[PATCH v2 0/7] arm64 / sched/preempt: support PREEMPT_DYNAMIC with static keys

Mark Rutland
2022-02-04T15:06:08Z

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.

[1] https://lore.kernel.org/all/20211109172408.49641-1-mark.rutland@arm.com/

Mark Rutland (7):
sched/preempt: move PREEMPT_DYNAMIC logic later
sched/preempt: refactor sched_dynamic_update()
sched/preempt: simplify irqentry_exit_cond_resched() callers
sched/preempt: decouple HAVE_PREEMPT_DYNAMIC from GENERIC_ENTRY
sched/preempt: add PREEMPT_DYNAMIC using static keys
arm64: entry: centralize premeption decision
arm64: support PREEMPT_DYNAMIC

arch/Kconfig | 37 +++-
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/preempt.h | 16 +-
arch/arm64/kernel/entry-common.c | 28 ++-
arch/x86/Kconfig | 2 +-
arch/x86/include/asm/preempt.h | 10 +-
include/linux/entry-common.h | 15 +-
include/linux/kernel.h | 7 +-
include/linux/sched.h | 10 +-
kernel/entry/common.c | 22 +-
kernel/sched/core.c | 346 ++++++++++++++++++-------------
11 files changed, 322 insertions(+), 172 deletions(-)

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2.30.2