Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer to improve scalability

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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