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! From tip; /** * migrate_disable - Prevent migration of the current task * * Maps to preempt_disable() which also disables preemption. Use * migrate_disable() to annotate that the intent is to prevent migration, * but not necessarily preemption. * * Can be invoked nested like preempt_disable() and needs the corresponding * number of migrate_enable() invocations. */ I assume that the rework is to document the distinction between migrate_disable and preempt_disable() because it may not be clear to some people why one should be used over another and the risk of cut&paste cargo cult programming. So I assume the rework is for the middle paragraph * Maps to preempt_disable() which also disables preemption. Use * migrate_disable() to annotate that the intent is to prevent migration, * but not necessarily preemption. The distinction is that preemption * disabling will protect a per-cpu structure from concurrent * modifications due to preemption. migrate_disable partially protects * the tasks address space and potentially preserves the TLB entries * even if preempted such as an needed for a local IO mapping or a * kmap_atomic() referenced by on-stack pointers to avoid interference * between user threads or kernel threads sharing the same address space. I know it can have other examples that are rt-specific and some tricks on percpu page alloc draining that relies on a combination of migrate_disable and interrupt disabling to protect the structures but the above example might be understandable to a non-RT audience. -- Mel Gorman SUSE Labs
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