Re: [PATCH v2 2/5] preempt/dynamic: Introduce preempt mode accessors

On Thu, 2021-11-11 at 10:36 +0100, Marco Elver wrote:

On Thu, 11 Nov 2021 at 04:47, Mike Galbraith [off-list ref] wrote:

quoted

On Thu, 2021-11-11 at 04:35 +0100, Mike Galbraith wrote:

quoted

On Thu, 2021-11-11 at 04:16 +0100, Mike Galbraith wrote:

quoted

On Wed, 2021-11-10 at 20:24 +0000, Valentin Schneider wrote:

quoted

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5f8db54226af..0640d5622496 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h

@@ -2073,6 +2073,22 @@ static inline void cond_resched_rcu(void)

 #endif
 }

+#ifdef CONFIG_PREEMPT_DYNAMIC
+
+extern bool is_preempt_none(void);
+extern bool is_preempt_voluntary(void);
+extern bool is_preempt_full(void);
+
+#else
+
+#define is_preempt_none() IS_ENABLED(CONFIG_PREEMPT_NONE)
+#define is_preempt_voluntary()
IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY)
+#define is_preempt_full() IS_ENABLED(CONFIG_PREEMPT)

I think that should be IS_ENABLED(CONFIG_PREEMPTION), see
c1a280b68d4e.

Noticed while applying the series to an RT tree, where tglx
has done that replacement to the powerpc spot your next patch
diddles.

Damn, then comes patch 5 properly differentiating PREEMPT/PREEMPT_RT.

So I suppose the powerpc spot should remain CONFIG_PREEMPT and become
CONFIG_PREEMPTION when the RT change gets merged, because that spot is
about full preemptibility, not a distinct preemption model.

That's rather annoying :-/

I guess the question is if is_preempt_full() should be true also if
is_preempt_rt() is true?

That's what CONFIG_PREEMPTION is.  More could follow, but it was added
to allow multiple models to say "preemptible".

Not sure all cases are happy with that, e.g. the kernel/trace/trace.c
case, which wants to print the precise preemption level.

Yeah, that's the "annoying" bit, needing one oddball model accessor
that isn't about a particular model.

To avoid confusion, I'd introduce another helper that says true if the
preemption level is "at least full", currently that'd be "full or rt".
Something like is_preempt_full_or_rt() (but might as well write
"is_preempt_full() || is_preempt_rt()"), or is_preemption() (to match
that Kconfig variable, although it's slightly confusing). The
implementation of that helper can just be a static inline function
returning "is_preempt_full() || is_preempt_rt()".

Would that help?

Yeah, as it sits two accessors are needed, one that says PREEMPT the
other PREEMPTION, spelling optional.

	-Mike