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sched_ext: initialize built-in idle state before ops.init()

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commit f0c6eab5e4 upstream.

A BPF scheduler may want to use the built-in idle cpumasks in ops.init()
before the scheduler is fully initialized, either directly or through a
BPF timer for example.

However, this would result in an error, since the idle state has not
been properly initialized yet.

This can be easily verified by modifying scx_simple to call
scx_bpf_get_idle_cpumask() in ops.init():

$ sudo scx_simple

DEBUG DUMP
===========================================================================

scx_simple[121] triggered exit kind 1024:
  runtime error (built-in idle tracking is disabled)
...

Fix this by properly initializing the idle state before ops.init() is
called. With this change applied:

$ sudo scx_simple
local=2 global=0
local=19 global=11
local=23 global=11
...

Fixes: d73249f887 ("sched_ext: idle: Make idle static keys private")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
[ Backport to 6.12:
  - Original commit doesn't apply cleanly to 6.12 since d73249f887 is
    not present.
  - This backport applies the same logical fix to prevent BPF scheduler
    failures while accessing idle cpumasks from ops.init(). ]
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Andrea Righi
2025-03-25 10:32:12 +01:00
committed by Greg Kroah-Hartman
parent 3c0d35b3ca
commit 3c06e9ad0b
1 changed files with 7 additions and 7 deletions
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14
kernel/sched/ext.c
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@@ -5220,6 +5220,13 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
for_each_possible_cpu(cpu)
cpu_rq(cpu)->scx.cpuperf_target = SCX_CPUPERF_ONE;
if (!ops->update_idle || (ops->flags & SCX_OPS_KEEP_BUILTIN_IDLE)) {
reset_idle_masks();
static_branch_enable(&scx_builtin_idle_enabled);
} else {
static_branch_disable(&scx_builtin_idle_enabled);
}
/*
* Keep CPUs stable during enable so that the BPF scheduler can track
* online CPUs by watching ->on/offline_cpu() after ->init().
@@ -5287,13 +5294,6 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
if (scx_ops.cpu_acquire || scx_ops.cpu_release)
static_branch_enable(&scx_ops_cpu_preempt);
if (!ops->update_idle || (ops->flags & SCX_OPS_KEEP_BUILTIN_IDLE)) {
reset_idle_masks();
static_branch_enable(&scx_builtin_idle_enabled);
} else {
static_branch_disable(&scx_builtin_idle_enabled);
}
/*
* Lock out forks, cgroup on/offlining and moves before opening the
* floodgate so that they don't wander into the operations prematurely.