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SubjectRe: [PATCH] mm, page_alloc: only use per-cpu allocator for irq-safe requests -fix
On Wed, Feb 08, 2017 at 03:56:22PM +0100, Thomas Gleixner wrote:
> 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.
>

Thanks for the education, I had missed it. The changelog should have been
"fix a dumb mistake and stick to preempt_enable". Assuming Andrew picks
this patch up, it'll be folded into the patch that introduced the problem
in the first place and will the broken usage will never hit mainline.

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

Understood, thanks.

--
Mel Gorman
SUSE Labs

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