Re: [linux-next:master] [mm/vmpressure] ea928e9e18: stress-ng.mremap.ops_per_sec 36.2% regression

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Aug 13, 2026 at 06:14:50PM +0100, Usama Arif wrote:
> 
> 
> On 13/08/2026 18:01, Shakeel Butt wrote:
> > On Thu, Aug 13, 2026 at 09:16:22PM +0800, kernel test robot wrote:
> >>
> >>
> >> Hello,
> >>
> >> kernel test robot noticed a 36.2% regression of stress-ng.mremap.ops_per_sec on:
> >>
> >> commit: ea928e9e18da682e9a5bc40aa862bff7ce5ae42e ("mm/vmpressure: move v1 userspace eventfd code into memcontrol-v1.c") https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> >>
> >> in testcase: stress-ng
> >> version: stress-ng-x86_64-29ce10a2c-1_20260712
> >> with following parameters:
> >>
> >> 	nr_threads: 100%
> >> 	testtime: 60s
> >> 	test: mremap
> >> 	cpufreq_governor: performance
> >>
> >>
> >>
> >> config: x86_64-rhel-9.4 (CONFIG_MEMCG=y and CONFIG_MEMCG_V1 is not set)
> >> compiler: gcc-14
> >> test machine: 256 threads 4 sockets INTEL(R) XEON(R) PLATINUM 8592+ (Emerald Rapids) with 256G memory
> >>
> >> (please refer to attached dmesg/kmsg for entire log/backtrace)
> >>
> > 
> > Hi there,
> > 
> > Can you please test the following patch and see if it fixes the regression?
> > 
> > 
> > From 84c0b05b3bc5cf73ee66ead75aafb1ad684462c3 Mon Sep 17 00:00:00 2001
> > From: Shakeel Butt <shakeel.butt@linux.dev>
> > Date: Thu, 13 Aug 2026 09:38:28 -0700
> > Subject: [PATCH] memcg: keep vmstats_percpu off the memory_events[] cacheline
> > 
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> > ---
> >  include/linux/memcontrol.h | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index e78bc98ab229..e25d5b9a1db8 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -246,8 +246,13 @@ struct mem_cgroup {
> >  	/* handle for "memory.swap.events" */
> >  	struct cgroup_file swap_events_file;
> >  
> > -	/* memory.stat */
> > +	/* Read-mostly. */
> >  	struct memcg_vmstats	*vmstats;
> > +	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
> > +	int			kmemcg_id;
> > +
> > +	/* Write-hot from here on; do not let it share with the above. */
> > +	CACHELINE_PADDING(_pad_);
> >  
> >  	/* memory.events */
> >  	atomic_long_t		memory_events[MEMCG_NR_MEMORY_EVENTS];
> > @@ -266,9 +271,6 @@ struct mem_cgroup {
> >  #if BITS_PER_LONG < 64
> >  	seqlock_t		socket_pressure_seqlock;
> >  #endif
> > -	int kmemcg_id;
> > -
> > -	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
> >  
> >  #ifdef CONFIG_CGROUP_WRITEBACK
> >  	struct list_head cgwb_list;
> 
> 
> I was currently testing this diff, not sure which one would be better.

I was just checking if false sharing of vmstats_percpu is the cause. If your
patch does not increase the struct size, we can go with that as a backportable
fix. I am planning to rearrange fields of struct mem_cgroup more drastically and
have it more stable as future work as we continuously see these regressions keep
popping up.

> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index e78bc98ab229b..215e2e87f42b2 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -268,10 +268,15 @@ struct mem_cgroup {
>  #endif
>         int kmemcg_id;
> 
> -       struct memcg_vmstats_percpu __percpu *vmstats_percpu;
> -
>  #ifdef CONFIG_CGROUP_WRITEBACK
>         struct list_head cgwb_list;
> +#endif
> +
> +       /* Keep the hot per-CPU stats pointer away from memory event counters. */
> +       struct memcg_vmstats_percpu __percpu *vmstats_percpu
> +               ____cacheline_aligned_in_smp;
> +
> +#ifdef CONFIG_CGROUP_WRITEBACK
>         struct wb_domain cgwb_domain;
>         struct memcg_cgwb_frn cgwb_frn[MEMCG_CGWB_FRN_CNT];
>  #endif
>