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[188.141.5.72]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48a820c865esm48160615e9.5.2026.04.30.02.27.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Apr 2026 02:27:50 -0700 (PDT) From: David Carlier To: Tejun Heo , David Vernet , Andrea Righi , Changwoo Min , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , sched-ext@lists.linux.dev (open list:SCHEDULER - SCHED_EXT), linux-kernel@vger.kernel.org (open list:SCHEDULER) Cc: David Carlier , Claude Subject: [PATCH] sched_ext: idle: Recheck prev_cpu after narrowing allowed mask Date: Thu, 30 Apr 2026 10:27:47 +0100 Message-ID: <20260430092747.73114-1-devnexen@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit scx_select_cpu_dfl() narrows @allowed to @cpus_allowed & @p->cpus_ptr when the BPF caller supplies a @cpus_allowed that differs from @p->cpus_ptr and @p doesn't have full affinity. However, @is_prev_allowed was computed against the original (wider) @cpus_allowed, so the prev_cpu fast paths could pick a @prev_cpu that is in @cpus_allowed but not in @p->cpus_ptr, violating the intended invariant that the returned CPU is always usable by @p. The kernel masks this via the SCX_EV_SELECT_CPU_FALLBACK fallback, but the behavior contradicts the documented contract. Move the @is_prev_allowed evaluation past the narrowing block so it tests against the final @allowed mask. Fixes: ee9a4e92799d ("sched_ext: idle: Properly handle invalid prev_cpu during idle selection") Assisted-by: Claude Signed-off-by: David Carlier --- kernel/sched/ext_idle.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c index 41785f65bbb2..bbb845f36a0a 100644 --- a/kernel/sched/ext_idle.c +++ b/kernel/sched/ext_idle.c @@ -464,12 +464,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, preempt_disable(); - /* - * Check whether @prev_cpu is still within the allowed set. If not, - * we can still try selecting a nearby CPU. - */ - is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed); - /* * Determine the subset of CPUs usable by @p within @cpus_allowed. */ @@ -486,6 +480,12 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, } } + /* + * Check whether @prev_cpu is still within the allowed set. If not, + * we can still try selecting a nearby CPU. + */ + is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed); + /* * This is necessary to protect llc_cpus. */ -- 2.53.0