[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: IRQ latency measurements in hypervisor
On Fri, 15 Jan 2021, Volodymyr Babchuk wrote: > >> ARMv8 platform. Namely Renesas Rcar H3 SoC on Salvator board. > >> > >> To accurately determine latency, I employed one of timer counter units > >> (TMUs) available on the SoC. This is 32-bit timer with auto-reload, > >> that can generate interrupt on underflow. I fed it with 33.275MHz > >> clock, which gave me resolution of about 30ns. I programmed the timer > >> to generate interrupt every 10ms. My ISR then read the current timer > >> value and determined how much time passed since last underrun. This > >> gave me time interval between IRQ generation and ISR invocation. > >> Those values were collected and every 10 seconds statistics was > >> calculated. There is an example of output from my Linux driver: > > > > It looks like a solid approach to collect results, similar to the one we > > used for the cache coloring work. Just make sure to collect very many > > results. > > > > A few of questions: did you use a single physical CPU? Are you using > > RTDS and schedule 2 vCPU on 1 pCPU? Is dom0 idle or busy? I take the > > results were measured in domU? > Yes, I used single pCPU, which ran 1-2 vCPUs depending on setup. At > first, this was only Dom0 with 1 vCPU, but in most cases it was Dom0 + > Zephyr DomU each with 1 vCPU. > > I quickly determined that 1 vCPU per 1 pCPU case meets my > requirements. On other hand, our real cases will always have more vCPUs > than pCPUs, so I was interested in 2 vCPUs - 1 pCPU case. > > > > >> [ 83.873162] rt_eval_tmu e6fc0000.tmu: Mean: 44 (1320 ns) stddev: 8 (240 > >> ns) > >> [ 94.136632] rt_eval_tmu e6fc0000.tmu: Mean: 44 (1320 ns) stddev: 8 (240 > >> ns) > >> [ 104.400098] rt_eval_tmu e6fc0000.tmu: Mean: 50 (1500 ns) stddev: 129 > >> (3870 ns) > >> [ 114.663828] rt_eval_tmu e6fc0000.tmu: Mean: 44 (1320 ns) stddev: 8 (240 > >> ns) > >> [ 124.927296] rt_eval_tmu e6fc0000.tmu: Mean: 56 (1680 ns) stddev: 183 > >> (5490 ns) > >> > >> This is the baremetal Linux. And there is Dom0: > >> > >> [ 237.431003] rt_eval_tmu e6fc0000.tmu: Mean: 306 (9180 ns) stddev: 25 > >> (750 ns) > >> [ 247.694506] rt_eval_tmu e6fc0000.tmu: Mean: 302 (9060 ns) stddev: 17 > >> (510 ns) > >> > >> Driver outputs both the raw timer value (eg. 4) and the same value > >> scaled to nanoseconds (eg. 1320 ns). As you can see baremetal setup is > >> much faster. But experiments showed that Linux does not provide > >> consistent values, even when running in baremetal mode. You can see > >> sporadic spikes in "std dev" values. > > > > So baremetal IRQ latency is 1320-1680ns and Linux IRQ latency is > > 9060-9180ns. I am not surprised that Linux results are inconsistent but > > I have a couple of observations: > > > > - 9us is high for Linux > > If the system is idle, the latency should be lower, around 2-3us. I > > imagine you are actually running some sort of interference from dom0? Or > > using RTDS and descheduling vCPUs? > > 9us was in idle state. Interestingly enough, I got latency if 3us while > Dom0 was doing some CPU-intensive tasks. So, under load latency is lower > than in idle state. I didn't investigated this, so I can't tell you what > causes this behavior. Did you use vwfi=native? You should definitely be able to see ~3us without interference and with 1vCPU <-> 1pCPU > > - the stddev of 3870ns is high for baremetal > > In the baremetal case the stddev should be minimal if the system is > > idle. > > This is what I expected too. But nevertheless there was spikes. I didn't > investigated this as well, so I can only speculate there. My rootfs is > on NFS, so maybe network driver caused this spikes. Yeah, maybe it would be best to reduce the sources of possible spikes and get rid of NFS. > > > > > >> So my next step was to use proper RT OS to do the measurements. I > >> chose Zephyr. My PR that adds Xen support to Zephyr can be found at > >> [1]. Support for RCAR Gen3 is not upstreamed, but is present on my > >> GitHub([2]). At [3] you can find the source code for application that > >> does the latency measurements. It behaves exactly as my linux driver, > >> but provides a bit more information: > >> > >> *** Booting Zephyr OS build zephyr-v2.4.0-2750-g0f2c858a39fc *** > >> RT Eval app > >> > >> Counter freq is 33280000 Hz. Period is 30 ns > >> Set alarm in 0 sec (332800 ticks) > >> Mean: 600 (18000 ns) stddev: 3737 (112110 ns) above thr: 0% [265 (7950 ns) > >> - 66955 (2008650 ns)] global [265 (7950 ns) 66955 (2008650 ns)] > >> Mean: 388 (11640 ns) stddev: 2059 (61770 ns) above thr: 0% [266 (7980 ns) > >> - 58830 (1764900 ns)] global [265 (7950 ns) 66955 (2008650 ns)] > >> Mean: 358 (10740 ns) stddev: 1796 (53880 ns) above thr: 0% [265 (7950 ns) > >> - 57780 (1733400 ns)] global [265 (7950 ns) 66955 (2008650 ns)] > > This is Zephyr running as DomU. > > >> ... > >> > >> So there you can see: mean time, standard deviation, % of interrupts > >> that was processed above 30us threshold, minimum and maximum latency > >> values for the current 10s run, global minimum and maximum. > >> > >> Zephyr running as baremetal showed very stable results (this is an > >> older build, so no extended statistics there): > >> > >> ## Starting application at 0x480803C0 ... > >> *** Booting Zephyr OS build zephyr-v2.4.0-1137-g5803ee1e8183 *** > >> RT Eval app > >> > >> Counter freq is 33280000 Hz. Period is 30 ns > >> Mean: 31 (930 ns) stddev: 0 (0 ns) > >> Mean: 31 (930 ns) stddev: 0 (0 ns) > >> Mean: 31 (930 ns) stddev: 0 (0 ns) > >> Mean: 31 (930 ns) stddev: 0 (0 ns) > >> Mean: 31 (930 ns) stddev: 0 (0 ns) > >> Mean: 31 (930 ns) stddev: 0 (0 ns) > > And this is Zephyr is running as baremetal. > > >> ... > >> > >> As Zephyr provided stable readouts with no jitter, I used it to do all > >> subsequent measurements. > > > > I am a bit confused here. Looking at the numbers above the stddev is > > 112110 ns in the first instance. That is pretty high. Am I looking at > > the wrong numbers? > > I added some clarification above. As for 112110ns in the very first instance > - I always ignored the first instance, assuming that things need to > settle after domain being created. > > But your comment is actually correct: what exacelt should "settle"? > Domain is already created. All should run smoothly. So, this is worth > investigating. It is fair to ignore the first 2 measurements. However, the numbers above have high stddev even at the 3rd measurement: 53us is high and above the target 30us. I take you didn't apply yet the alloc_heap_pages and the serial workarounds? > >> IMPORTANT! All subsequent tests was conducted with only 1 CPU core > >> enabled. My goal was to ensure that system can timely react to an > >> external interrupt even under load. > > > > All right. FYI I have no frame of reference for 2 vCPUs on 1 pCPUs, all > > my tests were done with 1vCPU <-> 1pCPU and the null scheduler. > > As I said, I had no issues with 1vCPU <-> 1pCPU setup, so I quickly > moved to cases which had issues. > > >> Test results and latency sources > >> > >> As you can see, baremetal OS provides latency of ~0.9us without any > >> deviations. The same code running as DomU on idle system shows mean > >> latency of 12us and deviation of about 10us. Range of latencies in a > >> 10s batch can vary from 8us to 25us. This fits into required 30us > >> threshold, so no big issue there. > >> > >> But this worsens under certain conditions. > >> > >> 1. Serial console. RCAR serial driver (scif) works in synchronous > >> mode, so any heavy serial console output leads to higher > >> latency. Tests shows mean latency of 1000us and deviation of 1332 > >> us. 54% of interrupts are handled outside of 30us period. Real > >> values may be even higher, because in some cases timer would do > >> full cycle and instead of say 11ms ISR would read 11 % 10 = 1ms > >> latency. I tried to enable asynchronous mode for the serial > >> console. This made things better, but it lead to gaps in output, so > >> I just turned the serial console off completely. > > > > That's very interesting. I wonder if other serial drivers would cause > > similar issues, e.g. PL011. > > They should. This behavior is programmed in serial.c. Driver can enable > async mode calling serial_async_transmit(). As I can see, only ns16550 > driver does this. > > Maybe you didn't saw problems there because you had more pCPU enabled > and Xen used some other pCPU to do UART operations. Good to know. A workaround would be to let Dom0/DomU have direct access to the UART, not giving any UART to Xen at all. > >> 2. RTDS scheduler. With console disabled, things like "hexdump -v > >> /dev/zero" didn't affected the latency so badly, but anyways, > >> sometimes I got ~600us spikes. This is not a surprise, because of > >> default RTDS configuration. I changed period for DomU from default > >> 10ms to 100us and things got better: with Dom0 burning CPU I am > >> rarely getting max latency of about ~30us with mean latency of ~9us > >> and deviation of ~0.5us. On other hand, when I tried to set period > >> to 30us, max latency rose up to ~60us. > > > > This is very interestingi too. Did you get any spikes with the period > > set to 100us? It would be fantastic if there were none. > > As far as I can remember, there wasn't any. At least in my setup. And > with workaround for alloc_heap_pages() issue. Of course, I didn't > covered all the possible use cases. But in settled state, when all > domains are created and they are just doing owns tasks there was no > spikes. That's very good to hear
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