Re: [PATCH] delayacct: track delays from ksm cow

David Hildenbrand
2022-03-16T14:56:31Z

On 16.03.22 14:34, cgel.zte@gmail.com wrote:
> From: Yang Yang <yang.yang29@zte.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