From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx0b-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com [148.163.158.5]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5801017993; Mon, 13 Jul 2026 12:36:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=148.163.158.5 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783946189; cv=none; b=fwAR6T3LqM7htY+n6YoFUc2ozY1vM3Fa9vYpbWJihAd3NGDxf3kxX75vJvuM63fLL2Q7atdiSfhIiUJCth1AV7XJimVtN79tKWEBh74zCCAfB0NUM9/9c7IS17wn5tC+zptqFfDb0wg53gDZHznvjHbDTTC2wmXgGM04Dl6/WJk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783946189; c=relaxed/simple; bh=hbHfMraj9Zu0XLWlOZ2AVI2IUd1K78ImETQ9jXUhcmw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=LRza7Le4cEAFKKREZ40D6bRq2uP4mqzwxMsNfHbioYB143HR+sbF+Ez93v3iVvNetotEmKUsH5fJCUYi1pVhV6PNN9Rt4RB861d7z2mruLlDS4lSU+Onc0DrYKSF/cJRwEPRKT5XkY2+rkkJlPPWk2hhmI8gYysIxbAjiGElDD0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.ibm.com; spf=pass smtp.mailfrom=linux.ibm.com; dkim=pass (2048-bit key) header.d=ibm.com header.i=@ibm.com header.b=dkdd0YIa; arc=none smtp.client-ip=148.163.158.5 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.ibm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.ibm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=ibm.com header.i=@ibm.com header.b="dkdd0YIa" Received: from pps.filterd (m0356516.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.18.1.11/8.18.1.11) with ESMTP id 66DCD0IM2375227; Mon, 13 Jul 2026 12:36:07 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=cc :content-transfer-encoding:content-type:date:from:in-reply-to :message-id:mime-version:references:subject:to; s=pp1; bh=cFn1OW FObKu2t4u85IHFVhr6jxWOF09rwqr8oGI+DSA=; b=dkdd0YIai7TNzzQmpTpP1c ZNLK7eXdUPa/gjqwxWepNV6ecgfum7qDNSVyClv3aWbkdUBCSquzPCytMPTQAQF/ R/SbEO5W/VtwBekc450XZ7Mg+7CXsLmUnfC1gbAp4Zb5keaALiyiMKAYHVejFmSY ZhccvtGVYVaewqXbUxEdZZR3tmiF/Y4uTHCNPVkk+KK6vS/q5fH5c5neW4wuuJx4 ZN7pFdDair2+wO9JRPvFki7SZ+2KQ4IOWPvBKCFB0lU7X55QxaEzw5TQwxZ1x6mM /ptPZr7QHJ5Z5Wxs5Q/veMF+A65g38/ocEqErrKqKj/akl0UIUIJXBLxR+kSBVfA == Received: from ppma13.dal12v.mail.ibm.com (dd.9e.1632.ip4.static.sl-reverse.com [50.22.158.221]) by mx0a-001b2d01.pphosted.com (PPS) with ESMTPS id 4fbf2a07pw-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 13 Jul 2026 12:36:06 +0000 (GMT) Received: from pps.filterd (ppma13.dal12v.mail.ibm.com [127.0.0.1]) by ppma13.dal12v.mail.ibm.com (8.18.1.7/8.18.1.7) with ESMTP id 66DCYcSM017768; Mon, 13 Jul 2026 12:36:05 GMT Received: from smtprelay02.fra02v.mail.ibm.com ([9.218.2.226]) by ppma13.dal12v.mail.ibm.com (PPS) with ESMTPS id 4fc2cg5d7s-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 13 Jul 2026 12:36:05 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (smtpav02.fra02v.mail.ibm.com [10.20.54.101]) by smtprelay02.fra02v.mail.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 66DCa1hJ39977422 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 13 Jul 2026 12:36:01 GMT Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C18C320040; Mon, 13 Jul 2026 12:36:01 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2C66320043; Mon, 13 Jul 2026 12:35:52 +0000 (GMT) Received: from [9.39.29.148] (unknown [9.39.29.148]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Mon, 13 Jul 2026 12:35:51 +0000 (GMT) Message-ID: Date: Mon, 13 Jul 2026 18:05:50 +0530 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 06/12] sched/core: Push current task from non preferred CPU To: Yury Norov Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, yury.norov@gmail.com, kprateek.nayak@amd.com, iii@linux.ibm.com, corbet@lwn.net, tglx@kernel.org, gregkh@linuxfoundation.org, pbonzini@redhat.com, seanjc@google.com, vschneid@redhat.com, huschle@linux.ibm.com, rostedt@goodmis.org, dietmar.eggemann@arm.com, maddy@linux.ibm.com, srikar@linux.ibm.com, hdanton@sina.com, chleroy@kernel.org, vineeth@bitbyteword.org, frederic@kernel.org, arighi@nvidia.com, pauld@redhat.com, christian.loehle@arm.com, tj@kernel.org, tommaso.cucinotta@gmail.com, maz@kernel.org, rafael@kernel.org, rdunlap@infradead.org, kernellwp@gmail.com, linux-doc@vger.kernel.org References: <20260709215648.1246821-1-sshegde@linux.ibm.com> <20260709215648.1246821-7-sshegde@linux.ibm.com> Content-Language: en-US From: Shrikanth Hegde In-Reply-To: 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-Info: AW1haW4tMjYwNzEzMDEyNyBTYWx0ZWRfX7lTRTbU+ESDQ NfnaT3LNAsLyM3xNrzfliKFKttPIcljwulbzISalFKQqQButJ7ZJ5Yz+HkGQoOrblddrZP38XkL 1on7/OjHMx9vjonMuZXIh5zDjTxTJuY= X-Proofpoint-GUID: juNevJpVuFfkabp6IzVhBASJr_lo9_Hq X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNzEzMDEyNyBTYWx0ZWRfX01NAkTet8+0s 9oYqxw1MKXcR3L6oX8/lKGwa34MHxaRBKsegADWv60iEbu8huLzNSbnanMVfmucoJOgXr7RW0dZ XA+fzfj6SzO2fU6ww9Yv2B0mMTKqAH0sOxzHKaUpZTKPpPrNpKClf15NmMCSTCKgh/rbCl+TT8R hIwCfkHNwSehjvE1A1AvZfbH9kguXXcrNNrvMTo+tfNI5P/NxBoH2elAosyyasVjXR6z3wIMJ2y bi4W2kpLqV+Wp/8F9COcuYC8qRMVgbtV+cmsWjbweLGFYbRRUiomH6sckCaJ82V8ZNdVUIjJ7Z+ t/5b/TfThkcqenfCgSYPRMmwNqaEqS/fm/ZiADnriIteTDG1MVBEn7DTL25zVcIlGhxm0a9xOJg hzYQeKppKFB9aJS/SycxnQOSGFTXsw1mzyxen+BgzeiKnPMzS3YleCVQSrveQwfXZutoM8Rn2i2 TrvlYumsTQzTjvrtw3A== X-Proofpoint-ORIG-GUID: wPHLajThYDDo8umGSBhdl7BnNGZ64epj X-Authority-Analysis: v=2.4 cv=PvajqQM3 c=1 sm=1 tr=0 ts=6a54dbb7 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=IkcTkHD0fZMA:10 a=RAioF0-LDSMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=Y2IxJ9c9Rs8Kov3niI8_:22 a=VnNF1IyMAAAA:8 a=8xdayRMsDemD69ekofUA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-07-13_03,2026-07-10_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 bulkscore=0 impostorscore=0 adultscore=0 clxscore=1015 lowpriorityscore=0 spamscore=0 suspectscore=0 phishscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2607130127 On 7/11/26 4:32 AM, Yury Norov wrote: > On Fri, Jul 10, 2026 at 03:26:42AM +0530, Shrikanth Hegde wrote: >> Actively push out task running on a non-preferred CPU. Since the task is >> running on the CPU, need to stop the cpu and push the task out. >> However, if the task is pinned only to non-preferred CPUs, it will continue >> running there. This will help in maintaining the userspace affinities >> unlike CPU hotplug or isolated cpusets. >> >> Though code is similar to __balance_push_cpu_stop and quite close to >> push_cpu_stop, it is being kept separate as it provides a cleaner >> implementation with CONFIG_PREFERRED_CPU. >> >> Add push_task_work_done flag to protect work buffer. >> Works only with FAIR class. >> >> For now, only current running task is pushed out. This keeps the code >> simpler. In future optimization maybe done to move all the queued >> task on the rq. >> >> Signed-off-by: Shrikanth Hegde >> --- >> v6->v7: >> - Moved is_migration_disabled >> - removed fair class check >> >> kernel/sched/core.c | 78 ++++++++++++++++++++++++++++++++++++++++++++ >> kernel/sched/sched.h | 8 +++++ >> 2 files changed, 86 insertions(+) >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index 9e8eec4451b6..74c93a88bf84 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -5774,6 +5774,9 @@ void sched_tick(void) >> unsigned long hw_pressure; >> u64 resched_latency; >> >> + if (!cpu_preferred(cpu)) >> + sched_push_current_non_preferred_cpu(rq); >> + >> if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE)) >> arch_scale_freq_tick(); >> >> @@ -11292,3 +11295,78 @@ void sched_change_end(struct sched_change_ctx *ctx) >> p->sched_class->prio_changed(rq, p, ctx->prio); >> } >> } >> + >> +#ifdef CONFIG_PREFERRED_CPU >> +static DEFINE_PER_CPU(struct cpu_stop_work, npc_push_task_work); >> + >> +static int sched_non_preferred_cpu_push_stop(void *arg) >> +{ >> + struct task_struct *p = arg; >> + struct rq *rq = this_rq(); >> + struct rq_flags rf; >> + int cpu; >> + >> + /* sanity checks and clear */ >> + if (cpu_preferred(rq->cpu) || is_migration_disabled(p)) { >> + scoped_guard(rq_lock, rq) >> + rq->push_task_work_done = false; >> + put_task_struct(p); >> + return 0; >> + } >> + >> + raw_spin_lock_irq(&p->pi_lock); >> + >> + /* This could take rq lock. So call it before rq lock is taken */ >> + cpu = select_fallback_rq(rq->cpu, p); >> + rq_lock(rq, &rf); >> + rq->push_task_work_done = false; >> + update_rq_clock(rq); >> + >> + context_unsafe_alias(rq); >> + >> + if (task_rq(p) == rq && task_on_rq_queued(p)) >> + rq = __migrate_task(rq, &rf, p, cpu); >> + >> + rq_unlock(rq, &rf); >> + raw_spin_unlock_irq(&p->pi_lock); >> + put_task_struct(p); >> + >> + return 0; >> +} >> + >> +/* >> + * Push the current task running on non-preferred CPU(npc). >> + * Using this non preferred CPU will lead to more vCPU preemptions >> + * in the host. So it is better not to use this CPU. >> + * >> + * Since task is running, call a stopper to push the task out. This is >> + * similar to how task moves during hotplug. In select_fallback_rq a >> + * preferred CPU will be chosen and henceforth task shouldn't come back to >> + * this CPU again. >> + * >> + * Works for FAIR class only >> + * >> + * If task is affined only non-preferred CPUs, it can't be moved out >> + */ >> +void sched_push_current_non_preferred_cpu(struct rq *rq) >> +{ >> + struct task_struct *push_task = rq->curr; >> + >> + /* Don't push the task if task's affinity doesn't allow */ >> + if (!task_can_sched_on_preferred(rq->cpu, push_task)) >> + return; > > Shouldn't you protect it with the rq lock against races with affinity or > policy changes? The task_can_sched_on_preferred() checks p->sched_class > and p->cpus_ptr. > Likely yes. I did below. It has survived light testing so far. i.e. - Moved checks under rq lock held. - There was a tiny window where task could have migrated. Moved it down further just before migrating. diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 6ed61182b19f..eda080ab3427 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -11311,7 +11311,7 @@ static int sched_non_preferred_cpu_push_stop(void *arg) int cpu; /* sanity checks and clear */ - if (cpu_preferred(rq->cpu) || is_migration_disabled(p)) { + if (cpu_preferred(rq->cpu)) { scoped_guard(rq_lock, rq) rq->push_task_work_done = false; put_task_struct(p); @@ -11328,7 +11328,8 @@ static int sched_non_preferred_cpu_push_stop(void *arg) context_unsafe_alias(rq); - if (task_rq(p) == rq && task_on_rq_queued(p)) { + if (task_rq(p) == rq && task_on_rq_queued(p) + && !is_migration_disabled(p)) { struct rq *dest_rq; dest_rq = __migrate_task(rq, &rf, p, cpu); @@ -11362,19 +11363,20 @@ void sched_push_current_non_preferred_cpu(struct rq *rq) { struct task_struct *push_task = rq->curr; - /* Don't push the task if task's affinity doesn't allow */ - if (!task_can_sched_on_preferred(rq->cpu, push_task)) - return; + scoped_guard(rq_lock, rq) { + /* Don't push the task if task's affinity doesn't allow */ + if (!task_can_sched_on_preferred(rq->cpu, push_task)) + return; - /* There is already a stopper thread. Don't race with it. */ - if (rq->push_task_work_done) - return; + /* There is already a stopper thread. Don't race with it. */ + if (rq->push_task_work_done) + return; - /* sched_tick runs with interrupts disabled. */ - get_task_struct(push_task); + /* sched_tick runs with interrupts disabled. */ + get_task_struct(push_task); - scoped_guard(rq_lock, rq) rq->push_task_work_done = true; + } stop_one_cpu_nowait(rq->cpu, sched_non_preferred_cpu_push_stop, push_task, this_cpu_ptr(&npc_push_task_work)); >> + >> + /* There is already a stopper thread. Don't race with it. */ >> + if (rq->push_task_work_done) >> + return; >> + >> + /* sched_tick runs with interrupts disabled. */ >> + get_task_struct(push_task); >> + >> + scoped_guard(rq_lock, rq) >> + rq->push_task_work_done = true; >> + >> + stop_one_cpu_nowait(rq->cpu, sched_non_preferred_cpu_push_stop, >> + push_task, this_cpu_ptr(&npc_push_task_work)); >> +} >> +#endif >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h >> index 6de6366f2faa..80c02e2c09eb 100644 >> --- a/kernel/sched/sched.h >> +++ b/kernel/sched/sched.h >> @@ -1277,6 +1277,8 @@ struct rq { >> >> struct list_head cfs_tasks; >> >> + bool push_task_work_done; >> + >> struct sched_avg avg_rt; >> struct sched_avg avg_dl; >> #ifdef CONFIG_HAVE_SCHED_AVG_IRQ >> @@ -4242,4 +4244,10 @@ static inline bool task_can_sched_on_preferred(int cpu, struct task_struct *p) >> return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask); >> } >> >> +#ifdef CONFIG_PREFERRED_CPU >> +void sched_push_current_non_preferred_cpu(struct rq *rq); >> +#else /* !CONFIG_PREFERRED_CPU */ >> +static inline void sched_push_current_non_preferred_cpu(struct rq *rq) { } >> +#endif >> + >> #endif /* _KERNEL_SCHED_SCHED_H */ >> -- >> 2.47.3