Re: [PATCH] sched/fair: Avoid false sharing in nohz struct

[Guo, Wangyang]

On 12/21/2025 9:05 PM, Shrikanth Hegde wrote:
> Hi Wangyang,
>
> On 12/11/25 11:26 AM, Wangyang Guo wrote:
> > There are two potential false sharing issue in nohz struct:
> > 1. idle_cpus_mask is a read-mostly field, but share the same cacheline
> >     with frequently updated nr_cpus.
>
> Updates to idle_cpus_mask is not same cacheline. it is updated alongside 
> nr_cpus.
>
> with CPUMASK_OFFSTACK=y, idle_cpus_mask is a pointer to the actual mask.
> Updates to it happen in another cacheline.
>
> with CPUMASK_OFFSTACK=n, idle_cpus_mask is on the stack and its length
> depends on NR_CPUS. typical value being 512/2048/8192 it can span a few
> cachelines. So updates to it likely in different cacheline compared to 
> nr_cpus.
>
> see  https://lore.kernel.org/all/aS6bK4ad-wO2fsoo@xxxxxxxxx/
This patch is mainly target for idle_cpus_mask as a pointer, which is 
default for many distro OS.

> Likely in your case, nr_cpus updates are the costly ones.
> Try below and see if it helps to fix your issue too.
> https://lore.kernel.org/all/20251201183146.74443-1-sshegde@xxxxxxxxxxxxx/
> I Should send out new version soon.
>
> > 2. Data followed by nohz still share the same cacheline and has
> >     potential false sharing issue.
>
> How does your patch handle this?
> I don't see any other struct apart from nohz being changed.

The data follow by nohz is implicit and determined by compiler.
For example, this is the layout from /proc/kallsyms in my machine:
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

What we can do is placing read-mostly `idle_cpus_mask` pointer in a new 
cacheline, so data followed by nohz would not be affected by nr_cpus.

> > This patch tries to resolve the above two problems by isolating the
> > frequently updated fields in a single cacheline.
> >
> > Reported-by: Benjamin Lei <benjamin.lei@xxxxxxxxx>
> > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Reviewed-by: Tianyou Li <tianyou.li@xxxxxxxxx>
> > Signed-off-by: Wangyang Guo <wangyang.guo@xxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5b752324270b..bcc2766b7986 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7193,13 +7193,14 @@ static DEFINE_PER_CPU(cpumask_var_t, 
> > should_we_balance_tmpmask);
> >   #ifdef CONFIG_NO_HZ_COMMON
> >   static struct {
> > -    cpumask_var_t idle_cpus_mask;
> > -    atomic_t nr_cpus;
> > +    /* Isolate frequently updated fields in a cacheline to avoid 
> > false sharing issue. */
> > +    atomic_t nr_cpus ____cacheline_aligned;
> >       int has_blocked;        /* Idle CPUS has blocked load */
> >       int needs_update;        /* Newly idle CPUs need their 
> > next_balance collated */
> >       unsigned long next_balance;     /* in jiffy units */
> >       unsigned long next_blocked;    /* Next update of blocked load in 
> > jiffies */
> > -} nohz ____cacheline_aligned;
> > +    cpumask_var_t idle_cpus_mask ____cacheline_aligned;
> > +} nohz;
>
> This can cause a lot of space wastage.
> for exp: powerpc has 128 byte cacheline.

nohz is global, only one exists. The size inflating is minimal, less 
than 1 cacheline.

BR
Wangyang