On 09/27/2012 01:23 PM, Raghavendra K T wrote: >> >> This gives us a good case for tracking preemption on a per-vm basis. As >> long as we aren't preempted, we can keep the PLE window high, and also >> return immediately from the handler without looking for candidates. > > 1) So do you think, deferring preemption patch ( Vatsa was mentioning > long back) is also another thing worth trying, so we reduce the chance > of LHP. Yes, we have to keep it in mind. It will be useful for fine grained locks, not so much so coarse locks or IPIs. I would still of course prefer a PLE solution, but if we can't get it to work we can consider preemption deferral. > > IIRC, with defer preemption : > we will have hook in spinlock/unlock path to measure depth of lock held, > and shared with host scheduler (may be via MSRs now). > Host scheduler 'prefers' not to preempt lock holding vcpu. (or rather > give say one chance. A downside is that we have to do that even when undercommitted. Also there may be a lot of false positives (deferred preemptions even when there is no contention). > > 2) looking at the result (comparing A & C) , I do feel we have > significant in iterating over vcpus (when compared to even vmexit) > so We still would need undercommit fix sugested by PeterZ (improving by > 140%). ? Looking only at the current runqueue? My worry is that it misses a lot of cases. Maybe try the current runqueue first and then others. Or were you referring to something else? > > So looking back at threads/ discussions so far, I am trying to > summarize, the discussions so far. I feel, at least here are the few > potential candidates to go in: > > 1) Avoiding double runqueue lock overhead (Andrew Theurer/ PeterZ) > 2) Dynamically changing PLE window (Avi/Andrew/Chegu) > 3) preempt_notify handler to identify preempted VCPUs (Avi) > 4) Avoiding iterating over VCPUs in undercommit scenario. (Raghu/PeterZ) > 5) Avoiding unnecessary spinning in overcommit scenario (Raghu/Rik) > 6) Pv spinlock > 7) Jiannan's proposed improvements > 8) Defer preemption patches > > Did we miss anything (or added extra?) > > So here are my action items: > - I plan to repost this series with what PeterZ, Rik suggested with > performance analysis. > - I ll go back and explore on (3) and (6) .. > > Please Let me know.. Undoubtedly we'll think of more stuff. But this looks like a good start. -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
