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Date: Wed, 8 Feb 2017 15:56:22 +0100 (CET)
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
In-Reply-To: <20170208143128.25ahymqlyspjcixu@techsingularity.net>
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References: <20170208143128.25ahymqlyspjcixu@techsingularity.net>
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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. It avoided introducing a preemption point
> where one didn't exist before which was marginal at best.

Actually local_irq_enable() _IS_ a preemption point, indirect but still:

   local_irq_disable()
   ....
--> HW interrupt is raised
   ....
   local_irq_enable()

   handle_irq()
	set_need_resched()
   ret_from_irq()
     preempt()

while with preempt_disable that looks like this:

   preempt_disable()
   ....
--> HW interrupt is raised
   handle_irq()
	set_need_resched()
   ret_from_irq()
   ....
   preempt_enable()
      preempt()

Now if you use preempt_enable_no_resched() then you miss the preemption and
depending on the actual code path you might run something which takes ages
without hitting a preemption point after that.

It's not only a problem for RT. It's also in mainline a violation of the
preemption mechanism.

Thanks,

	tglx