linux-kernel - Re: [PATCH 2/3] mm/swapfile: avoid confusing swap cache statistics

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Message-ID: <5c041ef1-05ec-f49f-7ada-fa58d5aec8ba@redhat.com>
Date:   Thu, 2 Jun 2022 10:41:56 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/3] mm/swapfile: avoid confusing swap cache statistics

On 02.06.22 09:29, Miaohe Lin wrote:
> On 2022/6/1 15:53, David Hildenbrand wrote:
>> On 01.06.22 04:11, Miaohe Lin wrote:
>>> On 2022/5/31 20:58, David Hildenbrand wrote:
>>>> On 31.05.22 04:55, Miaohe Lin wrote:
>>>>> On 2022/5/31 7:04, Andrew Morton wrote:
>>>>>> On Fri, 27 May 2022 17:26:25 +0800 Miaohe Lin <linmiaohe@...wei.com> wrote:
>>>>>>
>>>>>>> At swapoff time, we're going to swap in the pages continuously. So calling
>>>>>>> lookup_swap_cache would confuse statistics. We should use find_get_page
>>>>>>> directly here.
>>>>>>
>>>>>> Why is the existing behaviour wrong?  swapoff() has to swap stuff in to
>>>>>> be able to release the swap device.  Why do you believe that this
>>>>>> swapin activity should not be accounted?
>>>>>
>>>>> IMHO, statistics, e.g. swap_cache_info.find_success, are used to show the effectiveness
>>>>> of the swap cache activity. So they should only reflect the memory accessing activity
>>>>> of the user. I think swapoff can't reflect the effectiveness of the swap cache activity
>>>>> because it just swaps in pages one by one. Or statistics should reflect all the activity
>>>>> of the user including swapoff?
>>>>
>>>> I'm wondering who cares and why?
>>>
>>> I thought it's used to show the effectiveness of the swapcache readahead algorithm. If nobody
>>> ever cares about it now, I'm fine to drop this patch. And could these statistics be removed
>>> since nobody cares about it?
>>
>> IIUC, they are printed (via show_swap_cache_info()), which is called via
>> show_free_areas() -- primarily used via show_mem(). show_mem() is
>> primarily used when OOM, when allocation fails and we warn, from the OOM
>> killer, on panic().
>>
>> I am not sure how useful for (OOM ?) debugging the find_success vs.
>> find_total stats are at all. They are from ancient times. In
>> bb63be0a091c ("tmpfs: move swap_state stats update") we removed other
>> statistics that are "are relics of my 2.4.11 testing". Maybe
>> find_success and find_total can be similarly removed.
> 
> Maybe add_total, del_total, find_success and find_total should be similarly removed altogether?
> It seems those can't provide useful info when OOM occurs? And we can thus avoid touching the
> swap_cache_info cacheline.

At least makes sense to me, AFAIKU, these are not statistics one could
easily use to tune system performance because they are not easily
accessile. Maybe simply propose removal?


-- 
Thanks,

David / dhildenb