Re: [Xen-ia64-devel] [PATCH] NEW_TLBFLUSH_CLOCK_PERIOD_SOFTIRQ is notregistered.

[Isaku Yamahata <yamahata@xxxxxxxxxxxxx>]

On Tue, Jan 30, 2007 at 12:16:53PM +0800, Xu, Anthony wrote:
> Isaku Yamahata write on 2007年1月30日 11:36:
> > On Tue, Jan 30, 2007 at 09:46:04AM +0800, Xu, Anthony wrote:
> >> Isaku Yamahata write on 2007年1月29日 18:29:
> >>> 
> >>> How about the following example?
> >>> For simplicity, we consider only local_flush_tlb_all().
> >>> (The similar argument can be applied to vcpu_vhpt_flush())
> >>> 
> >>> suppose domM has two vcpus, vcpu0, vcpu1.
> >>>   domN has one vcpu, vcpu2.
> >>> 
> >>> - case 1
> >>>   vcpu0 and vcpu1 are running on same pcpu.
> >>>   vcpu0 runs.
> >>>   context switch <<<< local_flush_tlb_all() is necessry here  
> >>> vcpu1 runs. 
> >>> 
> >>> - case 2
> >>>   vcpu0, vcpu1 and vcpu2 are running on the same pcpu   vcpu0 runs
> >>>   context switch
> >>>   vcpu2 runs
> >>>   vcpu2 issues local_tlb_flush().
> >>>   context switch <<< local_flush_tlb_all() can be skipped.
> >> I can understand this. Yes, this local_flush_tlb_all can be skipped,
> >> But it is because vcpu2 issues local_tlb_flush.
> >> My question is why we need new_tlbflush_clock_period?
> > 
> > Because the counter is finite.
> > If we can ignore conter overflow, we can check only which counter
> > is bigger.
> > But when overflow comes in (i.e. counter == 0 after increment),
> > things become complicated. It's the reason of
> > new_tlbflush_clock_period. 
> > 
> > Probably another approach to address overflow is to use signed
> > comparison like Linux jiffies time_after().
> > But we can't assume the distance between two conters is near enough.
> > 
> > 
> Understand now.
> One more question
> 
> Why need local_vhpt_flush and vcpu_vhpt_flush call 
> tlbflush_clock_inc_and_return?
> 
> In per-CPU VHPT mode,
> tlbflush_clock_inc_and_return only needs to be called in local_flush_tlb_all.
> 
> Am I right?

Yes.
The tlb flush clock has existed since the per-vcpu vhpt mode patch wasn't
checked in.
So the tlb flush clock should co-exist with non per-vcpu VHPT mode.

-- 
yamahata

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