linux-kernel - Re: [PATCH] mm/khugepaged: Fix skipping of alloc sleep after second failure

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Message-ID: <20251125041503.GA113135@i85a15111.eu95sqa>
Date: Tue, 25 Nov 2025 12:15:03 +0800
From: Zhiheng Tao <junchuan.tzh@...group.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, ziy@...dia.com,
	baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
	npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
	baohua@...nel.org, lance.yang@...ux.dev, shy828301@...il.com,
	zokeefe@...gle.com, peterx@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/khugepaged: Fix skipping of alloc sleep after second
 failure

On Mon, Nov 24, 2025 at 10:14:20AM +0100, David Hildenbrand (Red Hat) wrote:
> On 11/24/25 07:19, Zhiheng Tao wrote:
> >In khugepaged_do_scan(), two consecutive allocation failures cause
> >the logic to skip the dedicated 60s throttling sleep
> >(khugepaged_alloc_sleep_millisecs), forcing a fallback to the
> >shorter 10s scanning interval via the outer loop
> >
> >Since fragmentation is unlikely to resolve in 10s, this results in
> >wasted CPU cycles on immediate retries.
> 
> Why shouldn't memory comapction be able to compact a single THP in 10s?
> 
> Why should it resolve in 60s?
> 
It may resolve in 10s or 60s. The problem is that the sleep controlled
by khugepaged_alloc_sleep_millisecs should not be skipped if allocation
fails.
> >
> >Reorder the failure logic to ensure khugepaged_alloc_sleep() is
> >always called on each allocation failure.
> >
> >Fixes: c6a7f445a272 ("mm: khugepaged: don't carry huge page to the next loop for !CONFIG_NUMA")
> 
> What are we fixing here? This sounds like a change that might be
> better on some systems, but worse on others?
> 
> We really need more information on when/how an issue was hit, and
> how this patch here really moves the needle in any way.
> 
It works better. The missing of khugepaged_alloc_sleep() is not
introduced by this change. Maybe I should remove "Fix".
> -- 
> Cheers
> 
> David