On Thu, Nov 19, 2020 at 12:14:11PM +0100, Peter Zijlstra wrote: > 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.". > Ah yes, that makes more sense. I was thinking in terms of what is protected but the PREEMPT_RT hazard is severe. > 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 guessed, it was the only thing that made sense. > 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. More scary then by adding this to the kerneldoc section for migrate_disable? * Usage of migrate_disable is heavily discouraged as it is extremely * hazardous on PREEMPT_RT kernels and any usage needs to be heavily * justified. Before even thinking about using this, read * "Migrate-Disable and why it is undesired" in * include/linux/preempt.h and include both a comment and document * in the changelog why the use case is an exception. It's not necessary for the current series because the interface hides it and anyone poking at the internals of kmap_atomic probably should be aware of the address space and TLB hazards associated with it. There are few in-tree users and presumably any future preempt-rt related merges already know why migrate_disable is required. However, with the kerneldoc, there is no excuse for missing it for new users that are not PREEMPT_RT-aware. It makes it easier to NAK/revert a patch without proper justification similar to how undocumented usages of memory barriers tend to get a poor reception. -- Mel Gorman SUSE Labs
