Messages in this thread | | | Date | Wed, 7 Oct 2009 11:41:11 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o interrupt disable |
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* Christoph Lameter (cl@linux-foundation.org) wrote: > On Wed, 7 Oct 2009, Mathieu Desnoyers wrote: > > > You are already calling the scheduler when ending the _fast_ path. I > > don't see the problem with calling it when you end the slow path > > execution. > > Well yes that gives rise to the thought of using > > preempt_enable_no_sched() > > at the end of the fastpath as well. Constant calls into the scheduler > could be a major performance issue
... but the opposite is a major RT behavior killer.
preempt_check_resched is basically:
a test TIF_NEED_RESCHED if true, call to preempt_schedule
So, it's not a call to the scheduler at each and every preempt_enable. It's _only_ if an interrupt came in during the preempt off section and the scheduler considered that it should re-schedule soon.
I really don't see what's bothering you here. Testing a thread flag is incredibly cheap. That's what is typically added to your fast path.
So, correct behavior would be:
preempt disable() fast path attempt if (fast path already taken) { local_irq_save(); slow path. local_irq_restore(); } preempt_enable()
Mathieu
> . I dont notice it here since I usually > cannot affort the preempt overhead and build kernels without support for > it. > >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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