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Wed, 12 Nov 2025 08:02:24 +0000 (GMT) Date: Wed, 12 Nov 2025 13:32:23 +0530 From: Srikar Dronamraju To: Tim Chen Cc: Peter Zijlstra , Ingo Molnar , Chen Yu , Doug Nelson , Mohini Narkhede , linux-kernel@vger.kernel.org, Vincent Guittot , Shrikanth Hegde , K Prateek Nayak Subject: Re: [PATCH v4] sched/fair: Skip sched_balance_running cmpxchg when balance is not due Message-ID: Reply-To: Srikar Dronamraju References: <6fed119b723c71552943bfe5798c93851b30a361.1762800251.git.tim.c.chen@linux.intel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <6fed119b723c71552943bfe5798c93851b30a361.1762800251.git.tim.c.chen@linux.intel.com> X-TM-AS-GCONF: 00 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMTA4MDAyMiBTYWx0ZWRfXx2y1LeaFrpVF aRAtQ2DH1ERbJjZZSaMzMp+SJlTmHOiMKQpiR+Ty3fah7x4/K5+xRXQttqDdDMk0xFypT1atgTA Z9yerqNGiyNdH1VSkTnzWPEAkneCHJEkEOmIygMcjTFD4stdAf0ymur0aXayBpqbY/k3fsfQzOQ c63fkWO+CvW9gGamJT0FWHSc6ChkKk+qAD7A6uJhmL4VFrmPkuZ821SXssuBCgN94wo9wRAeUai 2pozwT0s0yRnRNZfrNNsQ/qGOTIGMZ9baZT72njpHafOIjV+0kCbAitWHQCoLLOEd6HB7jcraFz kzzNxiLSnfeGCETdkBugoTvlva1Ehnzv5BsZXQu8aBJxDYXA2tp/Rtlv9mwMwYY2l4DalvDJ20p jlUrM3CrsiJT/BjJ5bc/RBETARM9og== X-Authority-Analysis: v=2.4 cv=ZK3aWH7b c=1 sm=1 tr=0 ts=69143f18 cx=c_pps a=5BHTudwdYE3Te8bg5FgnPg==:117 a=5BHTudwdYE3Te8bg5FgnPg==:17 a=8nJEP1OIZ-IA:10 a=6UeiqGixMTsA:10 a=VkNPw1HP01LnGYTKEx00:22 a=QyXUC8HyAAAA:8 a=VnNF1IyMAAAA:8 a=jY-qJ0Ov87OzJuVr7I4A:9 a=wPNLvfGTeEIA:10 a=cPQSjfK2_nFv0Q5t_7PE:22 X-Proofpoint-ORIG-GUID: X9_X7_9LL_0-Bk0O5VVs50yWk0AKfmFW X-Proofpoint-GUID: X9_X7_9LL_0-Bk0O5VVs50yWk0AKfmFW 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=2025-11-12_02,2025-11-11_03,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 lowpriorityscore=0 priorityscore=1501 suspectscore=0 phishscore=0 impostorscore=0 spamscore=0 bulkscore=0 adultscore=0 clxscore=1015 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2510240000 definitions=main-2511080022 * Tim Chen [2025-11-10 10:47:35]: > The NUMA sched domain sets the SD_SERIALIZE flag by default, allowing > only one NUMA load balancing operation to run system-wide at a time. > > @@ -11757,17 +11772,26 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq, > .fbq_type = all, > .tasks = LIST_HEAD_INIT(env.tasks), > }; > + bool need_unlock; > > cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask); > > schedstat_inc(sd->lb_count[idle]); > > redo: > + need_unlock = false; > if (!should_we_balance(&env)) { > *continue_balancing = 0; > goto out_balanced; > } > > + if (sd->flags & SD_SERIALIZE) { > + if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1)) { > + goto out_balanced; > + } > + need_unlock = true; > + } > + Moving the serialize check to sched_balance_rq is better since we only take when its really needed. Previously we could have skipped the balancing for the appropriate CPU. > group = sched_balance_find_src_group(&env); > if (!group) { > schedstat_inc(sd->lb_nobusyg[idle]); > @@ -11892,6 +11916,9 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq, > if (!cpumask_subset(cpus, env.dst_grpmask)) { > env.loop = 0; > env.loop_break = SCHED_NR_MIGRATE_BREAK; > + if (need_unlock) > + atomic_set_release(&sched_balance_running, 0); > + One nit: While the current code is good, would conditionally resetting the need_unlock just after resetting the atomic variable better than unconditional reset that we do now? > goto redo; Otherwise looks good to me. Reviewed-by: Srikar Dronamraju -- Thanks and Regards Srikar Dronamraju