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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 13 Jan 2026 06:45:56.8850 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 093d67ee-0923-4f4d-66a3-08de526f67c9 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: BL6PEPF0001AB55.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA1PR12MB9467 Hello Shrikanth, On 1/12/2026 10:34 AM, Shrikanth Hegde wrote: > Running on large systems nohz.nr_cpus cacheline was seen as contended. > There is atomic inc/dec and read happening on many > CPUs at a time and it is possible for this line to bounce often. > > 1st and 2nd patch are minor ones. Looks like correct things to do. > Not very important ones. > > 3rd patch: Main patch which is to get rid of nr_cpus.Instead, use the cpumask > which is always updated alongside with it. Functionally it should serve > the same purpose. Rest of the fields aren't updated that often. So this > line shouldn't bounce that often. > > Contention issue with nohz.idle_cpus_mask still remains. Mostly it is in > separate cacheline than nohz. There are ongoing efforts to mitigate it. It > is not addressed by this series. > > v3 -> v4: > - Added to changelog on one less cacheline being dirtied on idle > entry/exit (Valentin Schneider) I tested the v3 over the weekend and didn't spot any regressions (at least none that I can reproduce consistently) so feel free to include: Reviewed-and-tested-by: K Prateek Nayak If anyone is curious, following are results from my setup (3rd Generation EPYC, 2 socket x 64/128T, boost on, C2 disabled): Note: tbench hit some insane luck on higher utilization runs. I haven't been able to reproduce those regressions reliably. Most data points that show regression also have high run to run variance on both tip and tip + patch making them unreliable. ================================================================== Test : hackbench Units : Normalized time in seconds Interpretation: Lower is better Statistic : AMean ================================================================== Case: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV) 1-groups 1.00 [ -0.00]( 6.43) 1.04 [ -3.60](15.15) 2-groups 1.00 [ -0.00]( 5.42) 1.02 [ -2.17]( 3.57) 4-groups 1.00 [ -0.00]( 2.72) 0.99 [ 0.84]( 3.11) 8-groups 1.00 [ -0.00]( 3.65) 1.00 [ 0.31]( 2.50) 16-groups 1.00 [ -0.00]( 2.26) 1.02 [ -1.67]( 2.92) ================================================================== Test : tbench Units : Normalized throughput Interpretation: Higher is better Statistic : AMean ================================================================== Clients: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV) 1 1.00 [ 0.00]( 0.40) 1.00 [ -0.25]( 1.22) 2 1.00 [ 0.00]( 1.33) 0.99 [ -0.57]( 0.37) 4 1.00 [ 0.00]( 0.27) 1.00 [ 0.07]( 0.89) 8 1.00 [ 0.00]( 0.53) 0.99 [ -0.83]( 0.32) 16 1.00 [ 0.00]( 1.39) 1.00 [ 0.11]( 1.92) 32 1.00 [ 0.00]( 1.85) 0.99 [ -1.44]( 3.08) 64 1.00 [ 0.00]( 1.55) 0.98 [ -2.17]( 2.51) 128 1.00 [ 0.00]( 1.05) 0.94 [ -6.11]( 0.28) 256 1.00 [ 0.00]( 0.68) 0.94 [ -5.58]( 3.77) 512 1.00 [ 0.00]( 0.30) 0.95 [ -4.91]( 0.22) 1024 1.00 [ 0.00]( 0.19) 0.95 [ -4.86]( 0.21) ================================================================== Test : stream-10 Units : Normalized Bandwidth, MB/s Interpretation: Higher is better Statistic : HMean ================================================================== Test: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV) Copy 1.00 [ 0.00]( 8.08) 1.03 [ 2.91]( 4.84) Scale 1.00 [ 0.00]( 5.43) 1.04 [ 3.56]( 3.32) Add 1.00 [ 0.00]( 5.96) 1.04 [ 4.10]( 2.96) Triad 1.00 [ 0.00]( 6.36) 0.99 [ -1.23]( 5.83) ================================================================== Test : stream-100 Units : Normalized Bandwidth, MB/s Interpretation: Higher is better Statistic : HMean ================================================================== Test: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV) Copy 1.00 [ 0.00]( 3.78) 1.03 [ 3.17]( 1.90) Scale 1.00 [ 0.00]( 4.17) 1.02 [ 1.79]( 0.91) Add 1.00 [ 0.00]( 1.97) 1.01 [ 0.52]( 1.66) Triad 1.00 [ 0.00]( 2.28) 0.99 [ -1.49]( 4.44) ================================================================== Test : schbench Units : Normalized 99th percentile latency in us Interpretation: Lower is better Statistic : Median ================================================================== #workers: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV) 1 1.00 [ -0.00](33.02) 1.21 [-20.59]( 8.06) 2 1.00 [ -0.00](14.30) 1.14 [-14.29]( 6.45) 4 1.00 [ -0.00]( 2.22) 0.98 [ 2.22]( 4.55) 8 1.00 [ -0.00]( 4.63) 0.94 [ 5.56]( 1.96) 16 1.00 [ -0.00]( 1.67) 1.07 [ -6.67]( 1.82) 32 1.00 [ -0.00]( 5.58) 0.99 [ 1.04]( 2.11) 64 1.00 [ -0.00]( 6.03) 0.99 [ 0.52]( 5.25) 128 1.00 [ -0.00]( 7.09) 1.00 [ -0.49]( 5.11) 256 1.00 [ -0.00]( 3.14) 0.94 [ 6.06](13.53) 512 1.00 [ -0.00]( 0.86) 0.98 [ 2.23]( 1.53) ================================================================== Test : new-schbench-requests-per-second Units : Normalized Requests per second Interpretation: Higher is better Statistic : Median ================================================================== #workers: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV) 1 1.00 [ 0.00]( 0.14) 1.00 [ 0.00]( 0.52) 2 1.00 [ 0.00]( 0.14) 1.00 [ 0.28]( 0.00) 4 1.00 [ 0.00]( 0.14) 1.00 [ 0.00]( 0.00) 8 1.00 [ 0.00]( 0.00) 1.00 [ 0.00]( 0.14) 16 1.00 [ 0.00]( 0.00) 1.00 [ 0.00]( 0.00) 32 1.00 [ 0.00]( 5.05) 0.97 [ -3.11]( 1.91) 64 1.00 [ 0.00](10.41) 1.06 [ 5.60]( 3.79) 128 1.00 [ 0.00]( 0.30) 0.98 [ -2.38]( 0.31) 256 1.00 [ 0.00]( 1.43) 0.98 [ -1.73]( 1.38) 512 1.00 [ 0.00]( 1.45) 0.97 [ -3.33]( 1.48) ================================================================== Test : new-schbench-wakeup-latency Units : Normalized 99th percentile latency in us Interpretation: Lower is better Statistic : Median ================================================================== #workers: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV) 1 1.00 [ -0.00](24.99) 1.08 [ -8.33](16.90) 2 1.00 [ -0.00]( 0.00) 1.40 [-40.00](18.20) 4 1.00 [ -0.00](12.06) 1.27 [-27.27]( 7.75) 8 1.00 [ -0.00](14.13) 0.90 [ 10.00](23.66) 16 1.00 [ -0.00](15.96) 1.09 [ -9.09]( 7.45) 32 1.00 [ -0.00](12.06) 0.91 [ 9.09](18.23) 64 1.00 [ -0.00](15.78) 1.06 [ -6.25](13.18) 128 1.00 [ -0.00](10.57) 1.03 [ -3.41]( 5.15) 256 1.00 [ -0.00]( 0.32) 1.00 [ -0.00]( 0.21) 512 1.00 [ -0.00]( 0.00) 1.00 [ 0.38]( 0.20) ================================================================== Test : new-schbench-request-latency Units : Normalized 99th percentile latency in us Interpretation: Lower is better Statistic : Median ================================================================== #workers: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV) 1 1.00 [ -0.00]( 0.00) 1.00 [ -0.27]( 1.79) 2 1.00 [ -0.00]( 0.74) 0.96 [ 4.07]( 1.90) 4 1.00 [ -0.00]( 0.37) 0.96 [ 3.83]( 1.91) 8 1.00 [ -0.00]( 1.02) 1.00 [ -0.28]( 1.52) 16 1.00 [ -0.00]( 1.61) 1.00 [ 0.28]( 1.86) 32 1.00 [ -0.00]( 9.22) 1.04 [ -3.52]( 6.84) 64 1.00 [ -0.00]( 6.39) 1.06 [ -5.96](22.58) 128 1.00 [ -0.00]( 1.08) 1.12 [-12.43]( 4.61) 256 1.00 [ -0.00]( 6.10) 1.01 [ -0.77]( 4.87) 512 1.00 [ -0.00]( 1.41) 1.01 [ -1.03]( 1.27) -- Thanks and Regards, Prateek