Re: [PATCH sched_ext/for-7.1-fixes] sched_ext: Use HK_TYPE_DOMAIN_BOOT to detect isolcpus= domain isolation

[Frederic Weisbecker]

Le Wed, May 13, 2026 at 01:24:38PM +0200, Andrea Righi a écrit :
> scx_enable() refuses to attach a BPF scheduler when isolcpus=domain is
> in effect by comparing housekeeping_cpumask(HK_TYPE_DOMAIN) against
> cpu_possible_mask.
> 
> Since commit 27c3a5967f05 ("sched/isolation: Convert housekeeping
> cpumasks to rcu pointers"), HK_TYPE_DOMAIN's cpumask is RCU protected
> and dereferencing it requires either RCU read lock, the cpu_hotplug
> write lock, or the cpuset lock; scx_enable() holds none of these, so
> booting with isolcpus=domain and attaching any BPF scheduler triggers
> the following lockdep splat:
> 
>   =============================
>   WARNING: suspicious RCU usage
>   -----------------------------
>   kernel/sched/isolation.c:60 suspicious rcu_dereference_check() usage!
> 
>   1 lock held by scx_flash/281:
>    #0: ffffffff8379fce0 (update_mutex){+.+.}-{4:4}, at:
>        bpf_struct_ops_link_create+0x134/0x1c0
> 
>   Call Trace:
>    dump_stack_lvl+0x6f/0xb0
>    lockdep_rcu_suspicious.cold+0x37/0x70
>    housekeeping_cpumask+0xcd/0xe0
>    scx_enable.isra.0+0x17/0x120
>    bpf_scx_reg+0x5e/0x80
>    bpf_struct_ops_link_create+0x151/0x1c0
>    __sys_bpf+0x1e4b/0x33c0
>    __x64_sys_bpf+0x21/0x30
>    do_syscall_64+0x117/0xf80
>    entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> In addition, commit 03ff73510169 ("cpuset: Update HK_TYPE_DOMAIN cpumask
> from cpuset") made HK_TYPE_DOMAIN include cpuset isolated partitions as
> well, which means the current check also rejects BPF schedulers when a
> cpuset partition is active. That contradicts the original intent of
> commit 9f391f94a173 ("sched_ext: Disallow loading BPF scheduler if
> isolcpus= domain isolation is in effect"), which explicitly noted that
> cpuset partitions are honored through per-task cpumasks and should not
> be rejected.
> 
> Switch to housekeeping_enabled(HK_TYPE_DOMAIN_BOOT), which reads only
> the housekeeping flag bit (no RCU dereference) and reflects exactly the
> boot-time isolcpus= configuration that the error message refers to.
> 
> Fixes: 27c3a5967f05 ("sched/isolation: Convert housekeeping cpumasks to rcu pointers")

Ok reading the changelog of the above Fixes: head, I now understand why isolcpus
is excluded and not cpuset.

> Cc: stable@xxxxxxxxxxxxxxx # v7.0+
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>

Acked-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

-- 
Frederic Weisbecker
SUSE Labs