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Tue, 13 Jan 2026 09:01:07 GMT Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 097D22004E; Tue, 13 Jan 2026 09:01:07 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 1C82C20040; Tue, 13 Jan 2026 09:01:04 +0000 (GMT) Received: from [9.39.17.221] (unknown [9.39.17.221]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Tue, 13 Jan 2026 09:01:03 +0000 (GMT) Message-ID: <1c0641e1-38a0-42dc-ae70-393c93f2ca97@linux.ibm.com> Date: Tue, 13 Jan 2026 14:31:03 +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 v2] sched/clock: Avoid false sharing for sched_clock_irqtime To: Wangyang Guo , K Prateek Nayak Cc: linux-kernel@vger.kernel.org, Benjamin Lei , Tim Chen , Tianyou Li , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider References: <20260113074807.3404180-1-wangyang.guo@intel.com> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <20260113074807.3404180-1-wangyang.guo@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: Adi3QJz-sumDwjda94mZ1OBC4VfJNzED X-Authority-Analysis: v=2.4 cv=LLxrgZW9 c=1 sm=1 tr=0 ts=696609d6 cx=c_pps a=GFwsV6G8L6GxiO2Y/PsHdQ==:117 a=GFwsV6G8L6GxiO2Y/PsHdQ==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=VkNPw1HP01LnGYTKEx00:22 a=VwQbUJbxAAAA:8 a=QyXUC8HyAAAA:8 a=VnNF1IyMAAAA:8 a=3XoYeNF2x3M5Qf99c0oA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTEzMDA3MyBTYWx0ZWRfX3IPgDF/CMeCS HmpOdH4Fv5FIgaeCLl08++HBnmANjLKJRO/pUv8umLJl/ZcXc+JAiBNW0ATv3glsUGF/+VKYC1A +XPP37fEsO/uaFUBBH/QNMNcLE3bPgEvJA0ROczna+34BfcRe62zUOVYflwSuP6kTfh3+sFqxcg 370XpQ3FHMx7K2qvqZaEMV0m+xqc1v4jmLRcCFS+xHRGXAWOi1CM35aOXEe3UUWeuko9A7nO7HL 4UO8TTWG/3WAkkduC8sr8Y793eqBbHXWZqQBjfSB+DSiacAuZbqa4meIcgV4kwV7Y4IkNlgEO8D xMF3Rb59yg0AH5jdQfr2xa/2XKwCJnxi1MwUT3Xe1Poe1p4uowgYusmxecBQPYnlbPjxeNJZ7QT vwOtKMVILZq2GWv7KKUxE5hyL4/EpB/U+FencqtTi+73vgXMi4ZQFIjGmtzD4l6sqVd4007QDTF /+P7vprDh9GB+z2xsyg== X-Proofpoint-ORIG-GUID: ozrQT97ALQaxKVdTGRWWS4WHd8ScnChh 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=2026-01-13_01,2026-01-09_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 suspectscore=0 clxscore=1015 spamscore=0 impostorscore=0 malwarescore=0 phishscore=0 adultscore=0 lowpriorityscore=0 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2512120000 definitions=main-2601130073 On 1/13/26 1:18 PM, Wangyang Guo wrote: > Read-mostly sched_clock_irqtime may share the same cacheline with > frequently updated nohz struct. Mark it as __read_mostly 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) > * 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. > > Changes since V1: > - Use __read_mostly instead of __cacheline_aligned to avoid wasting > spaces. > > History: > v1: https://lore.kernel.org/all/20260113022958.3379650-1-wangyang.guo@intel.com/ > prev discussions: https://lore.kernel.org/all/20251211055612.4071266-1-wangyang.guo@intel.com/T/#u > > Suggested-by: Shrikanth Hegde I think it would appropriate to take prateek's tag here. > Reported-by: Benjamin Lei > Reviewed-by: Tim Chen > Reviewed-by: Tianyou Li > Signed-off-by: Wangyang Guo > --- > kernel/sched/cputime.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c > index 7097de2c8cda..fc8f8081c48f 100644 > --- a/kernel/sched/cputime.c > +++ b/kernel/sched/cputime.c > @@ -25,7 +25,7 @@ > */ > DEFINE_PER_CPU(struct irqtime, cpu_irqtime); > > -int sched_clock_irqtime; > +int sched_clock_irqtime __read_mostly; > > void enable_sched_clock_irqtime(void) > { Patch should be good on its own. Reviewed-by: Shrikanth Hegde