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As a result, schedulers > often implement their own idle CPU selection policies, which are typically > similar to one another, leading to a lot of code duplication. > > To address this, extend the built-in idle CPU selection policy introducing > the concept of allowed CPUs. > > With this concept, BPF schedulers can apply the built-in idle CPU selection > policy to a subset of allowed CPUs, allowing them to implement their own > hard/soft-affinity rules while still using the topology optimizations of > the built-in policy, preventing code duplication across different > schedulers. > > To implement this introduce a new helper kfunc scx_bpf_select_cpu_and() > that accepts a cpumask of allowed CPUs: > > s32 scx_bpf_select_cpu_and(struct task_struct *p, s32 prev_cpu, > u64 wake_flags, > const struct cpumask *cpus_allowed, u64 flags); > > Example usage > ============= > > s32 BPF_STRUCT_OPS(foo_select_cpu, struct task_struct *p, > s32 prev_cpu, u64 wake_flags) > { > const struct cpumask *cpus = task_allowed_cpus(p) ?: p->cpus_ptr; > s32 cpu; Andrea, I'm curious why cannot this expression simply be moved into the default select implementation? And then for those that need a more custom mask, we can do the scx_bpf_select_cpu_and() as a second step. Also I think I am missing, what is the motivation in the existing code to not do LLC/NUMA-only scans if the task is restrained? Thanks for clarifying. thanks, - Joel > > cpu = scx_bpf_select_cpu_and(p, prev_cpu, wake_flags, cpus, 0); > if (cpu >= 0) { > scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0); > return cpu; > } > > return prev_cpu; > } > > Results > ======= > > Load distribution on a 4 sockets / 4 cores per socket system, simulated > using virtme-ng, running a modified version of scx_bpfland that uses the > new helper scx_bpf_select_cpu_and() and 0xff00 as allowed domain: > > $ vng --cpu 16,sockets=4,cores=4,threads=1 > ... > $ stress-ng -c 16 > ... > $ htop > ... > 0[ 0.0%] 8[||||||||||||||||||||||||100.0%] > 1[ 0.0%] 9[||||||||||||||||||||||||100.0%] > 2[ 0.0%] 10[||||||||||||||||||||||||100.0%] > 3[ 0.0%] 11[||||||||||||||||||||||||100.0%] > 4[ 0.0%] 12[||||||||||||||||||||||||100.0%] > 5[ 0.0%] 13[||||||||||||||||||||||||100.0%] > 6[ 0.0%] 14[||||||||||||||||||||||||100.0%] > 7[ 0.0%] 15[||||||||||||||||||||||||100.0%] > > With scx_bpf_select_cpu_dfl() tasks would be distributed evenly across all > the available CPUs. > > ChangeLog v4 -> v5: > - simplify the code to compute (and) task's temporary cpumasks > > ChangeLog v3 -> v4: > - keep p->nr_cpus_allowed optimizations (skip cpumask operations when the > task can run on all CPUs) > - allow to call scx_bpf_select_cpu_and() also from ops.enqueue() and > modify the kselftest to cover this case as well > - rebase to the latest sched_ext/for-6.15 > > ChangeLog v2 -> v3: > - incrementally refactor scx_select_cpu_dfl() to accept idle flags and an > arbitrary allowed cpumask > - build scx_bpf_select_cpu_and() on top of the existing logic > - re-arrange scx_select_cpu_dfl() prototype, aligning the first three > arguments with select_task_rq() > - do not use "domain" for the allowed cpumask to avoid potential ambiguity > with sched_domain > > ChangeLog v1 -> v2: > - rename scx_bpf_select_cpu_pref() to scx_bpf_select_cpu_and() and always > select idle CPUs strictly within the allowed domain > - rename preferred CPUs -> allowed CPU > - drop %SCX_PICK_IDLE_IN_PREF (not required anymore) > - deprecate scx_bpf_select_cpu_dfl() in favor of scx_bpf_select_cpu_and() > and provide all the required backward compatibility boilerplate > > Andrea Righi (6): > sched_ext: idle: Extend topology optimizations to all tasks > sched_ext: idle: Explicitly pass allowed cpumask to scx_select_cpu_dfl() > sched_ext: idle: Accept an arbitrary cpumask in scx_select_cpu_dfl() > sched_ext: idle: Introduce scx_bpf_select_cpu_and() > selftests/sched_ext: Add test for scx_bpf_select_cpu_and() > sched_ext: idle: Deprecate scx_bpf_select_cpu_dfl() > > Documentation/scheduler/sched-ext.rst | 11 +- > kernel/sched/ext.c | 6 +- > kernel/sched/ext_idle.c | 196 ++++++++++++++++----- > kernel/sched/ext_idle.h | 3 +- > tools/sched_ext/include/scx/common.bpf.h | 5 +- > tools/sched_ext/include/scx/compat.bpf.h | 37 ++++ > tools/sched_ext/scx_flatcg.bpf.c | 12 +- > tools/sched_ext/scx_simple.bpf.c | 9 +- > tools/testing/selftests/sched_ext/Makefile | 1 + > .../testing/selftests/sched_ext/allowed_cpus.bpf.c | 121 +++++++++++++ > tools/testing/selftests/sched_ext/allowed_cpus.c | 57 ++++++ > .../selftests/sched_ext/enq_select_cpu_fails.bpf.c | 12 +- > .../selftests/sched_ext/enq_select_cpu_fails.c | 2 +- > tools/testing/selftests/sched_ext/exit.bpf.c | 6 +- > .../sched_ext/select_cpu_dfl_nodispatch.bpf.c | 13 +- > .../sched_ext/select_cpu_dfl_nodispatch.c | 2 +- > 16 files changed, 404 insertions(+), 89 deletions(-) > create mode 100644 tools/testing/selftests/sched_ext/allowed_cpus.bpf.c > create mode 100644 tools/testing/selftests/sched_ext/allowed_cpus.c