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

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Date:   Mon, 7 Feb 2022 16:29:43 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Nitesh Lal <nilal@...hat.com>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alex Belits <abelits@...its.com>, Peter Xu <peterx@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Oscar Shiang <oscar0225@...email.tw>
Subject: Re: [patch v11 07/13] task isolation: sync vmstats conditional on
 changes

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@...hat.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);
> +	set_thread_flag(TIF_TASK_ISOL);

This will force the return-to-user slow path on all CPUs,
whether the current task is isolated or not, right?

Also if we context switch to an isolated TASK B with a dirty vmstat,
we miss the TIF_TASK_ISOL.