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 > >
