scx_bpfland: small improvements #429
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Always assign the maximum time slice if there are idle CPUs in the system. Otherwise, double the task's unused time slice to reward tasks that use less CPU time and at the same time refill the time slice of the tasks every time they're dispatched. Signed-off-by: Andrea Righi <[email protected]>
Refine the safeguard mechanism to avoid generating too many interactive tasks in the system, which could nullify the effect of the interactive/regular task classification. The safeguard mechanism operates by pausing the promotion of new tasks to interactive status during the task wake-up process, whenever the number of interactive tasks in the priority queue exceeds a specific limit (set to 4x the number of online CPUs). Halting the promotion of additional interactive tasks allows to prioritize those already classified as interactive, thereby preventing potential "bursts" of excessive interactive tasks in the system. This refines the mitigation already provided by commit 640bd56 ("scx_bpfland: prevent tasks from abusing interactive priority boost"). Fixes: 640bd56 ("scx_bpfland: prevent tasks from abusing interactive priority boost") Signed-off-by: Andrea Righi <[email protected]>
Initialize the number of voluntary context switches metrics in the local task storage. Signed-off-by: Andrea Righi <[email protected]>
Instead of constantly checking the need to drain tasks from the DSQs of the offline CPUs, provide an atomic flag to notify when there are tasks to be drained from the offline CPUs. Signed-off-by: Andrea Righi <[email protected]>
We can rely on scx_bpf_nr_cpu_ids() to create all the possible per-CPU DSQs, eliminating the need for the hard-coded limit MAX_CPUS. In this way scx_bpfland can support the same amount of CPUs that the kernel can handle. Signed-off-by: Andrea Righi <[email protected]>
htejun
approved these changes
Jul 14, 2024
| * valid. | ||
| */ | ||
| static u64 cpu_to_dsq(s32 cpu) | ||
| { | ||
| if (cpu < 0 || cpu >= MAX_CPUS) { | ||
| u64 cpu_max = scx_bpf_nr_cpu_ids(); |
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Because this gets surprisingly confusing sometimes, I wonder whether it'd be beneficial if we all agree to use nr_cpu_ids consistently.
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Yes, good point, I'll change it to nr_cpu_ids, so it's more consistent with the rest of the code.
| offline = offline_cpumask; | ||
| if (!offline) | ||
| return 0; | ||
| if (bpf_cpumask_test_cpu(cpu, cast_mask(offline))) { |
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Hmm... this is a bit tricky. I think this can be a lot simpler if we had a way to wait a RCU grace period.
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It is quite ugly, also offline_cpumask is allocated once at init and it goes away only when the scheduler exits (it's never re-allocated), this is just to make the verifier happy...
We always use nr_cpu_ids to represent the maximum CPU id returned by scx_bpf_nr_cpu_ids(). Replace cpu_max with nr_cpu_ids to be more consistent with the rest of the code. Signed-off-by: Andrea Righi <[email protected]>
htejun
approved these changes
Jul 15, 2024
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Small improvements for the CPU hotplug support, time slice evaluation and in the logic to classify interactive/regular tasks. No major improvement in this PR, mostly small fixes/adjustments to enhance the quality of the code.