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27d47ebce4
Schedule work at regular intervals to implement the steal_governor
policy loop, which monitors steal time and takes action on the state of
preferred CPUs. The interval is determined by the interval_ms parameter.
schedule_delayed_work() is used since interval_ms is on the order of
milliseconds and the work does not need to happen instantly.
Periodic policy loop essentially does:
- Gets the total/delta steal values and cpus to use steal_ratio.
- Calculate the steal_ratio as below.
steal_ratio = (delta_steal * 100*100)/(delta_ns * num_cpus())
It is calculated this way to consider the fractional values of steal
time. I.e 10 means 0.1% steal time. A few tricks such as
divide by 10,000 are used to avoid possible overflow.
- If steal ratio is higher than high threshold, call the method to reduce
the preferred CPUs.
- If steal ratio is lower or equal to low threshold, call the method to
increase the preferred CPUs.
- If the steal ratio falls in between, no action is taken.
- Ensures design constraints always met.
1. At least one core/CPU must be there in preferred mask.
2. preferred CPUs is subset of active CPUs.
If not met, then restore preferred CPUs to active and stop
requeue of the work. Driver is effectively non-functional after that.
Note that design checks are always performed. This helps avoid placing
driver-specific design constraints inside the core CPU hotplug mechanism.
User may offline specific set of CPUs that could leave the preferred
mask as empty. With the design check performed always, driver gracefully
shuts down upon detecting that edge case.
In order to help the above loop, a few helper functions have been added.
1. get_system_steal_time()
- steal governor takes global view of steal time instead of individual
vCPU. Collect the steal values across the vCPUs of interest.
- Sum up steal time values across possible CPUs. This helps to keep it
a monotonically increasing number and avoids spikes due to CPU
hotplug.
2. decrease_preferred_cpus()
- Called when there is high steal time. It needs to decide which CPUs to
mark as non-preferred.
- Get first housekeeping CPU and its core mask. Mark it as
protected core. This helps to keep at least one core as preferred.
(kernel ensures at least one housekeeping CPU stays active.)
- Find the last CPU outside of this protected core mask. i.e target CPU
- Based on that target CPU, get its sibling and mark them as
non-preferred.
3. increase_preferred_cpus()
- Called when there is low steal time. It needs to decide which CPUs to
mark as preferred and set that state.
- Get the first active non-preferred CPUs. This likely is the last
set of CPUs being marked as non-preferred.
- get the siblings of that CPU and mark them as preferred.
4. get_system_cpus()
- informs how many CPUs needs to be considered for steal_ratio
calculations.
- Since only active CPUs effectively contribute to steal time delta,
returns number of active CPUs. This also helps to avoid dilution of
thresholds in sparsely populated systems.
Notes:
1. Using core instead of individual CPUs performs better as SMT is
quite common and some hypervisor such as powerVM does core scheduling.
2. This doesn't do any NUMA splicing to keep the code simpler and
minimal overhead. Current code expects CPUs spread uniformly
across NUMA nodes.
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260928053728.797539-13-sshegde@linux.ibm.com
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