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

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Date: Fri, 26 Jan 2024 12:12:38 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
	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 (23/09/21 17:41), Andrew Morton 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);

Shouldn't it leave lazy MMU mode here?

---

diff --git a/mm/madvise.c b/mm/madvise.c
index 0f222d464254..127f0c7b69ac 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -451,6 +451,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 		if (++batch_count == SWAP_CLUSTER_MAX) {
 			batch_count = 0;
 			if (need_resched()) {
+				arch_leave_lazy_mmu_mode();
 				pte_unmap_unlock(start_pte, ptl);
 				cond_resched();
 				goto restart;