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Wed, 9 Apr 2025 06:01:38 GMT Received: from smtpav03.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4C10C580DF; Wed, 9 Apr 2025 06:01:38 +0000 (GMT) Received: from smtpav03.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 31193580D7; Wed, 9 Apr 2025 06:01:33 +0000 (GMT) Received: from [9.43.21.2] (unknown [9.43.21.2]) by smtpav03.wdc07v.mail.ibm.com (Postfix) with ESMTP; Wed, 9 Apr 2025 06:01:32 +0000 (GMT) Message-ID: Date: Wed, 9 Apr 2025 11:31:31 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 1/1] sched: Remove unreliable wake_cpu check in proxy_needs_return To: hupu Cc: jstultz@google.com, linux-kernel@vger.kernel.org, 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, Madadi Vineeth Reddy References: <20250407134654.92841-1-hupu.gm@gmail.com> Content-Language: en-US Reply-To: 20250407134654.92841-1-hupu.gm@gmail.com From: Madadi Vineeth Reddy In-Reply-To: <20250407134654.92841-1-hupu.gm@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: 7KLj8l9Nty7nwh56AyH3noRUkiRSXAJ2 X-Proofpoint-GUID: UVxoA-B7r40WyAgbpA3huDgpKqZ_dbxc X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1095,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2025-04-09_02,2025-04-08_04,2024-11-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 bulkscore=0 spamscore=0 impostorscore=0 lowpriorityscore=0 priorityscore=1501 clxscore=1011 malwarescore=0 mlxscore=0 suspectscore=0 mlxlogscore=999 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2502280000 definitions=main-2504090020 On 07/04/25 19:16, hupu wrote: > 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); Which tree is this change based on? I don't see `proxy_needs_return` in tip/sched/core. Thanks, Madadi Vineeth Reddy