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Fri, 15 May 2026 17:24:59 +0000 (GMT) Received: from li-7bb28a4c-2dab-11b2-a85c-887b5c60d769.ibm.com.com (unknown [9.39.30.78]) by smtpav04.fra02v.mail.ibm.com (Postfix) with ESMTP; Fri, 15 May 2026 17:24:59 +0000 (GMT) From: Shrikanth Hegde To: mingo@kernel.org, peterz@infradead.org, vincent.guittot@linaro.org, linux-kernel@vger.kernel.org Cc: sshegde@linux.ibm.com, kprateek.nayak@amd.com, juri.lelli@redhat.com, vschneid@redhat.com, dietmar.eggemann@arm.com, tj@kernel.org, rostedt@goodmis.org, mgorman@suse.de, bsegall@google.com, arighi@nvidia.com, pauld@redhat.com Subject: [PATCH v4 0/4] sched: Simplify CONFIG_SCHED_SMT ifdef usage Date: Fri, 15 May 2026 22:54:52 +0530 Message-ID: <20260515172456.542799-1-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.51.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-ORIG-GUID: -7cGLu-WGRGt7NshCtKPe8lfRxSbMCmE X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTE1MDE3NSBTYWx0ZWRfX0F/zM7p90nEC X3HwagNhgpAtMPYWG7HE8a1CGe6J3bBWK6rB66khtPOCuV7iBcxP8B2hZzADFkDPIVzcZnekHdA n0/hQqx6vE8c3vIDt7LHifNsa54zpbGfz1V3LHpMdUWRwGNmd1uMvls7NbJn2MFkGfI94Kg+jgv BkkMAM9YoIgA0rK0gsO4mIHYjC99pGkNZYpYKiYSxKbZzLumP19ssxYlYMrcf+t37I5r7mtEo+k dj4lIM57lRiQv6I6rLFXfC9+7ctp9YSDHb0F7IglFtkeEoYi1cZgJ6jAg8bVNUQspVghCvCxjZQ QTUBb0jcnkbG58ad3Cm7G3SSXTdhGLbXWD52cLkceqgTYdYFA21A/ZXZ2lU+7Oy4DR0sLM4Uhdc aHWZmTID9KeqCQarYHuUH4WcXQR5Qi+HM1nxXti2L3AwYni/M9togbzn8Y5lTTEsRg42AfbI59Q c+snHeAJId/H1WITSSA== X-Authority-Analysis: v=2.4 cv=ZZEt8MVA c=1 sm=1 tr=0 ts=6a0756f3 cx=c_pps a=GFwsV6G8L6GxiO2Y/PsHdQ==:117 a=GFwsV6G8L6GxiO2Y/PsHdQ==:17 a=NGcC8JguVDcA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=uAbxVGIbfxUO_5tXvNgY:22 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=xr9nLQe3p8f6n9bC9yMA:9 X-Proofpoint-GUID: QGP1mv3sPOyXL7hZ3ONahCvEHgWQB2Ag 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=2026-05-15_04,2026-05-15_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 priorityscore=1501 malwarescore=0 bulkscore=0 impostorscore=0 phishscore=0 spamscore=0 lowpriorityscore=0 suspectscore=0 adultscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2605130000 definitions=main-2605150175 Semantics ========= - For CONFIG_SCHED_SMT=y: No functional change. - For CONFIG_SCHED_SMT=n: - cpu_smt_mask(cpu) becomes cpumask_of(cpu), effectively making it per CPU with no siblings. - sched_smt_present remains defined, but never becomes active Since cpumask_weight(cpumask_of(cpu)) == 1 Performance impact ================== - CONFIG_SCHED_SMT=y: No change in generated code. - CONFIG_SCHED_SMT=n: - Small increase in text size (~0.01%) due to removal of compile-time stubs. Most paths remain effectively dead due to static keys. - Fast paths are protected using IS_ENABLED(CONFIG_SCHED_SMT). With that, cpu_smt_mask() can be used unconditionally and reduces CONFIG_SCHED_SMT ifdef usage, improving readability and maintainability. This leaves the remaining use of CONFIG_SCHED_SMT mainly for topology handling bits. Also, remove the explicit use of sched_smt_present. Make use of sched_smt_active. It differs slightly for update_idle_core, which used with unlikely flag earlier. Now it will be likely. Should be ok, since most systems have SMT active today. Since v3[3]: - Minor re-order of sched_smt_active to bail out early for !smt (Valentin Schneider) - Collected tags. Thanks again. Since v2[2]: - Changed patch 3 significantly to use sched_smt_active check for fastpath. (K Prateek Nayak) - Collected the tags. Thanks to Phil Auld, Valentin Schneider, Tejun Heo, K Prateek Nayak. - Since patch 3 has changed by quite a bit, i have not applied the tags. Please review it again. Sorry for the inconvenience. - Added patch 4 to unify checking on SMT active using sched_smt_active(). Since v1[1]: - Dropped changes for stop_core_cpuslocked. intel_ifs is the only user and it gets compiled with SCHED_SMT always. Added comments around it why ifdefs are still being kept. - Decided not to add sched_smt_active checks for sched_core_cpu_deactivate and sched_core_cpu_starting, they bail out if weight(smt_mask) == 1. Hence core_lock won't be held for long. It is not fastpath either. - Thanks to Valentin Schneider and Tejun Heo for checking out the series and the feedback. [1]: https://lore.kernel.org/all/20260506110052.9974-1-sshegde@linux.ibm.com/ [2]: https://lore.kernel.org/all/20260512152125.308280-1-sshegde@linux.ibm.com/ [3]: https://lore.kernel.org/all/20260513133934.380347-1-sshegde@linux.ibm.com/ Shrikanth Hegde (4): topology: Introduce cpu_smt_mask for CONFIG_SCHED_SMT=n sched: Simplify ifdeffery around cpu_smt_mask sched/fair: Add sched_smt_active check for fastpaths sched: Unify SMT active check via sched_smt_active() include/linux/sched/smt.h | 4 ---- include/linux/topology.h | 15 ++++++++++++- kernel/sched/core.c | 6 ----- kernel/sched/core_sched.c | 2 +- kernel/sched/ext_idle.c | 6 ----- kernel/sched/fair.c | 46 +++++---------------------------------- kernel/sched/sched.h | 8 +------ kernel/sched/topology.c | 2 -- kernel/stop_machine.c | 5 +++++ kernel/workqueue.c | 4 ---- 10 files changed, 26 insertions(+), 72 deletions(-) -- 2.47.3