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

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Message-ID: <412dc01c-8829-eac2-52c7-3f704dbb5a98@redhat.com>
Date:   Wed, 16 Mar 2022 15:56:23 +0100
From:   David Hildenbrand <david@...hat.com>
To:     cgel.zte@...il.com, bsingharora@...il.com,
        akpm@...ux-foundation.org
Cc:     yang.yang29@....com.cn, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] delayacct: track delays from ksm cow

On 16.03.22 14:34, cgel.zte@...il.com wrote:
> From: Yang Yang <yang.yang29@....com.cn>
> 
> Delay accounting does not track the delay of ksm cow.  When tasks
> have many ksm pages, it may spend a amount of time waiting for ksm
> cow.
> 
> To get the impact of tasks in ksm cow, measure the delay when ksm
> cow happens. This could help users to decide whether to user 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
>     KSM             count    delay total  delay average
>                      3635      271567604              0ms
> 

TBH I'm not sure how particularly helpful this is and if we want this.

[...]

>  	struct vm_area_struct *vma = vmf->vma;
> +	vm_fault_t ret = 0;
> +	bool delayacct = false;
>  
>  	if (userfaultfd_pte_wp(vma, *vmf->pte)) {
>  		pte_unmap_unlock(vmf->pte, vmf->ptl);
> @@ -3294,7 +3296,11 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  		 *
>  		 * PageKsm() doesn't necessarily raise the page refcount.
>  		 */
> -		if (PageKsm(page) || page_count(page) > 3)
> +		if (PageKsm(page)) {
> +			delayacct = true;
> +			goto copy;
> +		}
> +		if (page_count(page) > 3)
>  			goto copy;
>  		if (!PageLRU(page))
>  			/*
> @@ -3308,7 +3314,12 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  			goto copy;
>  		if (PageSwapCache(page))
>  			try_to_free_swap(page);
> -		if (PageKsm(page) || page_count(page) != 1) {
> +		if (PageKsm(page)) {
> +			delayacct = true;
> +			unlock_page(page);
> +			goto copy;
> +		}
> +		if (page_count(page) != 1) {
>  			unlock_page(page);
>  			goto copy;
>  		}
> @@ -3328,10 +3339,18 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  	/*
>  	 * Ok, we need to copy. Oh, well..
>  	 */

Why not simply check for PageKsm() here? I dislike the added complexity
above.


-- 
Thanks,

David / dhildenb