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

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Message-Id: <20230910123335.36ebf58e46628eeffef612c3@linux-foundation.org>
Date:   Sun, 10 Sep 2023 12:33:35 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jiexun Wang <wangjiexun@...ylab.org>
Cc:     brauner@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, falcon@...ylab.org
Subject: Re: [PATCH 1/1] mm/madvise: add cond_resched() in
 madvise_cold_or_pageout_pte_range()

On Sat,  9 Sep 2023 13:33:08 +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.
> 
> ...
>
> @@ -441,6 +443,13 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>  	arch_enter_lazy_mmu_mode();
>  	for (; addr < end; pte++, addr += PAGE_SIZE) {
>  		ptent = ptep_get(pte);
> +		
> +		if (++batch_count == SWAP_CLUSTER_MAX) {
> +			pte_unmap_unlock(start_pte, ptl);
> +		 	cond_resched();
> +		 	start_pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
> +		 	batch_count = 0;
> +		}
>  
>  		if (pte_none(ptent))
>  			continue;

I doubt if we can simply drop the lock like this then proceed as if
nothing has changed while the lock was released.

Could be that something along these lines:

@@ -434,6 +436,7 @@ huge_unlock:
 regular_folio:
 #endif
 	tlb_change_page_size(tlb, PAGE_SIZE);
+restart:
 	start_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
 	if (!start_pte)
 		return 0;
@@ -441,6 +444,15 @@ regular_folio:
 	arch_enter_lazy_mmu_mode();
 	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;

would work, but more analysis would be needed.