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

On 16/11/21 14:29, Christophe Leroy wrote:

Le 10/11/2021 à 21:24, Valentin Schneider a écrit :

quoted

CONFIG_PREEMPT{_NONE, _VOLUNTARY} designate either:
o The build-time preemption model when !PREEMPT_DYNAMIC
o The default boot-time preemption model when PREEMPT_DYNAMIC

IOW, using those on PREEMPT_DYNAMIC kernels is meaningless - the actual
model could have been set to something else by the "preempt=foo" cmdline
parameter.

Introduce a set of helpers to determine the actual preemption mode used by
the live kernel.

Suggested-by: Marco Elver <elver@google.com>
Signed-off-by: Valentin Schneider <redacted>
---
  include/linux/sched.h | 16 ++++++++++++++++
  kernel/sched/core.c   | 11 +++++++++++
  2 files changed, 27 insertions(+)

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);

Those are trivial tests supposed to be used in fast pathes. They should
be static inlines in order to minimise the overhead.

quoted

+
+#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)

Would be better to use static inlines here as well instead of macros.

I realize I stripped all ppc folks from the cclist after dropping the ppc
snippet, but you guys might still be interested - my bad. That's done in
v3:

https://lore.kernel.org/lkml/20211112185203.280040-1-valentin.schneider@arm.com/ (local)

quoted

+
+#endif
+
+#define is_preempt_rt() IS_ENABLED(CONFIG_PREEMPT_RT)
+
  /*
   * Does a critical section need to be broken due to another
   * task waiting?: (technically does not depend on CONFIG_PREEMPTION,

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 97047aa7b6c2..9db7f77e53c3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -6638,6 +6638,17 @@ static void __init preempt_dynamic_init(void)
     }
  }

+#define PREEMPT_MODE_ACCESSOR(mode) \
+	bool is_preempt_##mode(void)						 \
+	{									 \
+		WARN_ON_ONCE(preempt_dynamic_mode == preempt_dynamic_undefined); \

Not sure using WARN_ON is a good idea here, as it may be called very
early, see comment on powerpc patch.

Bah, I was gonna say that you *don't* want users of is_preempt_*() to be
called before the "final" preemption model is set up (such users would need
to make use of static_calls), but I realize there's a debug interface to
flip the preemption model at will... Say an initcall sees
is_preempt_voluntary() and sets things up accordingly, and then the debug
knob switches to preempt_full. I don't think there's much we can really do
here though :/

quoted

+		return preempt_dynamic_mode == preempt_dynamic_##mode;		 \
+	}

I'm not sure that's worth a macro. You only have 3 accessors, 2 lines of
code each. Just define all 3 in plain text.

CONFIG_PREEMPT_DYNAMIC is based on using strategies like static_calls in
order to minimise the overhead. For those accessors you should use the
same kind of approach and use things like jump_labels in order to not
redo the test at each time and minimise overhead as much as possible.

That's a valid point, though the few paths that need patching up and don't
make use of static calls already (AFAICT the ppc irq path I was touching in
v2 needs to make use of irqentry_exit_cond_resched()) really seem like
slow-paths.

quoted

+
+PREEMPT_MODE_ACCESSOR(none)
+PREEMPT_MODE_ACCESSOR(voluntary)
+PREEMPT_MODE_ACCESSOR(full)
+
  #else /* !CONFIG_PREEMPT_DYNAMIC */

  static inline void preempt_dynamic_init(void) { }