linux-kernel - Re: [PATCH v7 1/3] mm/vmstat: Use per cpu variable to track a vmstat discrepancy

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Message-Id: <20220824132054.54ec0472496db153223282c4@linux-foundation.org>
Date:   Wed, 24 Aug 2022 13:20:54 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     atomlin@...hat.com, frederic@...nel.org, cl@...ux.com,
        tglx@...utronix.de, mingo@...nel.org, peterz@...radead.org,
        pauld@...hat.com, neelx@...hat.com, oleksandr@...alenko.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v7 1/3] mm/vmstat: Use per cpu variable to track a
 vmstat discrepancy

On Wed, 17 Aug 2022 16:13:47 -0300 Marcelo Tosatti <mtosatti@...hat.com> wrote:

> From: Aaron Tomlin <atomlin@...hat.com>
> 
> Add CPU-specific variable namely vmstat_dirty to indicate if
> a vmstat imbalance is present for a given CPU. Therefore, at the
> appropriate time, we can fold all the remaining differentials.
> 
> This speeds up quiet_vmstat in case no per-CPU differentials exist.
> 
> Based on 
> https://lore.kernel.org/lkml/20220204173554.763888172@fedora.localdomain/
> 
> Signed-off-by: Aaron Tomlin <atomlin@...hat.com>
> Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> 
> ---
>  mm/vmstat.c |   54 ++++++++++++++++++++----------------------------------
>  1 file changed, 20 insertions(+), 34 deletions(-)
> 
> Index: linux-2.6/mm/vmstat.c
> ===================================================================
> --- linux-2.6.orig/mm/vmstat.c
> +++ linux-2.6/mm/vmstat.c
> @@ -195,6 +195,12 @@ void fold_vm_numa_events(void)
>  #endif
>  
>  #ifdef CONFIG_SMP
> +static DEFINE_PER_CPU_ALIGNED(bool, vmstat_dirty);
> +
> +static inline void mark_vmstat_dirty(void)
> +{
> +	this_cpu_write(vmstat_dirty, true);
> +}

If we're to have a helper for this then how about helpers for clearing
it and reading it?

Also, vmstat_mark_dirty(), vmstat_clear_dirty() and vmstat_dirty()
would be better identifiers.

Then those helper functions become good sites for comments explaining
what's going on.