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Wed, 6 May 2026 11:01:03 +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, tglx@kernel.org, mgorman@suse.de, bsegall@google.com, arighi@nvidia.com Subject: [PATCH 0/3] sched: Simplify ifdeffery around CONFIG_SCHED_SMT Date: Wed, 6 May 2026 16:30:49 +0530 Message-ID: <20260506110052.9974-1-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-Spam-Details-Enc: AW1haW4tMjYwNTA2MDEwNCBTYWx0ZWRfX9M+5zCDa5c+D XShkcgKwL2R8tDcNQZpXLZ/LoCIQYs0SnBU4uOIYRBcuSrFWDYX/Ey68juGM4RE6Ex2aATAnSx0 BRGY38NSGm8tezhQvBqN4VKPNZmtKSkKscrwLZpQcCx0xbCc1t3Rw+PYpOVMamOUpyz3etsxYxe Yifm9SacSXnqreOwTAP+md8sBZGdO3XEUVTfk6YZK8xH0Cl2FdIcOzZ1bSJDsF/w1pYnExD8X1C qLnBKA4d/M2yfKVX+RV8hWMhlo26X0vd4yXTzCBZjwiZaXE88dZHoJvIE2ta9Bnwa1AJsoMU8yJ bu1dFVWbGOVeLGLwNVM5NRYJL97mOf705Rtx53/jLsdEa3PyJCcf9veOQBo+WK83UmVVjIhAAmX 3nVn3rHRrKw6E46fCAH/F97hgDEqeskwpyso6B2MjEKpbq+LAa7u0gLRV1SqJDg+l6ADP8nAlxl 0rusrBCaI+wfaK7JbOg== X-Proofpoint-GUID: Iqh7aq172P4yGTBcuPULi-3aNO2KOB3H X-Proofpoint-ORIG-GUID: 1khQey20vGWh4SmN7B8_zNcQfz26RAGa X-Authority-Analysis: v=2.4 cv=eu/vCIpX c=1 sm=1 tr=0 ts=69fb1f78 cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=NGcC8JguVDcA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=iQ6ETzBq9ecOQQE5vZCe:22 a=mkjBOvPYtRlXWoU6_aAA:9 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=2026-05-05_03,2026-04-30_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 phishscore=0 lowpriorityscore=0 clxscore=1015 adultscore=0 suspectscore=0 malwarescore=0 bulkscore=0 impostorscore=0 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2604200000 definitions=main-2605060104 While working on paravirt series on maintaining preferred CPUs, I wanted to add/remove a core. To do that, I had to access cpu_smt_mask. For that had to use CONFIG_SCHED_SMT around the code. That made me think, for CONFIG_SCHED_SMT=n, it effectively means to just add/remove a CPU. Thats when I thought of this idea of using cpumask_of(cpu) when CONFIG_SCHED_SMT=n. 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). Testing ======= - Did build/boot test on powerpc for CONFIG_SCHED_SMT=y/n. - Ran hackbench on powerpc for CONFIG_SCHED_SMT=y/n. Didn't observe any major difference. - Did build/boot test on x86 for CONFIG_SCHED_SMT=y/n. For x86 i had change the code for CONFIG_SCHED_SMT=n Plus, Major distros make CONFIG_SCHED_SMT=y for all major archs and few archs unconditionally make CONFIG_SCHED_SMT=y (x86, s390), So CONFIG_SCHED_SMT=n is a rare case. With that, cpu_smt_mask() to be used unconditionally and reduces CONFIG_SCHED_SMT-specific code paths, improving readability and maintainability. Please review and consider if this simplification is a worthwhile cleanup. Shrikanth Hegde (3): topology: Introduce cpu_smt_mask for CONFIG_SCHED_SMT=n sched: Simplify ifdeffery around cpu_smt_mask sched/fair: Add compile time check in fastpaths for CONFIG_SCHED_SMT=n include/linux/sched/smt.h | 4 ---- include/linux/topology.h | 15 +++++++++++++- kernel/sched/core.c | 6 ------ kernel/sched/ext_idle.c | 6 ------ kernel/sched/fair.c | 41 +++++---------------------------------- kernel/sched/sched.h | 6 ------ kernel/sched/topology.c | 2 -- kernel/stop_machine.c | 2 -- kernel/workqueue.c | 4 ---- 9 files changed, 19 insertions(+), 67 deletions(-) -- 2.51.0