linux-kernel - Re: [PATCH v3] sched/clock: Avoid false sharing for sched_clock

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Message-ID: <7277bf6a-eebf-4b76-801a-8d1bab16d911@linux.ibm.com>
Date: Fri, 16 Jan 2026 22:28:32 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Wangyang Guo <wangyang.guo@...el.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 v3] sched/clock: Avoid false sharing for
 sched_clock_irqtime

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.