From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <529EFB3D.6090900@xenomai.org> Date: Wed, 04 Dec 2013 10:51:57 +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> In-Reply-To: <529EF89E.6000302@xenomai.org> Content-Type: text/plain; charset="utf-8"; format="flowed" 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 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. -- Gilles.