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Mon, 07 Apr 2025 06:47:03 -0700 (PDT) Received: from localhost.localdomain ([202.43.239.100]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-af9bc35137csm7187016a12.44.2025.04.07.06.46.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 07 Apr 2025 06:47:02 -0700 (PDT) From: hupu To: jstultz@google.com, linux-kernel@vger.kernel.org Cc: juri.lelli@redhat.com, peterz@infradead.org, vschneid@redhat.com, mingo@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, hupu@transsion.com, hupu Subject: [RFC 1/1] sched: Remove unreliable wake_cpu check in proxy_needs_return Date: Mon, 7 Apr 2025 21:46:54 +0800 Message-ID: <20250407134654.92841-1-hupu.gm@gmail.com> X-Mailer: git-send-email 2.47.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The (p->wake_cpu != cpu_of(rq)) check in proxy_needs_return() is unsafe during early wakeup phase. When called via ttwu_runnable() path: |-- try_to_wake_up |-- ttwu_runnable |-- proxy_needs_return //we are here |-- select_task_rq |-- set_task_cpu //set p->wake_cpu here |-- ttwu_queue The p->wake_cpu at this point reflects the CPU where donor last ran before blocking, not the target migration CPU. During blocking period: 1. CPU affinity may have been changed by other threads 2. Proxy migrations might have altered the effective wake_cpu 3. set_task_cpu() hasn't updated wake_cpu yet in this code path This makes the wake_cpu vs current CPU comparison meaningless and potentially dangerous. Rely on find_proxy_task()'s later migration logic to handle CPU placement based on up-to-date affinity and scheduler state. Signed-off-by: hupu --- kernel/sched/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 3c4ef4c71cfd..ca4ca739eb85 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4047,7 +4047,7 @@ static inline bool proxy_needs_return(struct rq *rq, struct task_struct *p) raw_spin_lock(&p->blocked_lock); if (__get_task_blocked_on(p) && p->blocked_on_state & BO_NEEDS_RETURN) { - if (!task_current(rq, p) && (p->wake_cpu != cpu_of(rq))) { + if (!task_current(rq, p)) { if (task_current_donor(rq, p)) { put_prev_task(rq, p); rq_set_donor(rq, rq->idle); -- 2.47.0