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Tue, 7 Apr 2026 18:16:57 +0000 (GMT) Message-ID: <45999d3a-4a83-4b58-bfda-9639a27eb2e4@linux.ibm.com> Date: Tue, 7 Apr 2026 23:46:57 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] sched/fair: scale wake_wide() threshold by SMT width To: Dietmar Eggemann Cc: tanghui20@huawei.com, Zhang Qiao , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , linux-kernel@vger.kernel.org References: <20260407063915.2034198-1-zhangqiao22@huawei.com> <0aca0ee5-4102-426e-b290-6c50b6e819a7@arm.com> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <0aca0ee5-4102-426e-b290-6c50b6e819a7@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNDA3MDE2MSBTYWx0ZWRfX8LzAhsXN/s7k lE35zgJXjusV4jE3RxY4S3ewOrgMp9LrIZPdTW2/w8LxL7PlmuuyvP0V0QVzmPhqv385VNGfSef s8bbg+l4fOju3TM4ZWCHdE2qAUlK0/fqx45zsWCNNyOPVlYedW29sw4Fp4/S369TYvfhRWGVEyo Zu8d9OEKBwxpfEh37xAgg0cj299vXtaqjYs8fj9gMMoDUx4ltec0vYM0XJmf3bGDmdyN9oKCMH7 vBcqLFHFHIQLkDVKukYwBvExkr4qus5rdJ3pArqjRFJ3yQDf2T9HZ50kqZwojTuVCImKFmmLnkn 0lHZVD3NmsOjN/Xn3sSY2kkholzIuzdbWL+1i7RsZ5NJsFY7j2xJXEpN8RrBwFJIgmWn1oa1lLI 69oZHEhlf0H5O5pPEIFJ8FHQNxf1VtP6UM5jVrD4bqBwKRMA6YYfdfUGcQ9eziCmMD+6tNSsn1D 9IruBVljPydZRHsD8ew== X-Proofpoint-GUID: jX86ymyOrhbq1uwnztjFhTNO9RDqX9Hq X-Authority-Analysis: v=2.4 cv=a/wAM0SF c=1 sm=1 tr=0 ts=69d54a1f cx=c_pps a=5BHTudwdYE3Te8bg5FgnPg==:117 a=5BHTudwdYE3Te8bg5FgnPg==:17 a=IkcTkHD0fZMA:10 a=A5OVakUREuEA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=i0EeH86SAAAA:8 a=GGN0yvBJN0WIA8DzvZ0A:9 a=QEXdDO2ut3YA:10 X-Proofpoint-ORIG-GUID: paF4xsHFn8x04aJJ45hQm-BRboweOj81 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=2026-04-07_04,2026-04-07_05,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 clxscore=1015 spamscore=0 impostorscore=0 priorityscore=1501 phishscore=0 lowpriorityscore=0 adultscore=0 malwarescore=0 suspectscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2604010000 definitions=main-2604070161 On 4/7/26 8:08 PM, Dietmar Eggemann wrote: > On 07.04.26 08:39, Zhang Qiao wrote: >> wake_wide() uses sd_llc_size as the spreading threshold to detect wide >> waker/wakee relationships and to disable wake_affine() for those cases. >> >> On SMT systems, sd_llc_size counts logical CPUs rather than physical >> cores. This inflates the wake_wide() threshold, allowing wake_affine() >> to pack more tasks into one LLC domain than the actual compute capacity >> of its physical cores can sustain. The resulting SMT interference may >> cost more than the cache-locality benefit wake_affine() intends to gain. >> >> Scale the factor by the SMT width of the current CPU so that it >> approximates the number of independent physical cores in the LLC domain, >> making wake_wide() more likely to kick in before SMT interference >> becomes significant. On non-SMT systems the SMT width is 1 and behaviour >> is unchanged. >> >> Signed-off-by: Zhang Qiao >> --- >> kernel/sched/fair.c | 5 +++++ >> 1 file changed, 5 insertions(+) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index f07df8987a5ef..4896582c6e904 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -7334,6 +7334,11 @@ static int wake_wide(struct task_struct *p) >> unsigned int slave = p->wakee_flips; >> int factor = __this_cpu_read(sd_llc_size); >> >> + /* Scale factor to physical-core count to account for SMT interference. */ >> + if (sched_smt_active()) >> + factor = DIV_ROUND_UP(factor, >> + cpumask_weight(cpu_smt_mask(smp_processor_id()))); >> + >> if (master < slave) >> swap(master, slave); >> if (slave < factor || master < slave * factor) > > I assume not a lot of people care since this needs: wake_affine machinery needs SD_WAKE_AFFINE. No? > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c > index 5847b83d9d55..596c5d590532 100644 > --- a/kernel/sched/topology.c > +++ b/kernel/sched/topology.c > @@ -1691,7 +1691,7 @@ sd_init(struct sched_domain_topology_level *tl, > .flags = 1*SD_BALANCE_NEWIDLE > | 1*SD_BALANCE_EXEC > | 1*SD_BALANCE_FORK > - | 0*SD_BALANCE_WAKE > + | 1*SD_BALANCE_WAKE > | 1*SD_WAKE_AFFINE > | 0*SD_SHARE_CPUCAPACITY > | 0*SD_SHARE_LLC > > And then it's a trade-off between one busy thread per core vs. wakeup cost.