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

[Isaku Yamahata <yamahata@xxxxxxxxxxxxx>]

On Mon, Jan 29, 2007 at 05:58:44PM +0800, Xu, Anthony wrote:
> Isaku Yamahata write on 2007年1月29日 17:21:
> > On Mon, Jan 29, 2007 at 05:00:11PM +0800, Xu, Anthony wrote:
> > It doesn't optimize NEED_FLUSH itself.
> > The optimization path is executed when NEED_FLUSH return 0.
> > See flush_vtlb_for_context_switch() @ xen/arch/ia64/xehn/domain.c.
> > 
> > When CONFIG_XEN_IA64_TLBFLUSH_CLOCK is defined,
> > NEED_FLUSH() always returns 1. No optimization.
> > Suppose that CONFIG_XEN_IA64_TLBFLUSH_CLOCK is defined and
> > NEED_FLUSH() returns 0.
> > In that case, we can skip local_vhpt_flush() or local_flush_tlb_all().
> 
> Hi Isaku,
> 
> Thanks for your explanation.  
> 
> Suppose that CONFIG_XEN_IA64_TLBFLUSH_CLOCK is defined
> NEED_FLUSH() returns 0.
>  In that case, we can skip local_vhpt_flush() or local_flush_tlb_all().
> 
> But the skip is on the cost of new_tlbflush_clock_period calling 
> vcpu_vhpt_flush.
> 
> Anyway, vcpu_vhpt_flush must be called, the difference is where it is called.
> 
> I don't see the benefit of new_tlbflush_clock_period.
> 
> I must miss something.
> 
> 
> 
> Can you explain more?

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.
  vcpu1 runs

You can confirm its effect by the perf-counters,
tlbflush_clock_cswitch_skip, flush_vtlb_for_context_switch and
tlbflush_clock_cswitch_purge.
Please note that local_flush_tlb_all() (or vcpu_vhpt_flush()) is called
everytime grant table unmapping without tlb insert tracking optimization.
But they aren't so often called with tlb insert tracking optimization,
tlb flush clock optimization becomes less effetive than before.
-- 
yamahata

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