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
