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

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Message-ID: <7f3a973f-1f0c-4e50-2528-b0e9dc3e4cac@redhat.com>
Date:   Fri, 25 Mar 2022 21:19:51 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>, cgel.zte@...il.com
Cc:     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 25.03.22 21:09, Andrew Morton wrote:
> 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?

As raised previously (also when proposing this), I'd like to have
COW_ANON, COW_ZERO, COW_OTHER. Ideally, we'd have added all via a single
patch for them. They would at least be of value to me.

-- 
Thanks,

David / dhildenb