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Date:   Mon, 24 Oct 2022 13:54:26 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Aaron Tomlin <atomlin@redhat.com>
Cc:     mtosatti@redhat.com, cl@linux.com, tglx@linutronix.de,
        mingo@kernel.org, peterz@infradead.org, pauld@redhat.com,
        neelx@redhat.com, oleksandr@natalenko.name, atomlin@atomlin.com,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH v8 3/5] mm/vmstat: Do not queue vmstat_update if tick is
 stopped
Message-ID: <20221024115426.GA1287228@lothringen>
References: <20220924152227.819815-1-atomlin@redhat.com>
 <20220924152227.819815-4-atomlin@redhat.com>
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On Sat, Sep 24, 2022 at 04:22:25PM +0100, Aaron Tomlin wrote:
> From: Marcelo Tosatti <mtosatti@redhat.com>
> 
> From the vmstat shepherd, for CPUs that have the tick stopped, do not
> queue local work to flush the per-CPU vmstats, since in that case the
> flush is performed on return to userspace or when entering idle. Also
> cancel any delayed work on the local CPU, when entering idle on nohz
> full CPUs. Per-CPU pages can be freed remotely from housekeeping CPUs.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> ---
>  mm/vmstat.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 472175642bd9..3b9a497965b4 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -29,6 +29,7 @@
>  #include <linux/page_ext.h>
>  #include <linux/page_owner.h>
>  #include <linux/migrate.h>
> +#include <linux/tick.h>
>  
>  #include "internal.h"
>  
> @@ -1990,19 +1991,23 @@ static void vmstat_update(struct work_struct *w)
>   */
>  void quiet_vmstat(void)
>  {
> +	struct delayed_work *dw;
> +
>  	if (system_state != SYSTEM_RUNNING)
>  		return;
>  
>  	if (!is_vmstat_dirty())
>  		return;
>  
> +	refresh_cpu_vm_stats(false);
> +
>  	/*
> -	 * Just refresh counters and do not care about the pending delayed
> -	 * vmstat_update. It doesn't fire that often to matter and canceling
> -	 * it would be too expensive from this path.
> -	 * vmstat_shepherd will take care about that for us.
> +	 * If the tick is stopped, cancel any delayed work to avoid
> +	 * interruptions to this CPU in the future.
>  	 */
> -	refresh_cpu_vm_stats(false);
> +	dw = &per_cpu(vmstat_work, smp_processor_id());
> +	if (delayed_work_pending(dw) && tick_nohz_tick_stopped())
> +		cancel_delayed_work(dw);

This is doing the costly cancel_delayed_work() which is only necessary
right before entering entering in user.

There are places where the tick is stopped but it's not necessary to
cancel the work:

* nohz_full enter idle
* idle IRQs
* nohz_full exit idle
* nohz_full IRQ exit

I suggest having quiet_vmstat_enter_user() which does:

void quiet_vmstat_enter_user(void)
{
	quiet_vmstat();
	if (delayed_work_pending(dw) && tick_nohz_tick_stopped())
	   cancel_delayed_work(dw);
}

And call this one only before leaving the kernel. The rest can use quiet_vmstat().

Thanks.