The introduction of PREEMPT_LAZY was for multiple reasons:
- PREEMPT_RT suffered from over-scheduling, hurting performance compared to
!PREEMPT_RT.
- the introduction of (more) features that rely on preemption; like
folio_zero_user() which can do large memset() without preemption checks.
(Xen already had a horrible hack to deal with long running hypercalls)
- the endless and uncontrolled sprinkling of cond_resched() -- mostly cargo
cult or in response to poor to replicate workloads.
By moving to a model that is fundamentally preemptable these things become
managable and avoid needing to introduce more horrible hacks.
Since this is a requirement; limit PREEMPT_NONE to architectures that do not
support preemption at all. Further limit PREEMPT_VOLUNTARY to those
architectures that do not yet have PREEMPT_LAZY support (with the eventual goal
to make this the empty set and completely remove voluntary preemption and
cond_resched() -- notably VOLUNTARY is already limited to !ARCH_NO_PREEMPT.)
This leaves up-to-date architectures (arm64, loongarch, powerpc, riscv, s390,
x86) with only two preemption models: full and lazy.
While Lazy has been the recommended setting for a while, not all distributions
have managed to make the switch yet. Force things along. Keep the patch minimal
in case of hard to address regressions that might pop up.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://patch.msgid.link/20251219101502.GB1132199@noisy.programming.kicks-ass.net
7dadeaa6e8