Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler

[Avi Kivity <avi@redhat.com>]

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.


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