[Intel-gfx] [PATCH] drm/i915: Don't wait forever in drop_caches
John Harrison
john.c.harrison at intel.com
Tue Nov 8 19:37:41 UTC 2022
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 at Intel.com wrote:
>>>>>>>>>>>> From: John Harrison <John.C.Harrison at Intel.com>
>>>>>>>>>>>>
>>>>>>>>>>>> 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 at Intel.com>
>>>>>>>>>>>> ---
>>>>>>>>>>>> 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
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