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Tue, 4 Aug 2026 21:18:25 +0000 (GMT) Message-ID: <7e49236e-7f17-4583-8196-34ce9a2fa762@linux.ibm.com> Date: Wed, 5 Aug 2026 02:48:24 +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: [PATCH v4] sched/fair: Prefer fully idle cores for NOHZ balancing To: Andrea Righi Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , Christian Loehle , Phil Auld , Mete Durlu , linux-kernel@vger.kernel.org References: <20260804151324.918020-1-arighi@nvidia.com> <92dae374-14ec-4acb-b451-7ac81d9d262b@linux.ibm.com> Content-Language: en-US From: Shrikanth Hegde In-Reply-To: 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-ORIG-GUID: BJEcsxr0EA9NNc1lZ8CN8fHYT_0PyrlV X-Proofpoint-Spam-Info: AW1haW4tMjYwODA0MDE3MiBTYWx0ZWRfXzOaV91oTfPHO rdG2IAAheTGyVLk+4DTQCt5mprYX9TSol8bdy0dlhLRiOQ16y/66ZXROV1k3lVBsygPvtCQsUyl tBx+8hWaFRhWtd0UTNbTkTDPJ8M1+Y0= X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODA0MDE3MiBTYWx0ZWRfX6uEuj+NK/544 fYWboeH9y1XulCVVd+elvc4TWaQQmlYJOEsGvvB/W2Eci0Bw3lCxxORj0EX0heiOk04hw2jgBg5 3zdmx8zEkYULxsZMdN2B2/x8GzC+7GoDMa0UgC/MPMW5NF/pLltwaUk7y6Sn0PhvDeFb3hcFjjY bFVKoLehjRqcbX+xLDKPp4lv9WpHnRcoTmzZvTqt7OtzGW1VLueJ3kOoCzdm7SmGvSR0NtL6PLS QpMrY3dwIEsWXmPlcq0ID87Kt9Rcu51dceMHMDw25sU66i4WIkr8fIMj0UK8ze0oRyWPn0BfreN rUrqslzroYf3mLt+Fwc36rXTVMA81DOS7DypGBDsuUDEQUP4cULanJpvQw5bUy/a9icnsYlViF4 1WCRlPy/E3bIDXRuUvzBpxtxiY+H9GPe12dsjpMA+Vw6C/N9Mv9xTdgCVaAkUA9cPzJrnq8V7S0 6UHzMJCA0Wi30gcLisQ== X-Proofpoint-GUID: 4zCqVAw9fnkWmRcZbx_fFG5SrRXILDGq X-Authority-Analysis: v=2.4 cv=KfzidwYD c=1 sm=1 tr=0 ts=6a725728 cx=c_pps a=5BHTudwdYE3Te8bg5FgnPg==:117 a=5BHTudwdYE3Te8bg5FgnPg==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=RzCfie-kr_QcCd8fBx8p:22 a=IgSxoIpG7Lo4KQGFdqQA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-04_05,2026-08-04_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 bulkscore=0 impostorscore=0 suspectscore=0 malwarescore=0 adultscore=0 clxscore=1015 priorityscore=1501 phishscore=0 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608040172 On 8/4/26 11:39 PM, Andrea Righi wrote: > Hi Shrikanth, > >> Just Curious, making nohz_full= yeilds similar numbers? > > Yes, provided that the housekeeping core is also excluded from the workload. > > I tested this booting with: > > nohz_full=1-175,177-351 > > This leaves CPUs 0 and 176 for housekeeping. I excluded that core from the > workload and ran 87 OpenMP tasks on the remaining node-0 cores (1 task per core, > excluding the housekeeping one): > > $ env OMP_NUM_THREADS=87 \ > OMP_DYNAMIC=false \ > OPENBLAS_LOOPS=10 \ > OPENBLAS_PARAM_M=16384 \ > OPENBLAS_PARAM_N=16384 \ > OPENBLAS_PARAM_K=16384 \ > numactl -C 1-87,177-263 --membind=0 \ > ./benchmark/sgemm.goto 1 1 1 > > Results: > > unpatched : 5.246 TFLOP/s > unpatched+nohz_full : 6.953 TFLOP/s > patched : 6.861 TFLOP/s > patched+nohz_full : 6.984 TFLOP/s > > So, nohz_full seems to prevent the problematic ILB wakeups and can produce > similar results for this CPU-bound workload. However, it shouldn't be considered > a sobstiute for the ILB fix, since it requires explicit partitioning and > reserved housekeeping CPUs. Full dyntick also enables context tracking on the > isolated CPUs, adding kernel entry/exit overhead. > That's good to know. Thanks for running them. I put "just curious", knowing nohz_full is not a solution.