[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: IRQ latency measurements in hypervisor
+ Bertrand, Andrew (see comment on alloc_heap_pages()) Really great work, Volodymyr! Some comments below. On Tue, 12 Jan 2021, Volodymyr Babchuk wrote: > Hello community, > > Recently I was experimenting with context switching latencies in > Xen. I want to share my findings and discuss some issues I have found. > > My target was to ensure that no more 30us passes between interrupt > generation and handling in a guest VM. Main goal was to determine if > Xen is ready for soft RT and to identify main issues that prevent > this. > > TLDR: I was able to achieve mean 15us latency with tuned RTDS > scheduler, disabled serial console and some hacks to the Xen > toolstack. This was done on a setup with 1 PCPU. > > Test setup. > > 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? > [ 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? - the stddev of 3870ns is high for baremetal In the baremetal case the stddev should be minimal if the system is idle. > 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)] > ... > > 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) > ... > > 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? > 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. > 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. > 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. > 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. > Conclusions. > > I wanted to: > > 1. Share my approach to IRQ latency measurements. > > 2. Share results of said measurements. Thank you very much. I asked a couple of questions above to better understand your setup. > 3. Ask for other potential scenarios, which can affect IRQ latency so I > can test them on my setup. I'll follow up once I understood the above a bit better. > 4. Discuss the issue with alloc_domheap_pages(). I brooded over ways > to make it preemptible. But it is a) located deep in call chain and > b) used not only by hypercalls. So I can't see an easy way to make > it preemptible. The best solution would be of course to make the > whole hypervisor preemptible. I even heard about some work long > time ago... > > 5. Show that under certain conditions Xen can be used in a soft RT > system. Of course, additional tests are needed, but with some minor > tweaks it can provide ~15us IRQ latency on a typical workload. > > [1] https://github.com/zephyrproject-rtos/zephyr/pull/30164 > [2] https://github.com/lorc/zephyr/tree/renesas_salvator_x > [3] https://github.com/lorc/zephyr/blob/rt_eval/samples/rt_eval/src/main.c
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