Re: [patch v11 07/13] task isolation: sync vmstats conditional on changes

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Feb 04, 2022 at 02:35:44PM -0300, Marcelo Tosatti wrote:
> Rather than syncing VM-stats on every return to userspace
> (or VM-entry), keep track of changes through a per-CPU bool.
> 
> This improves performance when enabling task isolated
> for vcpu VMs.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> ---
> v11:
> - Add TIF_TASK_ISOL bit to thread info flags and use it
>   to decide whether to perform task isolation work on
>   return to userspace     
> 
>  include/linux/vmstat.h |   13 ++++++++++++-
>  mm/vmstat.c            |   29 ++++++++++++++++++++++++++++-
>  2 files changed, 40 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/include/linux/vmstat.h
> ===================================================================
> --- linux-2.6.orig/include/linux/vmstat.h
> +++ linux-2.6/include/linux/vmstat.h
> @@ -22,7 +22,18 @@ int sysctl_vm_numa_stat_handler(struct c
>  #endif
>  
>  #if defined(CONFIG_SMP) && defined(CONFIG_TASK_ISOLATION)
> -void sync_vmstat(void);
> +DECLARE_PER_CPU_ALIGNED(bool, vmstat_dirty);
> +
> +extern struct static_key vmstat_sync_enabled;
> +
> +void __sync_vmstat(void);
> +static inline void sync_vmstat(void)
> +{
> +	if (static_key_false(&vmstat_sync_enabled))
> +		__sync_vmstat();
> +}
> +
> +void init_sync_vmstat(void);
>  #else
>  static inline void sync_vmstat(void)
>  {
> Index: linux-2.6/mm/vmstat.c
> ===================================================================
> --- linux-2.6.orig/mm/vmstat.c
> +++ linux-2.6/mm/vmstat.c
> @@ -334,6 +334,31 @@ void set_pgdat_percpu_threshold(pg_data_
>  	}
>  }
>  
> +#ifdef CONFIG_TASK_ISOLATION
> +struct static_key vmstat_sync_enabled;
> +DEFINE_PER_CPU_ALIGNED(bool, vmstat_dirty);
> +
> +static inline void mark_vmstat_dirty(void)
> +{
> +	if (!static_key_false(&vmstat_sync_enabled))
> +		return;
> +
> +	raw_cpu_write(vmstat_dirty, true);

Why not __this_cpu_write() ? Shouldn't we make sure we are not
preemptible and not mark the wrong CPU?

Thanks.