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

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

On Fri, Aug 14, 2026 at 11:32:09AM +0100, Usama Arif wrote:
> 
> 
> On 13/08/2026 18:29, Shakeel Butt wrote:
> > 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.
> > 
> 
> 
> Yes this makes sense. I did not expect such a big change in a benchmark
> with my patch, although I feel like the microbenchmark is probably not
> that realistic.
> 
> I think another issue is that its a 4 socket system.
> I only have access to a single socket system, and I see a 4.38% regression.
> 
> Do you know if there a way for kernel test robot to test the below patch
> on its host?

+Oliver

I used to request Oliver to test a patch for performance regressions reported by
intel test robot.

> 
> 
> From b862e84e7bd6a54b1546b7f21a6cf991253def18 Mon Sep 17 00:00:00 2001
> From: Usama Arif <usama.arif@linux.dev>
> Date: Thu, 13 Aug 2026 11:42:05 -0700
> Subject: [PATCH] mm/memcontrol: avoid false sharing between vmstats and events
> 
> Moving v1 userspace eventfd handling into memcontrol-v1.c shrank
> struct vmpressure from 112 to 24 bytes when CONFIG_MEMCG_V1 is disabled.
> This moved memory_events_local[MEMCG_SWAP_FAIL] and the hot
> vmstats_percpu pointer onto the same cacheline.
> 
> The stress-ng mremap stressor exercises MADV_PAGEOUT with swap
> disabled, generating about 20 million MEMCG_SWAP_FAIL updates per
> 60-second run on a 176-CPU test system.  Those writes bounce the line
> while memcg statistics paths load vmstats_percpu.
> 
> Move cgwb_list into the existing alignment gap and cacheline-align
> vmstats_percpu.  This separates the pointer from the event counters
> without increasing the size of struct mem_cgroup in the tested
> configuration.
> 
> The blamed commit reduced median mremap throughput by 4.38% on the
> test system with one socket. The patched kernel brings the performance
> to within 0.5% of the parent which is within the observed boot-to-boot
> spread (up to 1.2%).
> 
> Fixes: ea928e9e18da ("mm/vmpressure: move v1 userspace eventfd code into memcontrol-v1.c")
> Reported-by: kernel test robot <yi1.lai@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202608131743.c6a7dda4-lkp@intel.com
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  include/linux/memcontrol.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> 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
> -- 
> 2.53.0-Meta
> 
> 
> 
> >>
> >> 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
> >>
>