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SubjectRe: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o interrupt disable
* Peter Zijlstra (peterz@infradead.org) wrote:
> On Tue, 2009-10-06 at 22:54 -0400, Mathieu Desnoyers wrote:
> > > + local_irq_save(flags);
> > > + preempt_enable(); /* Get rid of count */
> >
> > Ugh ? Is that legit ?
>
> Yeah, it reads rather awkward, and the comment doesn't make it any
> better, but what he's doing is:
>
> slab_alloc()
> preempt_disable();
> __slab_alloc()
> local_irq_save(flags);
> preempt_enable();
>

Yes, I understood this is what he was doing, but I wonder about the
impact on the scheduler. If we have:

* Jiffy 1 -- timer interrupt

* preempt disable
* Jiffy 2 -- timer interrupt
-> here, the scheduler is disabled, so the timer interrupt is skipped.
The scheduler depends on preempt_check_resched() at preempt_enable()
to execute in a bounded amount of time.
* local_irq_save
* preempt_enable
-> interrupts are disabled, scheduler execution is skipped.
* local_irq_restore
-> the interrupt line is low. The scheduler won't be called. There is
no preempt_check_resched() call.

* Jiffy 3 -- timer interrupt
-> Scheduler finally gets executed, missing a whole jiffy.

At the very least, I think an explicit preempt_check_resched should be
added after local_irq_restore().

Also, preempt_enable here should be replaced with
preempt_enable_no_resched().

Mathieu

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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