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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: IRQ latency measurements in hypervisor
Hi Stefano, Stefano Stabellini writes: [...] >> >> 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. > > - 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. > > >> 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. >> 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. >> 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. > >> 3. Huge latency spike during domain creation. I conducted some >> additional tests, including use of PV drivers, but this didn't >> affected the latency in my "real time" domain. But attempt to >> create another domain with relatively large memory size of 2GB led >> to huge spike in latency. Debugging led to this call path: >> >> XENMEM_populate_physmap -> populate_physmap() -> >> alloc_domheap_pages() -> alloc_heap_pages()-> huge >> "for ( i = 0; i < (1 << order); i++ )" loop. >> >> This loops handles struct page* for every one of 262144 pages that >> was allocated by calling populate_physmap(). This function is not >> preemptible and it takes about a second to complete on my >> setup. Needless to say that it completely screws any responsiveness >> of the hypervisor. >> >> I managed to overcome the issue #3 by commenting out all calls to >> populate_one_size() except the populate_one_size(PFN_4K_SHIFT) in >> xg_dom_arm.c. This lengthened domain construction, but my "RT" domain >> didn't experienced so big latency issues. Apparently all other >> hypercalls which are used during domain creation are either fast or >> preemptible. No doubts that my hack lead to page tables inflation and >> overall performance drop. > > I think we need to follow this up and fix this. Maybe just by adding > a hypercall continuation to the loop. As Julien pointed out, that is hardly possible with the current code. But we need to find some way to fix this. [...] -- Volodymyr Babchuk at EPAM
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