lkml.org 
[lkml]   [2012]   [Apr]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH RFC V6 0/11] Paravirtualized ticketlocks
On Sat, Mar 31, 2012 at 09:37:45AM +0530, Srivatsa Vaddagiri wrote:
> * Thomas Gleixner <tglx@linutronix.de> [2012-03-31 00:07:58]:
>
> > I know that Peter is going to go berserk on me, but if we are running
> > a paravirt guest then it's simple to provide a mechanism which allows
> > the host (aka hypervisor) to check that in the guest just by looking
> > at some global state.
> >
> > So if a guest exits due to an external event it's easy to inspect the
> > state of that guest and avoid to schedule away when it was interrupted
> > in a spinlock held section. That guest/host shared state needs to be
> > modified to indicate the guest to invoke an exit when the last nested
> > lock has been released.
>
> I had attempted something like that long back:
>
> http://lkml.org/lkml/2010/6/3/4
>
> The issue is with ticketlocks though. VCPUs could go into a spin w/o
> a lock being held by anybody. Say VCPUs 1-99 try to grab a lock in
> that order (on a host with one cpu). VCPU1 wins (after VCPU0 releases it)
> and releases the lock. VCPU1 is next eligible to take the lock. If
> that is not scheduled early enough by host, then remaining vcpus would keep
> spinning (even though lock is technically not held by anybody) w/o making
> forward progress.
>
> In that situation, what we really need is for the guest to hint to host
> scheduler to schedule VCPU1 early (via yield_to or something similar).
>
> The current pv-spinlock patches however does not track which vcpu is
> spinning at what head of the ticketlock. I suppose we can consider
> that optimization in future and see how much benefit it provides (over
> plain yield/sleep the way its done now).

Right. I think Jeremy played around with this some time?
>
> Do you see any issues if we take in what we have today and address the
> finer-grained optimization as next step?

I think that is the proper course - these patches show
that on baremetal we don't incur performance regressions and in
virtualization case we benefit greatly. Since these are the basic
building blocks of a kernel - taking it slow and just adding
this set of patches for v3.5 is a good idea - and then building on top
of that for further refinement.

>
> - vatsa


\
 
 \ /
  Last update: 2012-04-16 17:53    [W:0.131 / U:2.108 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site