isolcpus is our friend too
Isolating some CPUs on the kernel command line using the isolcpus= option, in order to prevent the load balancer from offloading in-band work to them is not only a good idea with PREEMPT_RT, but for any dual kernel configuration too.
By doing so, having some random in-band work evicting cache lines on a CPU where real-time threads briefly sleep is less likely, increasing the odds of costly cache misses, which translates positively into the latency numbers you can get. Even if EVL’s small footprint core has a limited exposure to such kind of disturbance, saving a handful of microseconds is worth it when the worst case figure is already within tenths of microseconds.
CONFIG_DEBUG_HARD_LOCKS is cool but ruins real-time guarantees
CONFIG_DEBUG_HARD_LOCKS is enabled, the lock dependency engine
CONFIG_LOCKDEP) which helps in tracking down deadlocks and other
locking-related issues is also enabled for Dovetail’s hard
which underpins most of the serialization mechanisms the EVL core
This is nice as it has the lock validator monitor the hard spinlocks
EVL uses too. However, this comes with a high price latency-wise:
seeing hundreds of microseconds spent in the validator with hard
interrupts off from time to time is not uncommon. Running the latency
monitoring utility (aka
latmus) which is part of
libevl in this
configuration should give you pretty ugly numbers.
In short, it is fine enabling
CONFIG_DEBUG_HARD_LOCKS for debugging
some locking pattern in EVL, but you won’t be able to meet real-time
requirements at the same time in such configuration.
Enabling the ondemand CPUFreq governor - or any governor performing dynamic adjustment of the CPU frequency - may induce significant latency for EVL on your system, from ten microseconds to more than a hundred depending on the hardware. Selecting the so-called performance governor is the safe option, which guarantees that no frequency transition ever happens, keeping the CPUs at their maximum processing speed.
In other words, if
CONFIG_CPU_FREQ has to be enabled in your
CONFIG_CPU_FREQ_GOV_PERFORMANCE exclusively is most often the best way
to prevent unexpectedly high latency peaks.
CONFIG_SMP for best latency on single-core systems
On single-core hardware, some out-of-line code may still be executed for dealing with various types of spinlock with a SMP build, which translates into additional CPU branches and cache misses. On low end hardware, this overhead may be noticeable.
Therefore, if you neither need SMP support nor kernel debug options
which depend on instrumenting the spinlock constructs (e.g.
CONFIG_DEBUG_PREEMPT), you may want to disable all the related kernel
options, starting with
GCC 10.x might generate code causing the SMP boot process to break
early, as reported by this
post. As a work-around, you
CONFIG_STACKPROTECTOR_STRONG from your kernel
CONFIG_ACPI_PROCESSOR_IDLE may increase the latency upon wakeup on
IRQ from idle on some SoC (up to 30 us observed) on x86. This option
is implicitly selected by the following configuration chain:
CONFIG_ACPI_PROCESSOR. If out-of-range latency figures are observed
on your x86 hardware, turning off this chain may help.
When the HPET is disabled, the watchdog which monitors the sanity of
the current clocksource for the kernel may use refined-jiffies as
the reference clocksource to compare with. Unfortunately, such
clocksource is fairly imprecise for timekeeping since timer
interrupts might be missed. This could in turn trigger false
positives with the watchdog, which would end up declaring the TSC
clocksource as ‘unstable’. For instance, it has been observed that
CONFIG_FUNCTION_GRAPH_TRACER on some legacy hardware would
systematically cause such behavior at boot. The following warning
splat appearing in the kernel log is symptomatic of this problem:
clocksource: timekeeping watchdog on CPU0: Marking clocksource
'tsc-early' as unstable because the skew is too large:
clocksource: 'refined-jiffies' wd_now: fffb7018 wd_last: fffb6e9d
clocksource: 'tsc-early' cs_now: 68a6a7070f6a0 cs_last: 68a69ab6f74d6
tsc: Marking TSC unstable due to clocksource watchdog
This is a problem because the TSC is the best-rated
clocksource and directly accessible from the vDSO, which speeds
up timestamping operations. If the TSC on your hardware is known to be
fine and face this issue nevertheless, you may want to pass
tsc=nowatchdog to the kernel to prevent it, or even
if all TSCs are reliable enough to be synchronized across CPUs. If
the TSC is really unstable on some legacy hardware and you cannot
ignore the watchdog alert, you can still leave it to other
clocksources such as acpi_pm. Calls to evl_read_clock() would be
slower compared to a direct syscall-less readout from the vDSO, but
the EVL core would nevertheless manage to get timestamps from its
built-in clocks at the expense of
an out-of-band system call, without involving the in-band stage
though. You definitely want to make sure everything is right on your
platform with respect to reading timestamps by running the
latmus test, which
can detect any related issue.
You can retrieve the current clocksource used by the kernel as follows:
# cat /sys/devices/system/clocksource/clocksource0/current_clocksource
CONFIG_PERF is not an option, passing
nmi_watchodg=0 on the
kernel command line at boot may help.
nmi_watchodg=0 turns off the hard lockup detection for the
in-band kernel. However, EVL will still detect runaway EVL threads
stuck in out-of-band execution if
CONFIG_EVL_WATCHDOG is enabled.