linux-kernel - [FIX PATCH] mm: pcp: fix pcp->free_count reduction on page allocation

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Message-ID: <20250107091724.35287-1-nikhil.dhama@amd.com>
Date: Tue, 7 Jan 2025 14:47:24 +0530
From: Nikhil Dhama <nikhil.dhama@....com>
To: <akpm@...ux-foundation.org>
CC: Nikhil Dhama <nikhil.dhama@....com>, Ying Huang
	<huang.ying.caritas@...il.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, Bharata B Rao <bharata@....com>, Raghavendra
	<raghavendra.kodsarathimmappa@....com>
Subject: [FIX PATCH] mm: pcp: fix pcp->free_count reduction on page allocation

In current PCP auto-tuning desgin, free_count was introduced to track
the consecutive page freeing with a counter, This counter is incremented
by the exact amount of pages that are freed, but reduced by half on
allocation. This is causing a 2-node iperf3 client to server's network
bandwidth to drop by 30% if we scale number of client-server pairs from 32
(where we achieved peak network bandwidth) to 64.

To fix this issue, on allocation, reduce free_count by the exact number
of pages that are allocated instead of halving it.

On a 2-node AMD server, one running iperf3 clients and other iperf3
sever, This patch restores the performance drop.

Fixes: 6ccdcb6d3a74 ("mm, pcp: reduce detecting time of consecutive high order page freeing")

Signed-off-by: Nikhil Dhama <nikhil.dhama@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Ying Huang <huang.ying.caritas@...il.com>
Cc: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org
Cc: Bharata B Rao <bharata@....com>
Cc: Raghavendra <raghavendra.kodsarathimmappa@....com>
---
 mm/page_alloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cae7b93864c2..e2a8ec5584f8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3037,10 +3037,10 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
 
 	/*
 	 * On allocation, reduce the number of pages that are batch freed.
-	 * See nr_pcp_free() where free_factor is increased for subsequent
+	 * See free_unref_page_commit() where free_count is increased for subsequent
 	 * frees.
 	 */
-	pcp->free_count >>= 1;
+	pcp->free_count -= (1 << order);
 	list = &pcp->lists[order_to_pindex(migratetype, order)];
 	page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list);
 	pcp_spin_unlock(pcp);
-- 
2.25.1