linux-kernel - Re: [PATCH v4] sched/clock: Avoid false sharing for sched_clock

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Message-ID: <d5c4d759-7f48-44f8-bad2-23577d6c4462@intel.com>
Date: Tue, 27 Jan 2026 11:24:49 +0800
From: "Guo, Wangyang" <wangyang.guo@...el.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, Benjamin Lei <benjamin.lei@...el.com>,
 Tim Chen <tim.c.chen@...ux.intel.com>, Tianyou Li <tianyou.li@...el.com>,
 K Prateek Nayak <kprateek.nayak@....com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH v4] sched/clock: Avoid false sharing for
 sched_clock_irqtime

On 1/27/2026 2:45 AM, Shrikanth Hegde wrote:
> Hi Wangyang.
> 
> On 1/26/26 7:44 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:
>> In kernel 6.14, 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.
>>
> 
> IMHO, this changelog is meant for latest sched/core. having 6.14 data 
> won't be
> that relevant.
> 
> Also, changelog should be mentioning essence of calling disable in
> sched_clock_init_late which was in v3 discussions.
> 
>> 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.
>>
>> ---
> 
> Please dont put a --- here.
> 
> git am will strip everything after this and actual diff. So the your
> signed-off-by and other details won't be captured.
> 
> If the intention was keep version history, put it after the signed-off-by
> for example:
> https://lore.kernel.org/lkml/20260120113246.27987-2-kprateek.nayak@amd.com/
Changelog updated in v5:

https://lore.kernel.org/all/20260127031602.1907377-1-wangyang.guo@intel.com/

BR
Wangyang