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SubjectRe: [patch V4 4/8] sched: Make migrate_disable/enable() independent of RT
On Wed, Nov 18, 2020 at 08:48:42PM +0100, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
>
> 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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