Re: [PATCH v7 2/2] mm/oom_kill: The OOM reaper traverses the VMA maple tree in reverse order

[Michal Hocko <mhocko@suse.com>]

On Wed 03-09-25 17:27:29, zhongjinji wrote:
> Although the oom_reaper is delayed and it gives the oom victim chance to
> clean up its address space this might take a while especially for
> processes with a large address space footprint. In those cases
> oom_reaper might start racing with the dying task and compete for shared
> resources - e.g. page table lock contention has been observed.
> 
> Reduce those races by reaping the oom victim from the other end of the
> address space.
> 
> It is also a significant improvement for process_mrelease(). When a process
> is killed, process_mrelease is used to reap the killed process and often
> runs concurrently with the dying task. The test data shows that after
> applying the patch, lock contention is greatly reduced during the procedure
> of reaping the killed process.

Thank you this is much better!

> Without the patch:
> |--99.74%-- oom_reaper
> |  |--76.67%-- unmap_page_range
> |  |  |--33.70%-- __pte_offset_map_lock
> |  |  |  |--98.46%-- _raw_spin_lock
> |  |  |--27.61%-- free_swap_and_cache_nr
> |  |  |--16.40%-- folio_remove_rmap_ptes
> |  |  |--12.25%-- tlb_flush_mmu
> |  |--12.61%-- tlb_finish_mmu
> 
> With the patch:
> |--98.84%-- oom_reaper
> |  |--53.45%-- unmap_page_range
> |  |  |--24.29%-- [hit in function]
> |  |  |--48.06%-- folio_remove_rmap_ptes
> |  |  |--17.99%-- tlb_flush_mmu
> |  |  |--1.72%-- __pte_offset_map_lock
> |  |--30.43%-- tlb_finish_mmu

Just curious. Do I read this correctly that the overall speedup is
mostly eaten by contention over tlb_finish_mmu?

> Signed-off-by: zhongjinji <zhongjinji@honor.com>

Anyway, the change on its own makes sense to me
Acked-by: Michal Hocko <mhocko@suse.com>

Thanks for working on the changelog improvements.

> ---
>  mm/oom_kill.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 3caaafc896d4..540b1e5e0e46 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -516,7 +516,7 @@ static bool __oom_reap_task_mm(struct mm_struct *mm)
>  {
>  	struct vm_area_struct *vma;
>  	bool ret = true;
> -	VMA_ITERATOR(vmi, mm, 0);
> +	MA_STATE(mas, &mm->mm_mt, ULONG_MAX, ULONG_MAX);
>  
>  	/*
>  	 * Tell all users of get_user/copy_from_user etc... that the content
> @@ -526,7 +526,13 @@ static bool __oom_reap_task_mm(struct mm_struct *mm)
>  	 */
>  	set_bit(MMF_UNSTABLE, &mm->flags);
>  
> -	for_each_vma(vmi, vma) {
> +	/*
> +	 * It might start racing with the dying task and compete for shared
> +	 * resources - e.g. page table lock contention has been observed.
> +	 * Reduce those races by reaping the oom victim from the other end
> +	 * of the address space.
> +	 */
> +	mas_for_each_rev(&mas, vma, 0) {
>  		if (vma->vm_flags & (VM_HUGETLB|VM_PFNMAP))
>  			continue;
>  
> -- 
> 2.17.1
> 

-- 
Michal Hocko
SUSE Labs