Messages in this thread | | | Date | Thu, 3 Jul 2014 22:05:41 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu] Parallelize and economize NOCB kthread wakeups |
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On Fri, Jul 04, 2014 at 05:23:56AM +0200, Mike Galbraith wrote: > On Thu, 2014-07-03 at 09:29 -0700, Paul E. McKenney wrote: > > On Thu, Jul 03, 2014 at 07:48:40AM +0200, Mike Galbraith wrote: > > > On Wed, 2014-07-02 at 22:21 -0700, Paul E. McKenney wrote: > > > > On Thu, Jul 03, 2014 at 05:31:19AM +0200, Mike Galbraith wrote: > > > > > > > > NO_HZ_FULL is a property of a set of CPUs. isolcpus is supposed to go > > > > > away as being a redundant interface to manage a single property of a set > > > > > of CPUs, but it's perfectly fine for NO_HZ_FULL to add an interface to > > > > > manage a single property of a set of CPUs. What am I missing? > > > > > > > > Well, for now, it can only be specified at build time or at boot time. > > > > In theory, it is possible to change a CPU from being callback-offloaded > > > > to not at runtime, but there would need to be an extremely good reason > > > > for adding that level of complexity. Lots of "fun" races in there... > > > > > > Yeah, understood. > > > > > > (still it's a NO_HZ_FULL wart though IMHO, would be prettier and more > > > usable if it eventually became unified with cpuset and learned how to > > > tap-dance properly;) > > > > Agreed, it would in some sense be nice. What specifically do you need > > it for? > > I personally have zero use for the thing (git/vi aren't particularly > perturbation sensitive;). I'm just doing occasional drive-by testing > from a distro perspective, how well does it work, what does it cost etc. > > > Are you really running workloads that generate large numbers of > > callbacks spread across most of the CPUs? It was this sort of workload > > that caused Rik's system to show scary CPU-time accumulation, due to > > the high overhead of frequent one-to-many wakeups. > > > > If your systems aren't running that kind of high-callback-rate workload, > > just set CONFIG_RCU_NOCB_CPU_ALL=y and don't worry about it. > > > > If your systems -are- running that kind of high-callback-rate workload, > > but your system has fewer than 200 CPUs, ensure that you have enough > > housekeeping CPUs to allow the grace-period kthread sufficient CPU time, > > set CONFIG_RCU_NOCB_CPU_ALL=y and don't worry about it. > > > > If your systems -are- running that kind of high-callback-rate workload, > > and your system has more than 200 CPUs, apply the following patch, > > set CONFIG_RCU_NOCB_CPU_ALL=y and once again don't worry about it. ;-) > > Turn it on and don't worry about it is exactly what distros want the > obscure feature with very few users to be. Last time I did a drive-by, > my boxen said I should continue to worry about it ;-)
Yep, which is the reason for the patch on the last email.
Then again, exactly which feature and which reason for worry?
Thanx, Paul
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