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Wed, 20 May 2026 17:04:36 +0000 (GMT) Received: from [9.39.30.77] (unknown [9.39.30.77]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 20 May 2026 17:04:36 +0000 (GMT) Message-ID: <310c1f04-562e-4e81-9d39-0f28e1cc414c@linux.ibm.com> Date: Wed, 20 May 2026 22:34:35 +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: [PATCH v2] sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT To: Steven Rostedt Cc: Valentin Schneider , LKML , Peter Zijlstra , Tejun Heo , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Ben Segall , Mel Gorman , K Prateek Nayak , Kyle McMartin References: <20260515103740.25ccbed8@gandalf.local.home> <61203cdb-af98-47e6-bb70-78af7f0f1cbc@linux.ibm.com> <20260520125226.50b83e76@gandalf.local.home> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <20260520125226.50b83e76@gandalf.local.home> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTIwMDE2MSBTYWx0ZWRfX7w1nz9rWgELu l/1HNKbgcgLKADQ4fAagnldR59ddzVG3pZM1Luf02/x9NPRJKOqLSDIeLdWOcAN8CcV2We2L9XQ 4wpS8MvA4mlYr+/3lQydayD8qSD0wI2GgshutoWCvqQgQV9rP/CEhb84UFrCcbn9tWa9R4PK2A7 hl/lxZeiFBBlXty48eziE2akRDx2H0VeXHOhpcCl4j5K3jwkgG3zJaTxae2XGjZrkSWf+qgZZLN SaBpZMDcxOChkV5W+9T9g/w7SyBDoULna6b6LSh+ZlvIV/6XtGujAninE3uDnBptAkVrWdHO/km gYS3+QaT9nslm3Zt6pLR96JyeFtU0prNR1KC8Ty2RDc+swHxXH3HNnYo66Z2V0jJ+fDgaC/KR0J VypSBstLBbxvcnJQgKJN5cvaeAjjP62mHwsrd4mZTL1pTa2bwuefAZyIKm/cqGoj3duZrykQCio vFZ+QLRYuGvqZr4XRnQ== X-Proofpoint-GUID: Ot_f56DcLcOQxLVxwhlwW0xCVQ5InXn4 X-Proofpoint-ORIG-GUID: s16mgsO3qrLp6cyyWlqzJqVyoLFRHW50 X-Authority-Analysis: v=2.4 cv=aYBRWxot c=1 sm=1 tr=0 ts=6a0de9aa cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=IkcTkHD0fZMA:10 a=NGcC8JguVDcA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=Y2IxJ9c9Rs8Kov3niI8_:22 a=VnNF1IyMAAAA:8 a=Uh4c-SwHJasI9CWpCSYA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-05-20_03,2026-05-18_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 adultscore=0 bulkscore=0 suspectscore=0 lowpriorityscore=0 clxscore=1015 spamscore=0 phishscore=0 priorityscore=1501 malwarescore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2605130000 definitions=main-2605200161 On 5/20/26 10:22 PM, Steven Rostedt wrote: > On Wed, 20 May 2026 12:03:37 +0530 > Shrikanth Hegde wrote: > >> On 5/18/26 2:17 PM, Valentin Schneider wrote: >>> On 15/05/26 10:37, Steven Rostedt wrote: >> >>> I got a bit confused here, please correct me if I didn't get this right: >>> >>> Per handle_softirqs(), we can't restart the softirq handling loop if >>> need_resched() is true (which would be the case here, per what we'd have >>> done it push_rt_task(@pull=true)). >>> I thought this meant softirqs couldn't be the issue, however this is only >>> valid within the scope of a single handle_softirqs() invocation. >>> >>> AIUI here we're being hammered by IRQs, thus under this sort of pattern: >>> >>> >>> __irq_exit_rcu() >>> invoke_softirq() >>> handle_softirqs() >>> // handle NET_RX_SOFTIRQ >>> // need_resched() is true so don't loop >>> >>> >> >> Wouldn't IRQ exit call schedule if need_resched is set? via irqentry_exit_cond_resched ? > > I haven't looked at the networking code, but does NAPI just continue > looping while there are packets to process? Or is there a check if > need_resched() is set, that it will exit out early? > > Yeah, I'm still wondering why this causes a live lock. > > -- Steve > > By any chance it is running with preempt=none/voluntary? If so it might never call schedule until it goes back to user space. >> >>> // Barely any progress made here towards actually executing __schedule() >>> >>> >>> __irq_exit_rcu() >>> invoke_softirq() >>> handle_softirqs() >>> // handle NET_RX_SOFTIRQ and wake up some tasks >>> // need_resched() is true so don't loop >>> >>> >>> & repeat ad nauseam. Did I get this right? >>> >