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Re: [Xen-devel] [PATCH for 4.7] xen: sched: avoid races on time values read from NOW()

On Thu, May 19, 2016 at 4:11 AM, Dario Faggioli
<dario.faggioli@xxxxxxxxxx> wrote:
> Hey Wei,
> Again, I'm using an otherwise unnecessary cover letter for my analysis about
> <<should we include this in 4.7 or not?>>. :-)
> I'd say yes, because the patch fixes an actual bug, in the form of a rather
> subtle race condition, which was all but trivial to spot.  I must say, though,
> that I've only found the bug guilty of being particularly nasty if we use
> Credit2.  Actually, I'm quite sure it has an effect on RTDS too (although I 
> did
> not trace that), but since both Credit2 and RTDS are still marked as
> experimental in 4.7, one may think it's not worthwhile putting in something
> like this to fix experimental only code.
> Just FYI, this bug is what was causing the issue I briefly chatted about on 
> with George, yesterday, i.e., it is what led Credit2 to emit (rather
> aggresively, actually) the debug printks showed here:
>  http://pastebin.com/gzYrNST5

In addition to the race condition in the bare metal,  actually I saw
this when I debug/run Xen in VirtualBox.
The situation is:
If we have nested virtualization or if we have heterogeneous cores
which have different speed/time, the RTDS scheduler (maybe credit2 as
well?) will have a problem in budget accounting. The "CPU" of Xen is
scheduled by the underlining hypervisor. One "CPU" of Xen could be
slower than another, showing the time is left behind.

We explicitly say that RTDS will have incorrect budget accounting for
nested virtualization situation, when the RTDS was upstreamed in Xen

Best Regards,

Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania

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