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scx_bpfland: skip idle CPU selection for tasks with changing affinity

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When tasks are changing CPU affinity it is pointless to try to find an
optimal idle CPU. In this case just skip the the idle CPU selection step
and let the task being dispatched to a global DSQ if needed.

Signed-off-by: Andrea Righi <andrea.righi@linux.dev>
This commit is contained in:
Andrea Righi 2024-09-03 09:54:29 +02:00
parent 802d104b46
commit 70b93ed641
1 changed files with 24 additions and 19 deletions
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43
scheds/rust/scx_bpfland/src/bpf/main.bpf.c
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@ -525,6 +525,30 @@ static s32 pick_idle_cpu(struct task_struct *p, s32 prev_cpu)
if (!turbo)
return -ENOENT;
/*
* If the task isn't allowed to use its previously used CPU it means
* that it's rapidly changing affinity. In this case it's pointless to
* find an optimal idle CPU, just return and let the task being
* dispatched to a global DSQ.
*/
if (!bpf_cpumask_test_cpu(prev_cpu, p->cpus_ptr))
return -ENOENT;
/*
* For tasks that can run only on a single CPU, we can simply verify if
* their only allowed CPU is still idle.
*
* Moreover, if local_kthreads is enabled, always allow to dispatch
* per-CPU kthreads directly to their target CPU, independently on the
* idle state.
*/
if (p->nr_cpus_allowed == 1) {
if (is_kthread(p) && local_kthreads ||
scx_bpf_test_and_clear_cpu_idle(prev_cpu))
return prev_cpu;
return -ENOENT;
}
/*
* Acquire the CPU masks to determine the online and idle CPUs in the
* system.
@ -533,25 +557,6 @@ static s32 pick_idle_cpu(struct task_struct *p, s32 prev_cpu)
idle_smtmask = scx_bpf_get_idle_smtmask();
idle_cpumask = scx_bpf_get_idle_cpumask();
/*
* For tasks that can run only on a single CPU, we can simply verify if
* their only allowed CPU is still usable, online and idle.
*
* Moreover, if local_kthreads is enabled, always dispatch per-CPU
* kthreads directly to their target CPU, independently on its idle
* state.
*/
if (p->nr_cpus_allowed == 1) {
if (bpf_cpumask_test_cpu(prev_cpu, p->cpus_ptr) &&
bpf_cpumask_test_cpu(prev_cpu, online_cpumask) &&
(is_kthread(p) && local_kthreads ||
scx_bpf_test_and_clear_cpu_idle(prev_cpu)))
cpu = prev_cpu;
else
cpu = -ENOENT;
goto out_put_cpumask;
}
/*
* Scheduling domains of the previously used CPU.
*/