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Mon, 10 Aug 2026 13:23:58 +0000 (GMT) Received: from [9.39.23.237] (unknown [9.39.23.237]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Mon, 10 Aug 2026 13:23:58 +0000 (GMT) Message-ID: Date: Mon, 10 Aug 2026 18:53:57 +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 v9 04/11] sched/core: Try to use a preferred CPU in is_cpu_allowed To: Mete Durlu , linux-kernel@vger.kernel.org Cc: 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, jgross@suse.com, virtualization@lists.linux.dev, 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 References: <20260724140732.2683314-1-sshegde@linux.ibm.com> <20260724140732.2683314-5-sshegde@linux.ibm.com> <31646728-5af6-4f3d-b461-8992c213e230@linux.ibm.com> Content-Language: en-US From: Shrikanth Hegde In-Reply-To: <31646728-5af6-4f3d-b461-8992c213e230@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODEwMDExMiBTYWx0ZWRfX2eQHCac+wRev 80O/qJpE7GLgFnQRfOGfSZWdri56cwtSLYGhM16LheEcGEO2vWGTHbT8zkdifNiuBD7pbXOHCQm ESS2lEEu96JjkWg7QiPLqiVYZuEVrrEgbAsDNVqXabELo0cStzPW43rjXww6hTalmO6LoFqT1vB cdtgPpdqCV1+llhKnhC1eiPZFJJM7Mw+HrJNsM7UHXdPm8B2wqv8DV5DMkqWguTrO9M5rQmrOMd GiGwowHYrsLmDSeq5U+c0N1wW4/C9Qb86+HAFcLkAjWQHFo721ukVfWWKXZRTfNSIHP3hf8REQv 8RpZl6f3AEph9tGCrA39CAtfKzvzquYipf3/SqU/pZeaYMv5/u+nSoOOeONnkHuEEYifyl2AakM wLBYJWTA0H6a2UFufIJ4HO/HG9S/uivdUTsD7iSnUUsgj1WrZ9OuIGvjMrlcP7orNcJYApncyv/ Rh8V1IgWhHvln5lwWvQ== X-Proofpoint-Spam-Info: AW1haW4tMjYwODEwMDExMiBTYWx0ZWRfXzrfM0ODdCebM bmjTR+x8LavNRsQVzwEHidJtIf4Gf1wqBIGTjK/XeHDe3bZCFEA3g7NgTHwfekzUOVMryxIKQm0 DrfHuAfibmsV5kzmwarUqMAAnU6xyWI= X-Authority-Analysis: v=2.4 cv=RqD16imK c=1 sm=1 tr=0 ts=6a79d0fb cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=uAbxVGIbfxUO_5tXvNgY:22 a=VnNF1IyMAAAA:8 a=2eUty9qkGphTp3J6qE4A:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-GUID: zTtBMhQId9jhfyaGKZchu1dPJTn-_J-r X-Proofpoint-ORIG-GUID: Gkq1fUHav5AqOl8Y7R49E8MdhM85LY_h X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-10_03,2026-08-07_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 priorityscore=1501 suspectscore=0 lowpriorityscore=0 clxscore=1015 adultscore=0 bulkscore=0 malwarescore=0 impostorscore=0 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608100112 Hi Mete, thanks for taking a look. On 8/10/26 5:28 PM, Mete Durlu wrote: > Hi, > > ... >> For majority of the cases this would still keep select_fallback_rq >> as O(N). task_has_preferred_cpus which is O(N) is called only if >> !cpu_preferred. Then task running there is expected to move out. >> So subsequent it should run on preferred CPU. This becomes O(N**2) >> only for tasks pinned only non preferred CPUs. That is rare case. >> >> Signed-off-by: Shrikanth Hegde >> --- >>   kernel/sched/core.c  | 12 ++++++++++-- >>   kernel/sched/sched.h | 12 ++++++++++++ >>   2 files changed, 22 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index a45f7c308329..9e8eec4451b6 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -2509,8 +2509,12 @@ static inline bool is_cpu_allowed(struct >> task_struct *p, int cpu) >>           return cpu_online(cpu); >>       /* Non kernel threads are not allowed during either online or >> offline. */ >> -    if (!(p->flags & PF_KTHREAD)) >> +    if (!(p->flags & PF_KTHREAD)) { >> +        /* Try to use preferred CPU if task's affinity allows */ >> +        if (task_can_sched_on_preferred(cpu, p)) >> +            return false; >>           return cpu_active(cpu); >> +    } >>       /* KTHREAD_IS_PER_CPU is always allowed. */ >>       if (kthread_is_per_cpu(p)) >> @@ -2520,7 +2524,11 @@ static inline bool is_cpu_allowed(struct >> task_struct *p, int cpu) >>       if (cpu_dying(cpu)) >>           return false; >> -    /* But are allowed during online. */ >> +    /* Try to keep unbound kthreads on a preferred CPU if possible. */ >> +    if (task_can_sched_on_preferred(cpu, p)) >> +        return false; >> + >> +    /* Otherwise, they are allowed to run on online CPU. */ >>       return cpu_online(cpu); >>   } >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h >> index 26ae13c86b69..6de6366f2faa 100644 >> --- a/kernel/sched/sched.h >> +++ b/kernel/sched/sched.h >> @@ -4230,4 +4230,16 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change) >>   #include "ext/ext.h" >> +static inline bool task_can_sched_on_preferred(int cpu, struct >> task_struct *p) > > > Sorry, I glanced to the earlier versions but I couldn't find > the answer to my question. Trivial one but still. > > FWICT, task_can_sched_on_preferred() is only being used in > core.c, is there a reason why it is defined in > kernel/sched/sched.h and not in core.c ? > Will move it out of sched.h. Thanks for pointing it out. > Thanks. > -Mete Also, thanks for checking that s390 change.