On Thu, Mar 05, 2015 at 05:00:05PM +0100, Frederic Weisbecker wrote:
> On Thu, Mar 05, 2015 at 07:56:59AM -0800, Paul E. McKenney wrote:
> > On Thu, Mar 05, 2015 at 04:35:09PM +0100, Frederic Weisbecker wrote:
> > > So, in the case we are calling that right after setting
> > > cputimer->running, I guess we are fine
> > > because we just updated cputimer with the freshest values.
> > >
> > > But if we are reading this a while after, say several ticks further,
> > > there is a chance that
> > > we read stale values since we don't lock anymore.
> > >
> > > I don't know if it matters or not, I guess it depends how stale it can be
> > > and how much precision
> > > we expect from posix cpu timers. It probably doesn't matter.
> > >
> > > But just in case, atomic64_read_return(&cputimer->utime, 0) would make
> > > sure we get the freshest
> > > value because it performs a full barrier, at the cost of more overhead of
> > > course.
> >
> > Well, if we are running within a guest OS, we might be delayed at any point
> > for quite some time. Even with interrupts disabled.
>
> You mean delayed because of the overhead of atomic_add_return() or the stale
> value
> of cptimer-> fields?
Because of preemption of the guest OS's VCPUs by the host OS.
Thanx, Paul
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