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scx_rustland: improve scheduler cpu selection #57

Merged
merged 2 commits into from
Dec 30, 2023
Merged

scx_rustland: improve scheduler cpu selection #57

merged 2 commits into from
Dec 30, 2023

Commits on Dec 30, 2023

  1. scx_rustland: improve scheduler's idle CPU selection 

    The current CPU selection logic in the scheduler presents some
inefficiencies.

When a task is drained from the BPF queue, the scheduler immediately
checks whether the CPU previously assigned to the task is still idle,
assigning it if it is. Otherwise, it iterates through available CPUs,
always starting from CPU #0, and selects the first idle one without
updating its state. This approach is consistently applied to the entire
batch of tasks drained from the BPF queue, resulting in all of them
being assigned to the same idle CPU (also with a higher likelihood of
allocation to lower CPU ids rather than higher ones).

While dispatching a batch of tasks to the same idle CPU is not
necessarily problematic, a fairer distribution among the list of idle
CPUs would be preferable.

Therefore change the CPU selection logic to distribute tasks equally
among the idle CPUs, still maintaining the preference for the previously
used one. Additionally, apply the CPU selection logic just before tasks
are dispatched, rather than assigning a CPU when tasks are drained from
the BPF queue. This adjustment is important, because tasks may linger in
the scheduler's internal structures for a bit and the idle state of the
CPUs in the system may change during that period.

Signed-off-by: Andrea Righi <[email protected]>
    Andrea Righi committed Dec 30, 2023
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  2. scx_rustland: check CPU selection validity 

    When the scheduler decides to assign a different CPU to the task always
make sure the assignment is valid according to the task cpumask. If it's
not valid simply dispatch the task to the global DSQ.

This prevents the scheduler from exiting with errors like this:

  09:11:02 [WARN] EXIT: SCX_DSQ_LOCAL[_ON] verdict target cpu 7 not allowed for gcc[440718]

In the future we may want move this check directly into the user-space
scheduler, but for now let's keep this check in the BPF dispatcher as a
quick fix.

Signed-off-by: Andrea Righi <[email protected]>
    Andrea Righi committed Dec 30, 2023
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