[Xen-devel] [PATCH v2 0/2] xen: credit1: fix a race when picking initial pCPU for a vCPU

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

Hey,

This is v2 of this
https://lists.xen.org/archives/html/xen-devel/2016-08/msg01679.html although it
became a small series, as I noticed in the process room for a small improvement
to the code and added a patch for that (that's patch 2).

Also, while adding the ASSERT() George asked for here
https://lists.xen.org/archives/html/xen-devel/2016-08/msg01694.html , I
discovered that things where a bit worse than they looked at the beginning
(I described such finding here:
https://lists.xen.org/archives/html/xen-devel/2016-08/msg01752.html).

Patch 1 of this series is my idea of a fix for the reported issue, that does
not explode in any other way and is also respectful of the code of
csched_tick(). In fact, while the most obvious) solution would be to just
always take the appropriate lock, doing that in csched_tick() is particularly
tricky (because of Credit1's lock nesting rule), and would result in a dance of
taking and releasing locks which I don't think is what we want.

I've tested this by repeatedly creating and destroying a VM, on both an idle
and a loaded system, as that seems to be what makes the system crash in both
Andrew's XenServer testing, and OSSTest flight 100518 (it's failing at the
guest-start/win.repeat step).

If people (George, mainly, I guess) agree this is the proper fix, I think this
(I mean, patch 1) needs to be backported to all the branches to which we
backported 9f358ddd69463 ("xen: Have schedulers revise initial placement").

Regards,
Dario
---
Dario Faggioli (2):
      xen: credit1: fix a race when picking initial pCPU for a vCPU
      xen: credit1: no need to check for is_idle_vcpu() in csched_vcpu_acct()

 xen/common/sched_credit.c |   45 +++++++++++++++++++++++++++++++++++----------
 1 file changed, 35 insertions(+), 10 deletions(-)
--
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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