Messages in this thread | | | Date | Wed, 7 Oct 2009 08:46:28 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o interrupt disable |
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* 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
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