Re: [PATCH v4 0/7] arm64 / sched/preempt: support PREEMPT_DYNAMIC with static keys

[Frederic Weisbecker <frederic@kernel.org>]

On Mon, Feb 14, 2022 at 04:52:09PM +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.

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

Thanks!