Re: [clocksource] 8901ecc231: stress-ng.lockbus.ops_per_sec -9.5% regression

[Paul E. McKenney]

On Thu, May 27, 2021 at 08:29:32PM +0100, Matthew Wilcox wrote:
> On Thu, May 27, 2021 at 12:19:23PM -0700, Paul E. McKenney wrote:
> > On Thu, May 27, 2021 at 12:01:23PM -0700, Andi Kleen wrote:
> > > 
> > > >      Nevertheless, it is quite possible that real-world use will result in
> > > >      some situation requiring that high-stress workloads run on hardware
> > > >      not designed to accommodate them, and also requiring that the kernel
> > > >      refrain from marking clocksources unstable.
> > > >      Therefore, provide an out-of-tree patch that reacts to this situation
> > > 
> > > out-of-tree means it will not be submitted?
> > > 
> > > I think it would make sense upstream, but perhaps guarded with some option.
> > 
> > The reason I do not intend to immediately upstream this patch is that
> > it increases the probability that a real clocksource read-latency issue
> > will be ignored, for example, during hardware bringup.  Furthermore,
> > the only known need from it comes from hardware that is, in the words
> > of the stress-ng man page, "poorly designed".  And the timing of this
> > email thread leads me to believe that such hardware is not easy to obtain.
> 
> I think you're placing a little too much weight on the documentation
> here.  It seems that a continuous stream of locked operations executed
> in userspace on a single CPU can cause this problem to occur.  If that's
> true all the way out to one guest in a hypervisor can cause problems
> for the hypervisor itself, I think cloud providers everywhere are
> going to want this patch?

Only those cloud provides making heavy use of the aforementioned "poorly
designed" hardware, correct?

							Thanx, Paul

> > My thought is therefore to keep this patch out of tree for now.
> > If it becomes clear that long-latency clocksource reads really are
> > a significant issue in their own right (as opposed to merely being a
> > symptom of a hardware or firmware bug), then this patch is available to
> > immediately respond to that issue.
> > 
> > And there would then be strong evidence in favor of me biting the bullet,
> > adding the complexity and the additional option (with your Suggested-by),
> > and getting that upstream and into -stable.
> > 
> > Seem reasonable?
> > 
> > 							Thanx, Paul
> >