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Wed, 8 May 2024 07:31:04 GMT Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E8D9E20043; Wed, 8 May 2024 07:31:01 +0000 (GMT) Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 9B9842004E; Wed, 8 May 2024 07:31:01 +0000 (GMT) Received: from tuxmaker.linux.ibm.com (unknown [9.152.85.9]) by smtpav01.fra02v.mail.ibm.com (Postfix) with ESMTPS; Wed, 8 May 2024 07:31:01 +0000 (GMT) From: Sven Schnelle To: Valentin Schneider Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Heiko Carstens , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched/core: Test online status in available_idle_cpu() In-Reply-To: (Valentin Schneider's message of "Thu, 02 May 2024 17:57:50 +0200") References: <20240429055415.3278672-1-svens@linux.ibm.com> Date: Wed, 08 May 2024 09:31:01 +0200 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain X-TM-AS-GCONF: 00 X-Proofpoint-GUID: zL9zjBJQejBI0Pl9aBSvDVgQHV1GXQ6J X-Proofpoint-ORIG-GUID: 7EJyJOe8lqN3UDWbP2TUSCtlnI3kCFJr X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1039,Hydra:6.0.650,FMLib:17.11.176.26 definitions=2024-05-08_04,2024-05-08_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 lowpriorityscore=0 phishscore=0 adultscore=0 malwarescore=0 mlxscore=0 spamscore=0 mlxlogscore=560 impostorscore=0 clxscore=1015 suspectscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2405010000 definitions=main-2405080054 Valentin Schneider writes: > On 29/04/24 07:54, Sven Schnelle wrote: >> The current implementation of available_idle_cpu() doesn't test >> whether a possible cpu is offline. On s390 this dereferences a >> NULL pointer in arch_vcpu_is_preempted() because lowcore is not >> allocated for offline cpus. On x86, tracing also shows calls to >> available_idle_cpu() after a cpu is disabled, but it looks like >> this isn't causing any (obvious) issue. Nevertheless, add a check >> and return early if the cpu isn't online. >> >> Signed-off-by: Sven Schnelle > > > So most of the uses of that function is in wakeup task placement. > o find_idlest_cpu() works on the sched_domain spans, so shouldn't deal with > offline CPUs. > o select_idle_sibling() may issue an available_idle_cpu(prev) with an > offline previous, which would trigger your issue. > > Currently, even if select_idle_sibling() picks an offline CPU, this will > get corrected by select_fallback_rq() at the end of > select_task_rq(). However, it would make sense to realize @prev isn't a > suitable pick before making it to the fallback machinery, in which case > your patch makes sense beyond just fixing s390. > > Reviewed-by: Valentin Schneider Thanks for the review! Ingo/Peter, gentle ping, are you planning to take this patch?