linux-kernel - Re: [PATCH] delayacct: track delays from COW

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Date:   Fri, 8 Apr 2022 09:31:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     CGEL <cgel.zte@...il.com>
Cc:     akpm@...ux-foundation.org, bsingharora@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Yang Yang <yang.yang29@....com.cn>,
        Ran Xiaokai <ran.xiaokai@....com.cn>
Subject: Re: [PATCH] delayacct: track delays from COW

On 06.04.22 09:37, CGEL wrote:
> On Wed, Mar 23, 2022 at 09:49:46AM +0100, David Hildenbrand wrote:
>> On 22.03.22 12:04, cgel.zte@...il.com wrote:
>>> From: Yang Yang <yang.yang29@....com.cn>
>>>
>>> Delay accounting does not track the delay of COW. When tasks trigger
>>> much COW, it may spend a amount of time waiting for it. To get the
>>> impact of tasks in COW, measure the delay when it happens. This
>>> could help users to do tunnings, such as decide whether to use
>>> ksm or not.
>>>
>>> Also update tools/accounting/getdelays.c:
>>>
>>>     / # ./getdelays -dl -p 231
>>>     print delayacct stats ON
>>>     listen forever
>>>     PID     231
>>>
>>>     CPU             count     real total  virtual total    delay total  delay average
>>>                      6247     1859000000     2154070021     1674255063          0.268ms
>>>     IO              count    delay total  delay average
>>>                         0              0              0ms
>>>     SWAP            count    delay total  delay average
>>>                         0              0              0ms
>>>     RECLAIM         count    delay total  delay average
>>>                         0              0              0ms
>>>     THRASHING       count    delay total  delay average
>>>                         0              0              0ms
>>>     COMPACT         count    delay total  delay average
>>>                         3          72758              0ms
>>>     COW             count    delay total  delay average
>>>                      3635      271567604              0ms
>>
>> You should also update Documentation/accounting/delay-accounting.rst
>> most probably.
>>
>> Overall LGTM and this might be of value not only for KSM but for anybody
>> using fork(). There will be collisions with [1], especially [2], which I
>> want to get in -next early after we have v5.18-rc1 (after rebasing [1]
>> on top of this).
>>
>> We'll have to decide if we want to also account hugetlb wp code
>> (hugetlb_cow), and if we want to account "unsharing" here as well under
>> cow (I tend to say that we want to for simplicity). For THP, we only
>> split and don't copy, so there isn't anything to account.
>>
> As for simplicity, what about account "PAGECOPY" instead of "COW"?
> "PAGECOPY" include COW and unsharing. And we may also account hugetlb
> wp in follow-up patches, based on this patch is sufficient reviewed.

PAGECOPY might be too generic. You actually want to express "potentially
shared page was copied by the write-fault handler while it was write
protected".

do_wp_page()->wp_page_copy()

Maybe simply "WP_COPY" as a prefix ("Write-protect copy") ?

-- 
Thanks,

David / dhildenb