Re: IRQ latency measurements in hypervisor

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

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