Re: [PATCH v4 10/10] KVM/x86/lbr: lazy save the guest lbr stack

[Andi Kleen <ak@linux.intel.com>]

On Fri, Dec 28, 2018 at 11:47:06AM +0800, Wei Wang wrote:
> On 12/28/2018 04:51 AM, Andi Kleen wrote:
> > Thanks. This looks a lot better than the earlier versions.
> > 
> > Some more comments.
> > 
> > On Wed, Dec 26, 2018 at 05:25:38PM +0800, Wei Wang wrote:
> > > When the vCPU is scheduled in:
> > > - if the lbr feature was used in the last vCPU time slice, set the lbr
> > >    stack to be interceptible, so that the host can capture whether the
> > >    lbr feature will be used in this time slice;
> > > - if the lbr feature wasn't used in the last vCPU time slice, disable
> > >    the vCPU support of the guest lbr switching.
> > time slice is the time from exit to exit?
> 
> It's the vCPU thread time slice (e.g. 100ms).

I don't think the time slices are that long, but ok.

> 
> > 
> > This might be rather short in some cases if the workload does a lot of exits
> > (which I would expect PMU workloads to do) Would be better to use some
> > explicit time check, or at least N exits.
> 
> Did you mean further increasing the lazy time to multiple host thread
> scheduling time slices?
> What would be a good value for "N"?

I'm not sure -- i think the goal would be to find a value that optimizes
performance (or rather minimizes overhead). But perhaps if it's as you say the
scheduler time slice it might be good enough as it is.

I guess it could be tuned later based on more experneice.

> > or partially cleared. This would be user visible.
> > 
> > In theory could try to detect if the guest is inside a PMI and
> > save/restore then, but that would likely be complicated. I would
> > save/restore for all cases.
> 
> Yes, it is easier to save for all the cases. But curious for the
> non-callstack
> mode, it's just ponit sampling functions (kind of speculative in some
> degree).
> Would rarely losing a few recordings important in that case?

In principle no for statistical samples, but I know some tools complain
for bogus samples (e.g. autofdo will). Also with perf report --branch-history it will
be definitely visible. I think it's easier to always safe now than to
handle the user complaints about this later.


-Andi