On Wed, Feb 09, 2022 at 03:35:28PM +0000, Mark Rutland wrote:

This series enables PREEMPT_DYNAMIC on arm64. To do so, it adds a new
mechanism allowing the preemption functions to be enabled/disabled using
static keys rather than static calls, with architectures selecting
whether they use static calls or static keys.

With non-inline static calls, each function call results in a call to
the (out-of-line) trampoline which either tail-calls its associated
callee or performs an early return.

The key idea is that where we're only enabling/disabling a single
callee, we can inline this trampoline into the start of the callee,
using a static key to decide whether to return early, and leaving the
remaining codegen to the compiler. The overhead should be similar to
(and likely lower than) using a static call trampoline. Since most
codegen is up to the compiler, we sidestep a number of implementation
pain-points (e.g. things like CFI should "just work" as well as they do
for any other functions).

The bulk of the diffstat for kernel/sched/core.c is shuffling the
PREEMPT_DYNAMIC code later in the file, and the actual additions are
fairly trivial.

I've given this very light build+boot testing so far.

Since v1 [1]:
* Rework Kconfig text to be clearer
* Rework arm64 entry code
* Clarify commit messages.

Since v2 [2]:
* Add missing includes
* Always provide prototype for preempt_schedule()
* Always provide prototype for preempt_schedule_notrace()
* Fix __cond_resched() to default to disabled
* Fix might_resched() to default to disabled
* Clarify example in commit message

Acked-by: Frederic Weisbecker <frederic@kernel.org>

Thanks!

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