linux-kernel - Re: [PATCH v2 1/1] mm/madvise: add cond_resched() in madvise_cold_or_pageout_pte

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Message-Id: <20230921174101.8e6271422a857af5414ce0a0@linux-foundation.org>
Date:   Thu, 21 Sep 2023 17:41:01 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jiexun Wang <wangjiexun@...ylab.org>
Cc:     brauner@...nel.org, falcon@...ylab.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        tangjinyu@...ylab.org
Subject: Re: [PATCH v2 1/1] mm/madvise: add cond_resched() in
 madvise_cold_or_pageout_pte_range()

On Thu, 21 Sep 2023 20:27:51 +0800 Jiexun Wang <wangjiexun@...ylab.org> wrote:

> Currently the madvise_cold_or_pageout_pte_range() function exhibits 
> significant latency under memory pressure, which can be effectively 
> reduced by adding cond_resched() within the loop.
> 
> When the batch_count reaches SWAP_CLUSTER_MAX, we reschedule 
> the task to ensure fairness and avoid long lock holding times.
> 
> ...
>
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -354,6 +354,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>  	struct folio *folio = NULL;
>  	LIST_HEAD(folio_list);
>  	bool pageout_anon_only_filter;
> +	unsigned int batch_count = 0;
>  
>  	if (fatal_signal_pending(current))
>  		return -EINTR;
> @@ -433,6 +434,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>  regular_folio:
>  #endif
>  	tlb_change_page_size(tlb, PAGE_SIZE);
> +restart:
>  	start_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);

The handling of start_pte looks OK.

>  	if (!start_pte)
>  		return 0;
> @@ -441,6 +443,15 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>  	for (; addr < end; pte++, addr += PAGE_SIZE) {
>  		ptent = ptep_get(pte);
>  
> +		if (++batch_count == SWAP_CLUSTER_MAX) {
> +			batch_count = 0;
> +			if (need_resched()) {
> +				pte_unmap_unlock(start_pte, ptl);
> +				cond_resched();
> +				goto restart;
> +			}
> +		}
> +
>  		if (pte_none(ptent))
>  			continue;
>  

I think this patch looks OK, but would appreciate careful review from
others, please.