Re: [Xen-devel] Arndale secondary CPU boot issue Was Re: [xen-unstable test] 60076: regressions - FAIL

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 30 Jul 2015, Ian Campbell wrote:
> On Wed, 2015-07-29 at 19:18 +0100, Julien Grall wrote:
> 
> As an aside from the issue you are seeing:
> 
> > The old implementation of spinlock is sending an event (via the assembly
> > instruction SEV) to the other physical CPUs. This will wake up the
> > others CPUs waiting on the assembly instruction WFE (Wait For Event).
> 
> Uh, I didn't notice this about the new implementation, sorry I should have
> done.
> 
> IMHO we should investigate (probably with some urgency) inserting a WFE and
> SEV pair into the lock/unlock paths, else power consumption will suck.
> 
> I think that probably means using something new to replace the cpu_relax()
> calls in the spinlocks with a WFE on ARM (we don't just want to change
> relax) and to add a arch specific hook for the SEV on the release path.

I agree: adding a WFE in cpu_relax() is too risky at this point.


> If it is too late for 4.6 (which would depend on the eventual complexity of
> the actual fix) then we should fix this ASAP in 4.7 and backport for 4.6.1.

I don't think we can release 4.6 without a WFE in the locks. We might
want to consider reverting to spin_locks on ARM (although I am aware
that the code is common at the moment).

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