From: Thomas Gleixner <tglx@linutronix.de>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Michal Hocko <mhocko@kernel.org>,
Vlastimil Babka <vbabka@suse.cz>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] mm, page_alloc: only use per-cpu allocator for irq-safe requests -fix v2
Date: Wed, 8 Feb 2017 16:27:47 +0100 (CET) [thread overview]
Message-ID: <alpine.DEB.2.20.1702081627380.3536@nanos> (raw)
In-Reply-To: <20170208152200.ydlvia2c7lm7ln3t@techsingularity.net>
On Wed, 8 Feb 2017, Mel Gorman wrote:
> preempt_enable_no_resched() was used based on review feedback that had
> no strong objection at the time. The thinking was that it avoided adding
> a preemption point where one didn't exist before so the feedback was
> applied. This reasoning was wrong.
>
> There was an indirect preemption point as explained by Thomas Gleixner where
> an interrupt could set_need_resched() followed by preempt_enable being
> a preemption point that matters. This use of preempt_enable_no_resched
> is bad from both a mainline and RT perspective and a violation of the
> preemption mechanism. Peter Zijlstra noted that "the only acceptable use
> of preempt_enable_no_resched() is if the next statement is a schedule()
> variant".
>
> The usage was outright broken and I should have stuck to preempt_enable()
> as it was originally developed. It's known from previous tests
> that there was no detectable difference to the performance by using
> preempt_enable_no_resched().
>
> 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@techsingularity.net>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> 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;
> }
>
>
> --
> Mel Gorman
> SUSE Labs
>
next prev parent reply other threads:[~2017-02-08 17:05 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-08 15:22 [PATCH] mm, page_alloc: only use per-cpu allocator for irq-safe requests -fix v2 Mel Gorman
2017-02-08 15:27 ` Thomas Gleixner [this message]
2017-02-09 6:22 ` Hillf Danton
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