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
