sched_ext: Use per-CPU DSQs instead of per-node global DSQs in bypass mode

Bypass mode routes tasks through fallback dispatch queues. Originally a single
global DSQ, b7b3b2dbae73 ("sched_ext: Split the global DSQ per NUMA node")
changed this to per-node DSQs to resolve NUMA-related livelocks.

Dan Schatzberg found per-node DSQs can still livelock when many threads are
pinned to different small CPU subsets: each CPU must scan many incompatible
tasks to find runnable ones, causing severe contention with high CPU counts.

Switch to per-CPU bypass DSQs. Each task queues on its current CPU. Default
idle CPU selection and direct dispatch handle most cases well.

This introduces a failure mode when tasks concentrate on one CPU in
over-saturated systems. If the BPF scheduler severely skews placement before
triggering bypass, that CPU's queue may be too long to drain, causing RCU
stalls. A load balancer in a future patch will address this. The bypass DSQ is
separate from local DSQ to enable load balancing: local DSQs use rq locks,
preventing efficient scanning and transfer across CPUs, especially problematic
when systems are already contended.

v2: Clarified why bypass DSQ is separate from local DSQ (Andrea Righi).

Reported-by: Dan Schatzberg <schatzberg.dan@gmail.com>
Reviewed-by: Dan Schatzberg <schatzberg.dan@gmail.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 60285c3d07cf..3d3216ff9188 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -57,6 +57,7 @@ enum scx_dsq_id_flags {
 	SCX_DSQ_INVALID		= SCX_DSQ_FLAG_BUILTIN | 0,
 	SCX_DSQ_GLOBAL		= SCX_DSQ_FLAG_BUILTIN | 1,
 	SCX_DSQ_LOCAL		= SCX_DSQ_FLAG_BUILTIN | 2,
+	SCX_DSQ_BYPASS		= SCX_DSQ_FLAG_BUILTIN | 3,
 	SCX_DSQ_LOCAL_ON	= SCX_DSQ_FLAG_BUILTIN | SCX_DSQ_FLAG_LOCAL_ON,
 	SCX_DSQ_LOCAL_CPU_MASK	= 0xffffffffLLU,
 };