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Tue, 7 Apr 2026 17:58:49 +0000 (GMT) Message-ID: <4276a8b7-141a-4e0b-bc35-47cb0a341801@linux.ibm.com> Date: Tue, 7 Apr 2026 23:28:48 +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: Zhang Qiao Cc: tanghui20@huawei.com, Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , linux-kernel@vger.kernel.org References: <20260407063915.2034198-1-zhangqiao22@huawei.com> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <20260407063915.2034198-1-zhangqiao22@huawei.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-Authority-Analysis: v=2.4 cv=KeridwYD c=1 sm=1 tr=0 ts=69d545de cx=c_pps a=GFwsV6G8L6GxiO2Y/PsHdQ==:117 a=GFwsV6G8L6GxiO2Y/PsHdQ==:17 a=IkcTkHD0fZMA:10 a=A5OVakUREuEA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=RzCfie-kr_QcCd8fBx8p:22 a=i0EeH86SAAAA:8 a=zRBEQ48W-6ZW6DrTdEYA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-ORIG-GUID: W2lE260H_6WTa9XJPOxPQCgPA0ZY7bgW X-Proofpoint-GUID: S3g6f947kNQQcdZ7sch22iQ7XE1g8FDe X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNDA3MDE2MSBTYWx0ZWRfX3e/EIOmFd/eW pp0wRZjj1AthBLaSyuFeq48tRMYsSjQ9KDiu+BNPox5dGLbRjmZY7SMvq/rQF449cb15SCdg0RD oggCH5sK6Ox5ZjnKtZWn/VGbOnG3REysff0qjcSAEx4jkM0IwoE7T5kUpmWOW/w5itLM3fGWQz5 jbAtOuZAp6Q8ooDehxB4i3Oqs+ZKuZfw7mhmoO0ctBTEexwems7gCqXri+HqwIVHoNyWALwqsEC k4Swsdj/PqSPwQot/GV08YnnVWI/5cPqCX09Cy5EKurh//F4xsziZNPJgEcvw47VGVZ+8r2DzWH iLlYfGDeMS3Rjrx/kaQwbFkF2G4dXlOak6qx9Ctc2qvwV+VK0MOEUTiFEMk2NuO7br1etk65rGG SaSxmPUBc3PpcAuXxHNj4zcGpXwPCOSVqu8sLu1V+U+GprldMGE+alAV1+emD0fGhFiEnicSilR 6Ge45D7hHxI8mSUqlAg== 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 suspectscore=0 malwarescore=0 lowpriorityscore=0 adultscore=0 impostorscore=0 clxscore=1011 phishscore=0 priorityscore=1501 spamscore=0 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2604010000 definitions=main-2604070161 Hi. On 4/7/26 12:09 PM, 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. > Isn't load balance to move it out? What does the workload do? > 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. > There are systems where LLC_SIZE == SMT_SIZE. i.e one core in the LLC. This would effectively disable wake_affine feature in such systems. Power10 being a major example. > 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)