On Thu, Nov 19, 2020 at 09:38:34AM +0000, Mel Gorman wrote: > On Wed, Nov 18, 2020 at 08:48:42PM +0100, Thomas Gleixner wrote: > > From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > > > Now that the scheduler can deal with migrate disable properly, there is no > > real compelling reason to make it only available for RT. > > > > There are quite some code pathes which needlessly disable preemption in > > order to prevent migration and some constructs like kmap_atomic() enforce > > it implicitly. > > > > Making it available independent of RT allows to provide a preemptible > > variant of kmap_atomic() and makes the code more consistent in general. > > > > FIXME: Rework the comment in preempt.h - Peter? > > > > I didn't keep up to date and there is clearly a dependency on patches in > tip for migrate_enable/migrate_disable . It's not 100% clear to me what > reworking you're asking for but then again, I'm not Peter! He's talking about the big one: "Migrate-Disable and why it is undesired.". I still hate all of this, and I really fear that with migrate_disable() available, people will be lazy and usage will increase :/ Case at hand is this series, the only reason we need it here is because per-cpu page-tables are expensive... I really do think we want to limit the usage and get rid of the implicit migrate_disable() in spinlock_t/rwlock_t for example. AFAICT the scenario described there is entirely possible; and it has to show up for workloads that rely on multi-cpu bandwidth for correctness. Switching from preempt_disable() to migrate_disable() hides the immediate / easily visible high priority latency, but you move the interference term into a place where it is much harder to detect, you don't lose the term, it stays in the system. So no, I don't want to make the comment less scary. Usage is discouraged.
