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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Authority-Analysis: v=2.4 cv=KvJAGGWN c=1 sm=1 tr=0 ts=696a6e3f cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=VkNPw1HP01LnGYTKEx00:22 a=vCJXZfvWvrYXnvdGxyQA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTE2MDEyMiBTYWx0ZWRfX5ONWW+zefy+R E7L9ZrDkHUCIh5i2ZkG+9GafGSPx9GJDh3Oob4JilU87gBl1MTpR+HMUiAyTa0f028CUjxk/Iso QdPPLooRbLN3XWKDkXuZuuymCZRk3/bIFgsTbCBWHotpykRTVU3k0JewUz/jwkqmwoSwX5hnthC //LrovdtXHfheTUNd1P6XUvOjCv+nAJZc4f2fjOvY+KgkYbD4T7pdknueDXlij6iCbxGTCU7wfu m/CGFNY6/YM8kNWd0qK7gMvYL6/S/8OkfFvCGzVzc7XJfDmgoQIfO1iThcj5gMu4Sgz9ClMl7y6 jdujO+5eSjfWsFsCX/yJSeREs1oZKgPp50eRFPMgWfu3j5m2VVBaxAfxQGdcWdD3p/D+AX8BQKt tftc/Ph3lLx9B59QEF+eDBVXfoncDpU37CaSOSF7+pP58aviz8XauM6lzPa35FzAUlsNsBszj/r cd00zkfqYUIre4DUBiA== X-Proofpoint-GUID: J82omWu3E63HMbKIIKnOgCNYkuiUFJ-d X-Proofpoint-ORIG-GUID: PnQ5H3OjjWGOqgiT-JF_qfg_Iw1SdrE5 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=2026-01-16_06,2026-01-15_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 malwarescore=0 suspectscore=0 impostorscore=0 phishscore=0 adultscore=0 clxscore=1015 spamscore=0 bulkscore=0 priorityscore=1501 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2512120000 definitions=main-2601160122 Hi Wangyang. On 1/16/26 8:09 AM, Wangyang Guo wrote: > Read-mostly sched_clock_irqtime may share the same cacheline with > frequently updated nohz struct. Make it as static_key to avoid > false sharing issue. > > Details: > We observed ~3% cycles hotspots in irqtime_account_irq when running > SPECjbb2015 in a 2-sockets system. Most of cycles spent in reading > sched_clock_irqtime, which is a read-mostly var. > > perf c2c (cachelien view) shows it has false sharing with nohz struct: > Num RmtHitm LclHitm Offset records Symbol > 6.25% 0.00% 0.00% 0x0 4 [k] _nohz_idle_balance.isra.0 > 18.75% 100.00% 0.00% 0x8 14 [k] nohz_balance_exit_idle > 6.25% 0.00% 0.00% 0x8 8 [k] nohz_balance_enter_idle > 6.25% 0.00% 0.00% 0xc 8 [k] sched_balance_newidle > 6.25% 0.00% 0.00% 0x10 31 [k] nohz_balancer_kick > 6.25% 0.00% 0.00% 0x20 16 [k] sched_balance_newidle > 37.50% 0.00% 0.00% 0x38 50 [k] irqtime_account_irq > 6.25% 0.00% 0.00% 0x38 47 [k] account_process_tick > 6.25% 0.00% 0.00% 0x38 12 [k] account_idle_ticks > > Offsets: > * 0x0 -- nohz.idle_cpu_mask (r) > * 0x8 -- nohz.nr_cpus (w) This is recently removed. > * 0x38 -- sched_clock_irqtime (r), not in nohz, but share cacheline > > The layout in /proc/kallsyms can also confirm that: > ffffffff88600d40 b nohz > ffffffff88600d68 B arch_needs_tick_broadcast > ffffffff88600d6c b __key.264 > ffffffff88600d6c b __key.265 > ffffffff88600d70 b dl_generation > ffffffff88600d78 b sched_clock_irqtime > > With the patch applied, irqtime_account_irq hotspot disappear. > This changelog likely needs to change with recent removal of nohz.nr_cpus. Please re-run it once with latest sched/core. (atleast offsets etc) The patch has merit on its own since the variable doesn't update often.