linux-kernel - [PATCH] mm, page_alloc: only use per-cpu allocator for irq-safe requests -fix

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Message-ID: <20170208143128.25ahymqlyspjcixu@techsingularity.net>
Date:   Wed, 8 Feb 2017 14:31:28 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>
Subject: [PATCH] mm, page_alloc: only use per-cpu allocator for irq-safe
 requests -fix

preempt_enable_no_resched() was used based on review feedback that had no
strong objection at the time. It avoided introducing a preemption point
where one didn't exist before which was marginal at best.

However, it is hazardous to the RT tree according to Thomas Gleixner
and is a violation of its expected use according to Peter Zijlstra. In
Peter's own words "the only acceptable use of preempt_enable_no_resched()
is if the next statement is a schedule() variant".

The impact of using preempt_enable in this particular
fast path is negligible. This is a fix to the mmotm patch
mm-page_alloc-only-use-per-cpu-allocator-for-irq-safe-requests.patch

Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
---
 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 eaecb4b145e6..2a36dad03dac 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2520,7 +2520,7 @@ void free_hot_cold_page(struct page *page, bool cold)
 	}

 out:
-	preempt_enable_no_resched();
+	preempt_enable();
 }

 /*
@@ -2686,7 +2686,7 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
 		__count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
 		zone_statistics(preferred_zone, zone);
 	}
-	preempt_enable_no_resched();
+	preempt_enable();
 	return page;
 }