diff options
| -rw-r--r-- | tools/sched_ext/scx_qmap.bpf.c | 40 |
1 files changed, 40 insertions, 0 deletions
diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c index f3587fb709c9..b68abb9e760b 100644 --- a/tools/sched_ext/scx_qmap.bpf.c +++ b/tools/sched_ext/scx_qmap.bpf.c @@ -471,6 +471,46 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 cpu, struct task_struct *prev) __sync_fetch_and_add(&nr_dispatched, 1); scx_bpf_dsq_insert(p, SHARED_DSQ, slice_ns, 0); + + /* + * scx_qmap uses a global BPF queue that any CPU's + * dispatch can pop from. If this CPU popped a task that + * can't run here, it gets stranded on SHARED_DSQ after + * consume_dispatch_q() skips it. Kick the task's home + * CPU so it drains SHARED_DSQ. + * + * There's a race between the pop and the flush of the + * buffered dsq_insert: + * + * CPU 0 (dispatching) CPU 1 (home, idle) + * ~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~ + * pop from BPF queue + * dsq_insert(buffered) + * balance: + * SHARED_DSQ empty + * BPF queue empty + * -> goes idle + * flush -> on SHARED + * kick CPU 1 + * wakes, drains task + * + * The kick prevents indefinite stalls but a per-CPU + * kthread like ksoftirqd can be briefly stranded when + * its home CPU enters idle with softirq pending, + * triggering: + * + * "NOHZ tick-stop error: local softirq work is pending, handler #N!!!" + * + * from report_idle_softirq(). The kick lands shortly + * after and the home CPU drains the task. This could be + * avoided by e.g. dispatching pinned tasks to local or + * global DSQs, but the current code is left as-is to + * document this class of issue -- other schedulers + * seeing similar warnings can use this as a reference. + */ + if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) + scx_bpf_kick_cpu(scx_bpf_task_cpu(p), 0); + bpf_task_release(p); batch--; |