From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <529F1974.60900@xenomai.org> Date: Wed, 04 Dec 2013 13:00:52 +0100 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <40A5BE95-8E78-4CD6-81D2-C97AA7A58FBB@open.ac.uk> <529DCF2F.1070702@xenomai.org> <1507DF58-4A8D-42E0-92B8-4A9EAB4289E3@open.ac.uk> <529DDB58.3090709@xenomai.org> <5B55252A-19D2-4A0D-82BE-FC77BFA6AEE1@open.ac.uk> <529DFEC3.1050106@xenomai.org> <90F2A7A6-5B5E-4A25-8D9D-3D50D0EC0826@open.ac.uk> <529E2801.5060505@xenomai.org> <529EEB7C.4090308@xenomai.org> <529EED06.4010108@xenomai.org> <529EF58A.8030003@xenomai.org> <529EF680.1040108@xenomai.org> <529EF89E.6000302@xenomai.org> <529EFB3D.6090900@xenomai.org> <529F03FC.8040409@xenomai.org> <529F04DD.2070201@xenomai.org> <529F0C48.20705@xenomai.org> <529F0DBC.9080905@xenomai.org> <529F13A1.5070403@xenomai.org> <529F1913.4030604@xenomai.org> In-Reply-To: <529F1913.4030604@xenomai.org> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Subject: Re: [Xenomai] latency spikes under load List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Philippe Gerum Cc: Kurijn Buys , Xenomai@xenomai.org On 12/04/2013 12:59 PM, Philippe Gerum wrote: > On 12/04/2013 12:36 PM, Philippe Gerum wrote: >> On 12/04/2013 12:10 PM, Gilles Chanteperdrix wrote: >>> On 12/04/2013 12:04 PM, Philippe Gerum wrote: >>>> On 12/04/2013 11:33 AM, Philippe Gerum wrote: >>>>> On 12/04/2013 11:29 AM, Philippe Gerum wrote: >>>>>> On 12/04/2013 10:51 AM, Gilles Chanteperdrix wrote: >>>>>>> On 12/04/2013 10:40 AM, Philippe Gerum wrote: >>>>>>>> On 12/04/2013 10:31 AM, Gilles Chanteperdrix wrote: >>>>>>>>> On 12/04/2013 10:27 AM, Philippe Gerum wrote: >>>>>>>>>> On 12/04/2013 09:51 AM, Gilles Chanteperdrix wrote: >>>>>>>>>>> On 12/04/2013 09:44 AM, Philippe Gerum wrote: >>>>>>>>>>>> On 12/03/2013 07:50 PM, Gilles Chanteperdrix wrote: >>>>>>>>>>>>> On 12/03/2013 05:49 PM, Kurijn Buys wrote: >>>>>>>>>>>>>> Op 3-dec.-2013, om 15:54 heeft Gilles Chanteperdrix het >>>>>>>>>>>>>> volgende >>>>>>>>>>>>>> geschreven: >>>>>>>>>>>>>> >>>>>>>>>>>>>>> On 12/03/2013 04:31 PM, Kurijn Buys wrote: >>>>>>>>>>>>>>>> Op 3-dec.-2013, om 13:23 heeft Gilles Chanteperdrix het >>>>>>>>>>>>>>>> volgende >>>>>>>>>>>>>>>> geschreven: >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> On 12/03/2013 02:07 PM, Kurijn Buys wrote: >>>>>>>>>>>>>>>>>> Thanks for the quick response, ACPI is enabled, I only >>>>>>>>>>>>>>>>>> disabled >>>>>>>>>>>>>>>>>> "Processor" in there... -1 was a typo indeed, it is at >>>>>>>>>>>>>>>>>> 1... I >>>>>>>>>>>>>>>>>> see SCHED_SMT [=y] in my kernel config... shall I >>>>>>>>>>>>>>>>>> recompile >>>>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>>>> kernel with this disabled then... no other things to try >>>>>>>>>>>>>>>>>> first/at >>>>>>>>>>>>>>>>>> the same time? >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> To remove hyperthreading, either: - disable it in the BIOS >>>>>>>>>>>>>>>>> configuration; - or disable CONFIG_SMP (not SCHED_SMPT) >>>>>>>>>>>>>>>>> in the >>>>>>>>>>>>>>>>> kernel configuration. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Ah I see, CONFIG_SMP is also enabled... I've disabled it in >>>>>>>>>>>>>>>> BIOS, but >>>>>>>>>>>>>>>> no success (tell me if it is worth trying to disable it in >>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>> kernel >>>>>>>>>>>>>>>> config in stead). >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> When you say "no success", you mean you still have 2 cpus >>>>>>>>>>>>>>> ? Or >>>>>>>>>>>>>>> you >>>>>>>>>>>>>>> still >>>>>>>>>>>>>>> have latency pikes? If the former, then yes, try without >>>>>>>>>>>>>>> CONFIG_SMP, or >>>>>>>>>>>>>>> pass nr_cpus=1 on the command line. If the latter, then no, >>>>>>>>>>>>>>> testing >>>>>>>>>>>>>>> without CONFIG_SMP is useless. >>>>>>>>>>>>>> >>>>>>>>>>>>>> the second: still latency... >>>>>>>>>>>>>> (lscpu says there is only 1 cpu now) >>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> I realized that the test with sched_rt_runtime_us on -1 I >>>>>>>>>>>>>>>>>> performed was with an earlier set-up. When I set it now to >>>>>>>>>>>>>>>>>> -1, I >>>>>>>>>>>>>>>>>> have better performance, but: 1) still spikes of up to >>>>>>>>>>>>>>>>>> 87us >>>>>>>>>>>>>>>>>> under >>>>>>>>>>>>>>>>>> load with ./latency 2) still some completely shifted >>>>>>>>>>>>>>>>>> occurrences >>>>>>>>>>>>>>>>>> with the other latency test, with a 1000µs period (but now >>>>>>>>>>>>>>>>>> only 2 >>>>>>>>>>>>>>>>>> out of 890814), and the rest of the distribution lies in >>>>>>>>>>>>>>>>>> [861-1139]µs, which is also rather large I suppose. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> sched_rt_runtime_us should not make any difference. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Something else you should try is to disable root thread >>>>>>>>>>>>>>>>> priority >>>>>>>>>>>>>>>>> coupling. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> I have tried a config with priority coupling support >>>>>>>>>>>>>>>> disabled >>>>>>>>>>>>>>>> before, >>>>>>>>>>>>>>>> but then the system was even more vulnerable for such >>>>>>>>>>>>>>>> latency >>>>>>>>>>>>>>>> peaks >>>>>>>>>>>>>>>> (however the mean latency was a little lower!) (I still have >>>>>>>>>>>>>>>> the >>>>>>>>>>>>>>>> kernel, but unfortunately the I-pipe tracer isn't installed >>>>>>>>>>>>>>>> there) >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Please keep priority coupling disabled in further tests. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> The ipipe trace after test (1) was similar to the one I >>>>>>>>>>>>>>>>>> posted, >>>>>>>>>>>>>>>>>> where this line seems to be the problem I suppose: :| >>>>>>>>>>>>>>>>>> #end >>>>>>>>>>>>>>>>>> 0x80000001 -179! 149.235 ipipe_check_context+0x87 >>>>>>>>>>>>>>>>>> (add_preempt_count+0x15) >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> ...I hoped the I-pipe trace would help..? >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Unfortunately the trace is not helping much. >>>>>>>>>>>>>> >>>>>>>>>>>>>> If it would help, I've another trace (joint as txt) where the >>>>>>>>>>>>>> following line seems to indicate a problem: >>>>>>>>>>>>>> : +func -141! 117.825 >>>>>>>>>>>>>> i915_gem_flush_ring+0x9 >>>>>>>>>>>>>> [i915] (i915_gem_do_execbuffer+0xb46 [i915]) >>>>>>>>>>>>>> -- The Open University is incorporated by Royal Charter (RC >>>>>>>>>>>>>> 000391), >>>>>>>>>>>>>> an exempt charity in England & Wales and a charity >>>>>>>>>>>>>> registered in >>>>>>>>>>>>>> Scotland (SC 038302). >>>>>>>>>>>>> >>>>>>>>>>>>> Ah this is a known issue then. I traced back this issue some >>>>>>>>>>>>> time >>>>>>>>>>>>> ago, >>>>>>>>>>>>> and from what I understood on the rt-users mailing list it is >>>>>>>>>>>>> fixed on >>>>>>>>>>>>> more recent kernels. So, I would advise to update to 3.10.18 >>>>>>>>>>>>> branch, >>>>>>>>>>>>> available here by git: >>>>>>>>>>>> >>>>>>>>>>>> Incidentally, I've been chasing a latency issue on x86 involving >>>>>>>>>>>> the >>>>>>>>>>>> i915 chipset recently on 3.10, >>>>>>>>>>> >>>>>>>>>>> was it 3.10 or 3.10.18 ? >>>>>>>>>>> >>>>>>>>>> >>>>>>>>>> http://git.xenomai.org/ipipe.git/log/?h=ipipe-3.10 >>>>>>>>>> >>>>>>>>>> which is currently 3.10.18. >>>>>>>>>> >>>>>>>>>>>> and it turned out that we were still >>>>>>>>>>>> badly hit by wbinvd instructions, emitted on _all_ cores via an >>>>>>>>>>>> IPI in >>>>>>>>>>>> the GEM control code, when the LLC cache is present. >>>>>>>>>>>> >>>>>>>>>>>> The jitter incurred by invalidating all internal caches exceeds >>>>>>>>>>>> 300 us >>>>>>>>>>>> in my test case, so it seems that we are not there yet. >>>>>>>>>>> >>>>>>>>>>> Ok, maybe the preempt_rt workaround is only enabled for >>>>>>>>>>> CONFIG_PREEMPT_RT? In which case we can try and import the >>>>>>>>>>> patch in >>>>>>>>>>> the >>>>>>>>>>> I-pipe. >>>>>>>>>>> >>>>>>>>>> >>>>>>>>>> Looking at the comment in the GEM code, this invalidation is >>>>>>>>>> required to >>>>>>>>>> flush transactions before updating the fence register. >>>>>>>>>> >>>>>>>>> >>>>>>>>> From what I understood, the preempt_rt patch asks users to pin >>>>>>>>> the X >>>>>>>>> server on one cpu and disables the IPI, so the invalidation can >>>>>>>>> be run >>>>>>>>> on only one cpu. That said, if that had solved the issue, Kurijn >>>>>>>>> would >>>>>>>>> not have observed the latency spikes when running with only one >>>>>>>>> cpu. >>>>>>>>> >>>>>>>> >>>>>>>> if (HAS_LLC(obj->base.dev)) >>>>>>>> on_each_cpu(i915_gem_write_fence__ipi, NULL, 1); >>>>>>>> >>>>>>>> So this will run on every CPU regardless of the number of CPUs, in >>>>>>>> sync >>>>>>>> mode. In addition, this section is interrupt-enabled. Some of my >>>>>>>> tests >>>>>>>> were conducted in UP mode to make sure we did not face a locking >>>>>>>> latency >>>>>>>> inherited from another core, like we had with the APIC madness in >>>>>>>> the >>>>>>>> early days, and the jitter was still right there. I don't see much >>>>>>>> hope. >>>>>>>> >>>>>>> >>>>>>> I have not read the preempt_rt patch, only the announces. But for >>>>>>> instance, in the 3.8.13-rt12 patch announce, I read: >>>>>>> >>>>>>> - added an option to the i915 driver to disable the expensive >>>>>>> wbinvd. A >>>>>>> warning is printed once on RT if wbinvd is not disabled to let >>>>>>> the >>>>>>> user know about this problem. This problem was decoded by Carsten >>>>>>> Emde. >>>>>>> >>>>>>> >>>>>> >>>>>> This is documented as a plain reversal of the former change aimed at >>>>>> fixing non-coherence issues with fence updates: >>>>>> >>>>>> From 22d61b535bbb5f2b65bfe564d16b0d2b4413535a Mon Sep 17 00:00:00 >>>>>> 2001 >>>>>> From: Chris Wilson >>>>>> Date: Wed, 10 Jul 2013 13:36:24 +0100 >>>>>> Subject: [PATCH 003/293] Revert "drm/i915: Workaround incoherence >>>>>> between >>>>>> fences and LLC across multiple CPUs" >>>>>> >>>>>> This reverts commit 25ff119 and the follow on for Valleyview commit >>>>>> 2dc8aae. >>>>>> >>>>> >>>>> That one seems to be suggested as a cheaper replacement for the ugly >>>>> wbinvd, we should have a look at it: >>>>> >>>>> drm/i915: Fix incoherence with fence updates on Sandybridge+ >>>>> >>>> >>>> We do have this one in 3.10.18, but not the reversal of the former >>>> workaround which produces jitter. >>>> >>>> http://www.spinics.net/lists/stable-commits/msg27025.html >>>> >>> From here: >>> http://www.osadl.org/Examples-of-latency-regressions.latest-stable-test-latency.0.html >>> >>> >>> >>> It seems this patch is even creating a regression. >>> >> >> Yes, in addition according to Chris Wilson, it did not actually fix the >> root issue, but only papered over it, making the bug less likely to >> happen when serializing the fence register updates among CPUs. It looks >> like we really want to drop it in ipipe-3.8, unless it is queued in >> -stable there. Did not check. >> > > I have a smoke test running over a patched kernel implementing the right > fixup instead of the former workaround. Latency is ok so far. I'll leave > this running a few hours more and see what happens. > Ok, could you push the branch somewhere so that I can try it? -- Gilles.