we can't tell
GuC to reset just one engine and we fudged tP just to "account" for
heartbeats.
You are missing the whole point of the patch series which is that the
last heartbeat period is '2 * tP' not '2 * tH'.
+ longer = READ_ONCE(engine->props.preempt_timeout_ms) * 2;
By making the last period double the pre-emption timeout, it is
guaranteed that the FULL RESET stage cannot be hit before the
hardware has attempted and timed-out on at least one pre-emption.
Oh well :) that probably means the overall scheme is too odd for me.
tp = 3tH and last pulse after 2tP I mean.
To be accurate, it is 'tP(RCS) = 3 * tH(default); tH(final) =
tP(current) * 2;'. Seems fairly straight forward to me. It's not a
recursive definition or anything like that. It gives us a total
heartbeat timeout that is close to the original version but still allows
at least one pre-emption event.
[snip]
<-- [tH] --> Pulse1 <-- [tH] --> Pulse2 <-- [tH] --> Pulse3 <---- [2
* tH] ----> full reset would be here
|
\- preemption triggered, tP = 3 * tH ----------------\
\-> Preempt timeout reset
Here is is kind of least worse, but question is why we fudged tP
when it gives us nothing good in this case.
The point of fudging tP(RCS) is to give compute workloads longer to
reach a pre-emptible point (given that EU walkers are basically not
pre-emptible). The reason for doing the fudge is not connected to the
heartbeat at all. The fact that it causes problems for the heartbeat
is an undesired side effect.
Note that the use of 'tP(RCS) = tH * 3' was just an arbitrary
calculation that gave us something that all interested parties were
vaguely happy with. It could just as easily be a fixed, hard coded
value of 7.5s but having it based on something configurable seemed
more sensible. The other option was 'tP(RCS) = tP * 12' but that felt
more arbitrary than basing it on the average heartbeat timeout. As
in, three heartbeat periods is about what a normal prio task gets
before it gets pre-empted by the heartbeat. So using that for general
purpose pre-emptions (e.g. time slicing between multiple user apps)
seems reasonable.
I think the fact you say tP fudge is not related to heartbeats and
then go to mention heartbeat even in the "formula" which uses no tH is
saying something (at least that's how I read the 7.5s option). :)
I said the tP fudge is not because of the heartbeat. It is obviously
related.
As per comment above, the fudge factor is based on the heartbeat because
the heartbeat is the ultimate limit. But the *reason* for the fudge fact
has nothing to do with the heartbeat. It is required even if heartbeats
are disabled.
B)
Instead, my idea to account for preempt timeout when calculating
when to schedule next hearbeat would look like this:
First of all tP can be left at a large value unrelated to tH. Lets
say tP = 640ms. tH stays 2.5s.
640ms is not 'large'. The requirement is either zero (disabled) or
region of 7.5s. The 640ms figure is the default for non-compute
engines. Anything that can run EUs needs to be 'huge'.
1) Background load = I915_PRIORITY_DISPLAY
<-- [tH + tP] --> Pulse1 <-- [tH + tP] --> Pulse2 <-- [tH + tP] -->
Pulse3 <-- [tH + tP] --> full reset would be here
Sure, this works but each period is now 2.5 + 7.5 = 10s. The full
five periods is therefore 50s, which is practically a minute.
No, in my proposal it is 3 * (2.5s + 640ms) =~ 9.3s.
Not good enough. After 2.5s, we send a pulse. After a further 640ms we
perform an engine reset. That means your compute workload had only 640ms
after being told to pre-empt to reach a pre-emption point. That won't
work. It needs to be multiple seconds.
[snip]
Am I missing some requirement or you see another problem with this
idea?
On a related topic, if GuC engine resets stop working when preempt
timeout is set to zero - I think we need to somehow let the user
know if they try to tweak it via sysfs. Perhaps go as far as
-EINVAL in GuC mode, if i915.reset has not explicitly disabled
engine resets.
Define 'stops working'. The definition of the sysfs interface is
that a value of zero disables pre-emption. If you don't have
pre-emption and your hang detection mechanism relies on pre-emption
then you don't have a hang detection mechanism either. If the user
really wants to allow
By stops working I meant that it stops working. :)
With execlist one can disable preempt timeout and "stopped
heartbeat" can still reset the stuck engine and so avoid collateral
damage. With GuC it appears this is not possible. So I was thinking
this is something worthy a log notice.
their context to run forever and never be pre-empted then that
means they also don't want it to be reset arbitrarily. Which means
they would also be disabling the heartbeat timer as well. Indeed,
this is what we
I don't think so. Preempt timeout is disabled already on TGL/RCS
upstream but hearbeat is not and so hangcheck still works.
The pre-emption disable in upstream is not a valid solution for
compute customers. It is a worst-of-all-worlds hack for general
usage. As noted already, any actual compute specific customer is
advised to disable all forms of reset and do their hang detection
manually. A slightly less worse hack for customers that are not
actually running long compute workloads (i.e. the vast majority of
end users) is to just use a long pre-emption timeout.
If disabled preemption timeout is worst of all words and compute needs
to disable heartbeat as well then why did we put it in? Perhaps it was
not know at the time it would not be good enough. But anyway, do I
read you correct that current thinking is it would be better to leave
it at default 640ms?
No. We cannot have the RCS default to 640ms.
Note that there is a difference between 'general end user who might run
some compute' and 'compute focused customer'. The driver defaults
(disabled or 7500ms) are for the general user who gets the
out-of-the-box experience and expects to be able to run 'normal'
workloads without hitting problems. I.e. they expect hung tasks to get
reset in a timely manner and while they might run some AI or other
compute workloads, they are not a HPC datacenter. Whereas the compute
datacenter customer expects their workloads to run for arbitrarily long
times (minutes, hours, maybe even days) without being arbitrarily
killed. Those customers will be explicitly configuring their datacenter
server for that scenario and thus don't care what the defaults are.
If so, if we went with my proposal, would everyone be happy? If yes,
isn't it a simpler scheme? No special casing when setting the preempt
timeout, no special casing of the last heartbeat pulse. Works
predictably regardless of the priority of the hypothetical
non-preemptible workload.
No, we have to have the increased pre-emption timeout. And that has
ripple effects of making very long heartbeats or risking races with the
heartbeat beating the per engine reset.
advise compute customers to do. It is then up to the user
themselves to spot a hang and to manually kill (Ctrl+C, kill ###,
etc.) their task. Killing the CPU task will automatically clear up
any GPU resources allocated to that task (excepting context
persistence, which is a) broken and b) something we also tell
compute customers to disable).
What is broken with context persistence? I noticed one patch
claiming to be fixing something in that area which looked suspect.
Has it been established no userspace relies on it?
One major issue is that it has hooks into the execlist scheduler
backend. I forget the exact details right now. The implementation as
a whole is incredibly complex and convoluted :(. But there's stuff
about what happens when you disable the heartbeat after having closed
a persistence context's handle (and thus made it persisting). There's
also things like it sends a super high priority heartbeat pulse at
the point of becoming persisting. That plays havoc for platforms with
dependent engines and/or compute workloads. A context becomes
persisting on RCS and results in your unrealted CCS work being reset.
It's a mess.
The comment from Daniel Vetter is that persistence should have no
connection to the heartbeat at all. All of that dynamic behaviour and
complexity should just be removed.
Dependent engines is definitely a topic on it's own, outside
hearbeats, persistence and all.
Except that it has implications for persistence which the current driver
does not take into account.
Otherwise there is definitely complexity in the execlists backend but
I am not sure if logic persistence and heartbeats are so very
connected. It does send a pulse when heartbeat interval is changed,
which if going to zero, it will kick of closed contexts if it can:
static struct intel_engine_cs *
__execlists_schedule_in(struct i915_request *rq)
{
struct intel_engine_cs * const engine = rq->engine;
struct intel_context * const ce = rq->context;
intel_context_get(ce);
if (unlikely(intel_context_is_closed(ce) &&
!intel_engine_has_heartbeat(engine)))
intel_context_set_banned(ce);
if (unlikely(intel_context_is_banned(ce) || bad_request(rq)))
reset_active(rq, engine);
Is this what you mean? The point of this is to make sure persistent
context does not hog the engine forever if hangcheck has been disabled.
Reminds me of my improvement to customer experience which never got in
(https://patchwork.freedesktop.org/patch/451491/?series=93420&rev=2).
Point of that one was to avoid engine reset (or worse) after user
presses "Ctrl-C" if something takes just over 1ms to cleanly complete.
The plan is that the persistent contexts would still get the default
grace period (pre-emption timeout at least) to finish but Ctrl+C will
kill it within a timely manner if it does not finish.
Persistence itself can stay. There are valid UMD use cases. It is
just massively over complicated and doesn't work in all corner cases
when not using execlist submission or on newer platforms. The
simplification that is planned is to allow contexts to persist until
the associated DRM master handle is closed. At that point, all
contexts associated with that DRM handle are killed. That is what AMD
and others apparently implement.
Okay, that goes against one recent IGT patch which appeared to work
around something by moving the position of _context_ close.
No it does not. The context close is not the trigger. The trigger is
closing the top level DRM handle. If your context has persistence
enabled (the default) then closing the context handle will have no
effect. No pulse, no pre-emption, no kill, nothing. But when the top
level handle is closed (application exit through whatever mechanism)
then all GPU resources will be cleaned up. As above, with at least a
pre-emption timeout grace period, but after that it is termination time.
The media use cases for persistence are all happy with this scheme. I
don't actually recall if we got a reply back from the OGL people. They
were definitely on the email thread/Jira task and did not complain. OCL
obviously don't care as their first action is to explicitly disable
persistence.
John.
Regards,
Tvrtko
