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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