linux-kernel - Re: [PATCH v3] mm/vmstat: add events for ksm cow

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Message-Id: <20220325130957.171a68dee88118082ab841c5@linux-foundation.org>
Date:   Fri, 25 Mar 2022 13:09:57 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     cgel.zte@...il.com
Cc:     david@...hat.com, yang.yang29@....com.cn, ran.xiaokai@....com.cn,
        yang.shi@...ux.alibaba.com, dave.hansen@...ux.intel.com,
        minchan@...nel.org, saravanand@...com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xu xin <xu.xin16@....com.cn>
Subject: Re: [PATCH v3] mm/vmstat: add events for ksm cow

On Thu, 24 Mar 2022 10:43:33 +0000 cgel.zte@...il.com wrote:

> From: Yang Yang <yang.yang29@....com.cn>
> 
> Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want
> to save memory, it's a tradeoff by suffering delay on ksm cow. Users can
> get to know how much memory ksm saved by reading
> /sys/kernel/mm/ksm/pages_sharing, but they don't know what's the costs
> of ksm cow, and this is important of some delay sensitive tasks.
> 
> So add ksm cow events to help users evaluate whether or how to use ksm.

It's unclear (to me) how anyone will actually use this, how they will
interpret the output.

Some tutorial words added to Documentation/vm/ksm.rst would be helpful.
While in there, please check for any other /proc/vmstat fields which
we forgot to document.

> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -131,6 +131,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  		SWAP_RA_HIT,
>  #ifdef CONFIG_KSM
>  		KSM_SWPIN_COPY,
> +		COW_KSM,

I agree that this name looks unpleasingly backwards.  Do we have an
expectation that we actually will be adding more COW_* fields?