On 11/7/2022 06:09, Tvrtko Ursulin wrote:
On 04/11/2022 17:45, John Harrison wrote:
On 11/4/2022 03:01, Tvrtko Ursulin wrote:
On 03/11/2022 19:16, John Harrison wrote:
On 11/3/2022 02:38, Tvrtko Ursulin wrote:
On 03/11/2022 09:18, Tvrtko Ursulin wrote:
On 03/11/2022 01:33, John Harrison wrote:
On 11/2/2022 07:20, Tvrtko Ursulin wrote:
On 02/11/2022 12:12, Jani Nikula wrote:
On Tue, 01 Nov 2022, John.C.Harrison@xxxxxxxxx wrote:
From: John Harrison <John.C.Harrison@xxxxxxxxx>
At the end of each test, IGT does a drop caches call via
sysfs with
sysfs?
Sorry, that was meant to say debugfs. I've also been working on
some sysfs IGT issues and evidently got my wires crossed!
special flags set. One of the possible paths waits for idle
with an
infinite timeout. That causes problems for debugging issues
when CI
catches a "can't go idle" test failure. Best case, the CI
system times
out (after 90s), attempts a bunch of state dump actions and then
reboots the system to recover it. Worst case, the CI system
can't do
anything at all and then times out (after 1000s) and simply
reboots.
Sometimes a serial port log of dmesg might be available,
sometimes not.
So rather than making life hard for ourselves, change the
timeout to
be 10s rather than infinite. Also, trigger the standard
wedge/reset/recover sequence so that testing can continue with a
working system (if possible).
Signed-off-by: John Harrison <John.C.Harrison@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_debugfs.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/i915_debugfs.c
b/drivers/gpu/drm/i915/i915_debugfs.c
index ae987e92251dd..9d916fbbfc27c 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -641,6 +641,9 @@
DEFINE_SIMPLE_ATTRIBUTE(i915_perf_noa_delay_fops,
DROP_RESET_ACTIVE | \
DROP_RESET_SEQNO | \
DROP_RCU)
+
+#define DROP_IDLE_TIMEOUT (HZ * 10)
I915_IDLE_ENGINES_TIMEOUT is defined in i915_drv.h. It's also
only used
here.
So move here, dropping i915 prefix, next to the newly proposed
one?
Sure, can do that.
I915_GEM_IDLE_TIMEOUT is defined in i915_gem.h. It's only used in
gt/intel_gt.c.
Move there and rename to GT_IDLE_TIMEOUT?
I915_GT_SUSPEND_IDLE_TIMEOUT is defined and used only in
intel_gt_pm.c.
No action needed, maybe drop i915 prefix if wanted.
These two are totally unrelated and in code not being touched by
this change. I would rather not conflate changing random other
things with fixing this specific issue.
I915_IDLE_ENGINES_TIMEOUT is in ms, the rest are in jiffies.
Add _MS suffix if wanted.
My head spins.
I follow and raise that the newly proposed DROP_IDLE_TIMEOUT
applies to DROP_ACTIVE and not only DROP_IDLE.
My original intention for the name was that is the 'drop caches
timeout for intel_gt_wait_for_idle'. Which is quite the mouthful
and hence abbreviated to DROP_IDLE_TIMEOUT. But yes, I realised
later that name can be conflated with the DROP_IDLE flag. Will
rename.
Things get refactored, code moves around, bits get left behind,
who knows. No reason to get too worked up. :) As long as people
are taking a wider view when touching the code base, and are
not afraid to send cleanups, things should be good.
On the other hand, if every patch gets blocked in code review
because someone points out some completely unrelated piece of
code could be a bit better then nothing ever gets fixed. If you
spot something that you think should be improved, isn't the
general idea that you should post a patch yourself to improve it?
There's two maintainers per branch and an order of magnitude or
two more developers so it'd be nice if cleanups would just be
incoming on self-initiative basis. ;)
For the actual functional change at hand - it would be nice if
code paths in question could handle SIGINT and then we could
punt the decision on how long someone wants to wait purely to
userspace. But it's probably hard and it's only debugfs so
whatever.
The code paths in question will already abort on a signal won't
they? Both intel_gt_wait_for_idle() and
intel_guc_wait_for_pending_msg(), which is where the
uc_wait_for_idle eventually ends up, have an 'if(signal_pending)
return -EINTR;' check. Beyond that, it sounds like what you are
asking for is a change in the IGT libraries and/or CI framework
to start sending signals after some specific timeout. That seems
like a significantly more complex change (in terms of the number
of entities affected and number of groups involved) and
unnecessary.
If you say so, I haven't looked at them all. But if the code path
in question already aborts on signals then I am not sure what is
the patch fixing? I assumed you are trying to avoid the write
stuck in D forever, which then prevents driver unload and
everything, requiring the test runner to eventually reboot. If
you say SIGINT works then you can already recover from userspace,
no?
Whether or not 10s is enough CI will hopefully tell us. I'd
probably err on the side of safety and make it longer, but at
most half from the test runner timeout.
This is supposed to be test clean up. This is not about how long
a particular test takes to complete but about how long it takes
to declare the system broken after the test has already
finished. I would argue that even 10s is massively longer than
required.
I am not convinced that wedging is correct though. Conceptually
could be just that the timeout is too short. What does wedging
really give us, on top of limiting the wait, when latter AFAIU
is the key factor which would prevent the need to reboot the
machine?
It gives us a system that knows what state it is in. If we can't
idle the GT then something is very badly wrong. Wedging
indicates that. It also ensure that a full GT reset will be
attempted before the next test is run. Helping to prevent a
failure on test X from propagating into failures of unrelated
tests X+1, X+2, ... And if the GT reset does not work because
the system is really that badly broken then future tests will
not run rather than report erroneous failures.
This is not about getting a more stable system for end users by
sweeping issues under the carpet and pretending all is well. End
users don't run IGTs or explicitly call dodgy debugfs entry
points. The sole motivation here is to get more accurate results
from CI. That is, correctly identifying which test has hit a
problem, getting valid debug analysis for that test (logs and
such) and allowing further testing to complete correctly in the
case where the system can be recovered.
I don't really oppose shortening of the timeout in principle,
just want a clear statement if this is something IGT / test
runner could already do or not. It can apply a timeout, it can
also send SIGINT, and it could even trigger a reset from outside.
Sure it is debugfs hacks so general "kernel should not implement
policy" need not be strictly followed, but lets have it clear
what are the options.
One conceptual problem with applying this policy is that the code is:
if (val & (DROP_IDLE | DROP_ACTIVE)) {
ret = intel_gt_wait_for_idle(gt, MAX_SCHEDULE_TIMEOUT);
if (ret)
return ret;
}
if (val & DROP_IDLE) {
ret = intel_gt_pm_wait_for_idle(gt);
if (ret)
return ret;
}
So if someone passes in DROP_IDLE and then why would only the
first branch have a short timeout and wedge. Yeah some bug happens
to be there at the moment, but put a bug in a different place and
you hang on the second branch and then need another patch. Versus
perhaps making it all respect SIGINT and handle from outside.
The pm_wait_for_idle is can only called after gt_wait_for_idle has
completed successfully. There is no route to skip the GT idle or to
do the PM idle even if the GT idle fails. So the chances of the PM
idle failing are greatly reduced. There would have to be something
outside of a GT keeping the GPU awake and there isn't a whole lot
of hardware left at that point!
Well "greatly reduced" is beside my point. Point is today bug is
here and we add a timeout, tomorrow bug is there and then the same
dance. It can be just a sw bug which forgets to release the pm ref
in some circumstances, doesn't really matter.
Huh?
Greatly reduced is the whole point. Today there is a bug and it
causes a kernel hang which requires the CI framework to reboot the
system in an extremely unfriendly way which makes it very hard to
work out what happened. Logs are likely not available. We don't even
necessarily know which test was being run at the time. Etc. So we
replace the infinite timeout with a meaningful timeout. CI now
correctly marks the single test as failing, captures all the correct
logs, creates a useful bug report and continues on testing more stuff.
So what is preventing CI to collect logs if IGT is forever stuck in
interruptible wait? Surely it can collect the logs at that point if
the kernel is healthy enough. If it isn't then I don't see how wedging
the GPU will make the kernel any healthier.
Is i915 preventing better log collection or could test runner be
improved?
Sure, there is still the chance of hitting an infinite timeout. But
that one is significantly more complicated to remove. And the chances
of hitting that one are significantly smaller than the chances of
hitting the first one.
This statement relies on intimate knowledge implementation details and
a bit too much white box testing approach but that's okay, lets move
past this one.
So you are arguing that because I can't fix the last 0.1% of possible
failures, I am not allowed to fix the first 99.9% of the failures?
I am clearly not arguing for that. But we are also not talking about
"fixing failures" here. Just how to make CI cope better with a class
of i915 bugs.
Regarding signals, the PM idle code ends up at
wait_var_event_killable(). I assume that is interruptible via at
least a KILL signal if not any signal. Although it's not entirely
clear trying to follow through the implementation of this code.
Also, I have no idea if there is a safe way to add a timeout to
that code (or why it wasn't already written with a timeout
included). Someone more familiar with the wakeref internals would
need to comment.
However, I strongly disagree that we should not fix the driver just
because it is possible to workaround the issue by re-writing the CI
framework. Feel free to bring a redesign plan to the IGT WG and
whatever equivalent CI meetings in parallel. But we absolutely
should not have infinite waits in the kernel if there is a trivial
way to not have infinite waits.
I thought I was clear that I am not really opposed to the timeout.
The rest of the paragraph I don't really care - point is moot
because it's debugfs so we can do whatever, as long as it is not
burdensome to i915, which this isn't. If either wasn't the case then
we certainly wouldn't be adding any workarounds in the kernel if it
can be achieved in IGT.
Also, sending a signal does not result in the wedge happening. I
specifically did not want to change that code path because I was
assuming there was a valid reason for it. If you have been
interrupted then you are in the territory of maybe it would have
succeeded if you just left it for a moment longer. Whereas, hitting
the timeout says that someone very deliberately said this is too
long to wait and therefore the system must be broken.
I wanted to know specifically about wedging - why can't you
wedge/reset from IGT if DROP_IDLE times out in quiescent or
wherever, if that's what you say is the right thing?
Huh?
DROP_IDLE has two waits. One that I am trying to change from infinite
to finite + wedge. One that would take considerable effort to change
and would be quite invasive to a lot more of the driver and which can
only be hit if the first timeout actually completed successfully and
is therefore of less importance anyway. Both of those time outs
appear to respect signal interrupts.
That's a policy decision so why would i915 wedge if an arbitrary
timeout expired? I915 is not controlling how much work there is
outstanding at the point the IGT decides to call DROP_IDLE.
Because this is a debug test interface that is used solely by IGT
after it has finished its testing. This is not about wedging the
device at some random arbitrary point because an AI compute workload
takes three hours to complete. This is about a very specific test
framework cleaning up after testing is completed and making sure the
test did not fry the system.
And even if an IGT test was calling DROP_IDLE in the middle of a test
for some reason, it should not be deliberately pushing 10+ seconds of
work through and then calling a debug only interface to flush it out.
If a test wants to verify that the system can cope with submitting a
minutes worth of rendering and then waiting for it to complete then
the test should be using official channels for that wait.
Plus, infinite wait is not a valid code path in the first place so
any change in behaviour is not really a change in behaviour. Code
can't be relying on a kernel call to never return for its correct
operation!
Why infinite wait wouldn't be valid? Then you better change the
other one as well. ;P
In what universe is it ever valid to wait forever for a test to
complete?
Well above you claimed both paths respect SIGINT. If that is so then
the wait is as infinite as the IGT wanted it to be.
See above, the PM code would require much more invasive changes. This
was low hanging fruit. It was supposed to be a two minute change to a
very self contained section of code that would provide significant
benefit to debugging a small class of very hard to debug problems.
Sure, but I'd still like to know why can't you do what you want from
the IGT framework.
Have the timeout reduction in i915, again that's fine assuming 10
seconds it enough to not break something by accident.
CI showed no regressions. And if someone does find a valid reason why a
post test drop caches call should legitimately take a stupidly long time
then it is easy to track back where the ETIME error came from and bump
the timeout.
With that change you already have broken the "infinite wait". It makes
the debugfs write return -ETIME in time much shorter than the test
runner timeout(s). What is the thing that you cannot do from IGT at
that point is my question? You want to wedge then? Send
DROP_RESET_ACTIVE to do it for you? If that doesn't work add a new
flag which will wedge explicitly.
We are again degrading into a huge philosophical discussion and all I
wanted to start with is to hear how exactly things go bad.
I have no idea what you are wanting. I am trying to have a technical
discussion about improving the stability of the driver during CI
testing. I have no idea if you are arguing that this change is good,
bad, broken, wrong direction or what.
Things go bad as explained in the commit message. The CI framework does
not use signals. The IGT framework does not use signals. There is no
watchdog that sends a TERM or KILL signal after a specified timeout. All
that happens is the IGT sits there forever waiting for the drop caches
IOCTL to return. The CI framework eventually gives up waiting for the
test to complete and tries to recover. There are many different CI
frameworks in use across Intel. Some timeout quickly, some timeout
slowly. But basically, they all eventually give up and don't bother
trying any kind of remedial action but just hit the reset button
(sometimes by literally power cycling the DUT). As result, background
processes that are saving dmesg, stdout, etc do not necessarily
terminate cleanly. That results in logs that are at best truncated, at
worst missing entirely. It also results in some frameworks aborting
testing at that point. So no results are generated for all the other
tests that have yet to be run. Some frameworks also run tests in
batches. All they log is that something, somewhere in the batch died. So
you don't even know which specific test actually hit the problem.
Can the CI frameworks be improved? Undoubtedly. In very many ways. Is
that something we have the ability to do with a simple patch? No. Would
re-writing the IGT framework to add watchdog mechanisms improve things?
Yes. Can it be done with a simple patch? No. Would a simple patch to
i915 significantly improve the situation? Yes. Will it solve every
possible CI hang? No. Will it fix any actual end user visible bugs? No.
Will it introduce any new bugs? No. Will it help us to debug at least
some CI failures? Yes.
John.
Regards,
Tvrtko
