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Thu, 4 Dec 2025 17:55:17 GMT Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 501FF20043; Thu, 4 Dec 2025 17:55:17 +0000 (GMT) Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BD5CF20040; Thu, 4 Dec 2025 17:55:13 +0000 (GMT) Received: from sapthagiri.in.ibm.com (unknown [9.39.29.188]) by smtpav01.fra02v.mail.ibm.com (Postfix) with ESMTP; Thu, 4 Dec 2025 17:55:13 +0000 (GMT) From: Srikar Dronamraju To: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Peter Zijlstra Cc: Ben Segall , Christophe Leroy , Dietmar Eggemann , Ingo Molnar , Juri Lelli , K Prateek Nayak , Madhavan Srinivasan , Mel Gorman , Michael Ellerman , Nicholas Piggin , Shrikanth Hegde , Srikar Dronamraju , Steven Rostedt , Swapnil Sapkal , Thomas Huth , Valentin Schneider , Vincent Guittot , virtualization@lists.linux.dev, Yicong Yang , Ilya Leoshkevich Subject: [PATCH 01/17] sched/fair: Enable group_asym_packing in find_idlest_group Date: Thu, 4 Dec 2025 23:23:49 +0530 Message-ID: <20251204175405.1511340-2-srikar@linux.ibm.com> X-Mailer: git-send-email 2.51.1 In-Reply-To: <20251204175405.1511340-1-srikar@linux.ibm.com> References: <20251204175405.1511340-1-srikar@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-TM-AS-GCONF: 00 X-Authority-Analysis: v=2.4 cv=dK+rWeZb c=1 sm=1 tr=0 ts=6931cb0b cx=c_pps a=GFwsV6G8L6GxiO2Y/PsHdQ==:117 a=GFwsV6G8L6GxiO2Y/PsHdQ==:17 a=wP3pNCr1ah4A:10 a=VkNPw1HP01LnGYTKEx00:22 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=ckJkcgXifS0MwuEMmiIA:9 X-Proofpoint-GUID: p4N8cC9Ej_pjMTdg3tr_-fwWHO81rS5E X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMTI5MDAyMCBTYWx0ZWRfX0K8Nzru4dAWO R21IXfvwbKS6T23j3HTsfBhZDUgKtDtSPvyQwaLvwmvQoWurdWjAK1orhWzXRb6zLZkMkqtagbh cwQQSnroNmxxRlKg5UpvOyEFMz71lfnTcFvu+LsFfltRoXaEBU6S4ogQySOieivCEEaswIQkoyr 6N1yeWfz8uPTXAD2Z0/bzmRK7eqh96GJYdOrhiIgGZwHSeDQhoa/YUQBNsz4w0i8L91ha7qCUCU J1A7miRZn29e9BCBXq11G1DzPLzNJKPoN8KO3NPfL75CFcykn1lNsswjHJC0lLz1tsmsOtjFFzY Pgh2RdtQUmwSSC2lum0ojc2AloRjrQt1SEGN191igbdM3zJq5+kQa+fxBZAFlJWaiIwf6vYaWgK xPCRX7MjiFjLn/yXO3dMMSVh3KfXow== X-Proofpoint-ORIG-GUID: 343_fIdgCxRbnqSTtODuZ3sGy1Qu_OmJ X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.100.49 definitions=2025-12-04_04,2025-12-04_03,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 lowpriorityscore=0 clxscore=1011 priorityscore=1501 bulkscore=0 adultscore=0 phishscore=0 impostorscore=0 spamscore=0 malwarescore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2510240000 definitions=main-2511290020 Content-Type: text/plain; charset="utf-8" Current scheduler code doesn't handle SD_ASYM_PACKING in the find_idlest_cpu path. On few architectures, like Powerpc, cache is at a core. Moving threads across cores may end up in cache misses. While asym_packing can be enabled above SMT level, enabling Asym packing across cores could result in poorer performance due to cache misses. However if the initial task placement via find_idlest_cpu does take asym_packing into consideration, then scheduler can avoid asym_packing migrations. This will result in lesser migrations and better packing and better overall performance. Previous version was posted at https://lore.kernel.org/all/20231018155036.2314342-1-srikar@linux.vnet.ibm.= com/t Signed-off-by: Srikar Dronamraju --- kernel/sched/fair.c | 33 ++++++++++++++++++++++++++++----- 1 file changed, 28 insertions(+), 5 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5b752324270b..979c3e333fba 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -10664,11 +10664,13 @@ static int idle_cpu_without(int cpu, struct task_= struct *p) * @group: sched_group whose statistics are to be updated. * @sgs: variable to hold the statistics for this group. * @p: The task for which we look for the idlest group/CPU. + * @this_cpu: current cpu */ static inline void update_sg_wakeup_stats(struct sched_domain *sd, struct sched_group *group, struct sg_lb_stats *sgs, - struct task_struct *p) + struct task_struct *p, + int asym_prefer_cpu) { int i, nr_running; =20 @@ -10705,6 +10707,12 @@ static inline void update_sg_wakeup_stats(struct s= ched_domain *sd, =20 } =20 + if (asym_prefer_cpu !=3D READ_ONCE(group->asym_prefer_cpu) && + sched_asym(sd, READ_ONCE(group->asym_prefer_cpu), + READ_ONCE(asym_prefer_cpu))) { + sgs->group_asym_packing =3D 1; + } + sgs->group_capacity =3D group->sgc->capacity; =20 sgs->group_weight =3D group->group_weight; @@ -10721,7 +10729,8 @@ static inline void update_sg_wakeup_stats(struct sc= hed_domain *sd, sgs->group_capacity; } =20 -static bool update_pick_idlest(struct sched_group *idlest, +static bool update_pick_idlest(struct sched_domain *sd, + struct sched_group *idlest, struct sg_lb_stats *idlest_sgs, struct sched_group *group, struct sg_lb_stats *sgs) @@ -10745,8 +10754,11 @@ static bool update_pick_idlest(struct sched_group = *idlest, return false; break; =20 - case group_imbalanced: case group_asym_packing: + return sched_asym(sd, READ_ONCE(group->asym_prefer_cpu), + READ_ONCE(idlest->asym_prefer_cpu)); + + case group_imbalanced: case group_smt_balance: /* Those types are not used in the slow wakeup path */ return false; @@ -10790,6 +10802,7 @@ sched_balance_find_dst_group(struct sched_domain *s= d, struct task_struct *p, int .avg_load =3D UINT_MAX, .group_type =3D group_overloaded, }; + int asym_prefer_cpu; =20 do { int local_group; @@ -10812,10 +10825,12 @@ sched_balance_find_dst_group(struct sched_domain = *sd, struct task_struct *p, int } else { sgs =3D &tmp_sgs; } + if (!local || local_group) + asym_prefer_cpu =3D READ_ONCE(group->asym_prefer_cpu); =20 - update_sg_wakeup_stats(sd, group, sgs, p); + update_sg_wakeup_stats(sd, group, sgs, p, asym_prefer_cpu); =20 - if (!local_group && update_pick_idlest(idlest, &idlest_sgs, group, sgs))= { + if (!local_group && update_pick_idlest(sd, idlest, &idlest_sgs, group, s= gs)) { idlest =3D group; idlest_sgs =3D *sgs; } @@ -10845,6 +10860,14 @@ sched_balance_find_dst_group(struct sched_domain *= sd, struct task_struct *p, int if (local_sgs.group_type > idlest_sgs.group_type) return idlest; =20 + if (idlest_sgs.group_type =3D=3D group_asym_packing) { + if (sched_asym(sd, READ_ONCE(idlest->asym_prefer_cpu), + READ_ONCE(local->asym_prefer_cpu))) { + return idlest; + } + return NULL; + } + switch (local_sgs.group_type) { case group_overloaded: case group_fully_busy: --=20 2.43.7