On 11/8/2022 01:08, Tvrtko Ursulin wrote:
On 07/11/2022 19:45, John Harrison wrote:
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.
To unblock, I suggest you go with the patch which caps the wait only,
and propose a wedging as an IGT patch to gem_quiescent_gpu(). That
should involve the CI/IGT folks into discussion on what logs will be,
or will not be collected once gem_quiescent_gpu() fails due -ETIME. In
fact probably you should copy CI/IGT folks on the v2 of the i915 patch
as well since I now think their acks would be good to have - from the
point of view of the current test runner behaviour with hanging tests.
Simply returning -ETIME without wedging will actually make the situation
worse. At the moment, you get 'all testing stopped due to machine not
responding' bugs being logged. Which is a right pain and has very little
useful information, but at least is not claiming random tests are broken
when they are not. If you return ETIME without wedging then test A will
hang and return ETIME. CI will log an ETIME bug against test A. CI will
then try test B, which will fail with ETIME because the system is still
broken but claiming to be working. So log a new bug against test B. Move
on to test C, oh look, ETIME - log another bug and move on to test D...
That is far worse, a whole slew of pointless and incorrect bugs have
just been logged.
And how is it possibly considered a backwards breaking or dangerous
change to wedge instead of hanging forever? Reboot versus wedge.
Absolutely no defined behaviour at all because the system has simply
stopped versus marking the system as broken and having a best effort at
handling the situation. Yup, that's definitely a very dangerous change
that could break all sorts of random user applications.
Re 'IGT folks' - whom? Ashutosh had already agreed to the original patch.
And CI folks are certainly aware of such issues. There are any number of
comments in Jiras about 'no logs available, cannot analyse'.
John.
Regards,
Tvrtko
