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

[Thomas Gleixner <tglx@linutronix.de>]

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



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