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Date: Wed, 14 Feb 2024 15:57:27 -0800
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Ankur Arora <ankur.a.arora@oracle.com>
Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, peterz@infradead.org,
	torvalds@linux-foundation.org, akpm@linux-foundation.org,
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	konrad.wilk@oracle.com
Subject: Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling
Message-ID: <a7e785f8-d4c3-4b1b-9abe-36ac0b971e44@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <20240213055554.1802415-1-ankur.a.arora@oracle.com>
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In-Reply-To: <20240213055554.1802415-1-ankur.a.arora@oracle.com>

On Mon, Feb 12, 2024 at 09:55:24PM -0800, Ankur Arora wrote:
> Hi,
> 
> This series adds a new scheduling model PREEMPT_AUTO, which like
> PREEMPT_DYNAMIC allows dynamic switching between a none/voluntary/full
> preemption model. However, unlike PREEMPT_DYNAMIC, it doesn't depend
> on explicit preemption points for the voluntary models.
> 
> The series is based on Thomas' original proposal which he outlined
> in [1], [2] and in his PoC [3].
> 
> An earlier RFC version is at [4].

This uncovered a couple of latent bugs in RCU due to its having been
a good long time since anyone built a !SMP preemptible kernel with
non-preemptible RCU.  I have a couple of fixes queued on -rcu [1], most
likely for the merge window after next, but let me know if you need
them sooner.

I am also seeing OOM conditions during rcutorture testing of callback
flooding, but I am still looking into this.  The full diff on top of
your series on top of v6.8-rc4 is shown below.  Please let me know if
I have messed up the Kconfig options.

							Thanx, Paul

[1] 6a4352fd1418 ("rcu: Update lockdep while in RCU read-side critical section")
    1b85e92eabcd ("rcu: Make TINY_RCU depend on !PREEMPT_RCU rather than !PREEMPTION")

------------------------------------------------------------------------

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0746b1b0b6639..b0b61b8598b03 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -778,9 +778,9 @@ static inline void rcu_read_unlock(void)
 {
 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
 			 "rcu_read_unlock() used illegally while idle");
+	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
 	__release(RCU);
 	__rcu_read_unlock();
-	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
 }
 
 /**
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index d0ecc8ef17a72..6bf969857a85b 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -31,7 +31,7 @@ config PREEMPT_RCU
 
 config TINY_RCU
 	bool
-	default y if !PREEMPTION && !SMP
+	default y if !PREEMPT_RCU && !SMP
 	help
 	  This option selects the RCU implementation that is
 	  designed for UP systems from which real-time response
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/SRCU-N b/tools/testing/selftests/rcutorture/configs/rcu/SRCU-N
index 07f5e0a70ae70..737389417c7b3 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/SRCU-N
+++ b/tools/testing/selftests/rcutorture/configs/rcu/SRCU-N
@@ -3,8 +3,10 @@ CONFIG_SMP=y
 CONFIG_NR_CPUS=4
 CONFIG_HOTPLUG_CPU=y
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 #CHECK#CONFIG_RCU_EXPERT=n
 CONFIG_KPROBES=n
 CONFIG_FTRACE=n
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/SRCU-T b/tools/testing/selftests/rcutorture/configs/rcu/SRCU-T
index c70cf0405f248..c9aca21d02f8c 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/SRCU-T
+++ b/tools/testing/selftests/rcutorture/configs/rcu/SRCU-T
@@ -1,5 +1,6 @@
 CONFIG_SMP=n
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
@@ -10,3 +11,4 @@ CONFIG_PROVE_LOCKING=y
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_DEBUG_ATOMIC_SLEEP=y
 #CHECK#CONFIG_PREEMPT_COUNT=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TASKS02 b/tools/testing/selftests/rcutorture/configs/rcu/TASKS02
index 2f9fcffff5ae3..472259f9e0a6a 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TASKS02
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TASKS02
@@ -1,8 +1,10 @@
 CONFIG_SMP=n
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
 #CHECK#CONFIG_TASKS_RCU=y
 CONFIG_FORCE_TASKS_RCU=y
 CONFIG_RCU_EXPERT=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TINY02 b/tools/testing/selftests/rcutorture/configs/rcu/TINY02
index 30439f6fc20e6..df408933e7013 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TINY02
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TINY02
@@ -1,5 +1,6 @@
 CONFIG_SMP=n
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
@@ -13,3 +14,4 @@ CONFIG_DEBUG_LOCK_ALLOC=y
 CONFIG_DEBUG_OBJECTS=y
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
 CONFIG_DEBUG_ATOMIC_SLEEP=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TRACE01 b/tools/testing/selftests/rcutorture/configs/rcu/TRACE01
index 85b407467454a..2f75c7349d83a 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TRACE01
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TRACE01
@@ -2,6 +2,7 @@ CONFIG_SMP=y
 CONFIG_NR_CPUS=5
 CONFIG_HOTPLUG_CPU=y
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
@@ -12,3 +13,4 @@ CONFIG_FORCE_TASKS_TRACE_RCU=y
 #CHECK#CONFIG_TASKS_TRACE_RCU=y
 CONFIG_TASKS_TRACE_RCU_READ_MB=y
 CONFIG_RCU_EXPERT=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE04 b/tools/testing/selftests/rcutorture/configs/rcu/TREE04
index dc4985064b3ad..9ef845d54fa41 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE04
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE04
@@ -2,6 +2,7 @@ CONFIG_SMP=y
 CONFIG_NR_CPUS=8
 CONFIG_PREEMPT_NONE=n
 CONFIG_PREEMPT_VOLUNTARY=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
 #CHECK#CONFIG_TREE_RCU=y
@@ -16,3 +17,4 @@ CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_RCU_EXPERT=y
 CONFIG_RCU_EQS_DEBUG=y
 CONFIG_RCU_LAZY=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE05 b/tools/testing/selftests/rcutorture/configs/rcu/TREE05
index 9f48c73709ec3..31afd943d85ef 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE05
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE05
@@ -1,6 +1,7 @@
 CONFIG_SMP=y
 CONFIG_NR_CPUS=8
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 #CHECK#CONFIG_TREE_RCU=y
@@ -18,3 +19,4 @@ CONFIG_PROVE_LOCKING=y
 CONFIG_PROVE_RCU_LIST=y
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_RCU_EXPERT=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE06 b/tools/testing/selftests/rcutorture/configs/rcu/TREE06
index db27651de04b8..1180fe36a3a12 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE06
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE06
@@ -1,6 +1,7 @@
 CONFIG_SMP=y
 CONFIG_NR_CPUS=8
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 #CHECK#CONFIG_TREE_RCU=y
@@ -17,3 +18,4 @@ CONFIG_PROVE_LOCKING=y
 CONFIG_DEBUG_OBJECTS=y
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
 CONFIG_RCU_EXPERT=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE07 b/tools/testing/selftests/rcutorture/configs/rcu/TREE07
index d30922d8c8832..969e852bd618b 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE07
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE07
@@ -1,6 +1,7 @@
 CONFIG_SMP=y
 CONFIG_NR_CPUS=16
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
@@ -15,3 +16,4 @@ CONFIG_RCU_FANOUT_LEAF=2
 CONFIG_DEBUG_LOCK_ALLOC=n
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_RCU_EXPERT=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE10 b/tools/testing/selftests/rcutorture/configs/rcu/TREE10
index a323d8948b7cf..4af22599f13ed 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE10
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE10
@@ -1,6 +1,7 @@
 CONFIG_SMP=y
 CONFIG_NR_CPUS=56
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
@@ -16,3 +17,4 @@ CONFIG_PROVE_LOCKING=n
 CONFIG_DEBUG_OBJECTS=n
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_RCU_EXPERT=n
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL b/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL
index 5d546efa68e83..7b2c9fb0cd826 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL
@@ -1,6 +1,7 @@
 CONFIG_SMP=y
 CONFIG_NR_CPUS=8
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_HZ_PERIODIC=n
@@ -9,3 +10,4 @@ CONFIG_NO_HZ_FULL=n
 CONFIG_DEBUG_LOCK_ALLOC=n
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_RCU_EXPERT=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcuscale/TINY b/tools/testing/selftests/rcutorture/configs/rcuscale/TINY
index 0fa2dc086e10c..80230745e9dc7 100644
--- a/tools/testing/selftests/rcutorture/configs/rcuscale/TINY
+++ b/tools/testing/selftests/rcutorture/configs/rcuscale/TINY
@@ -1,5 +1,6 @@
 CONFIG_SMP=n
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
@@ -14,3 +15,4 @@ CONFIG_RCU_BOOST=n
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_RCU_EXPERT=y
 CONFIG_RCU_TRACE=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcuscale/TRACE01 b/tools/testing/selftests/rcutorture/configs/rcuscale/TRACE01
index 0059592c7408a..eb47f36712305 100644
--- a/tools/testing/selftests/rcutorture/configs/rcuscale/TRACE01
+++ b/tools/testing/selftests/rcutorture/configs/rcuscale/TRACE01
@@ -1,5 +1,6 @@
 CONFIG_SMP=y
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
@@ -14,3 +15,4 @@ CONFIG_RCU_BOOST=n
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_RCU_EXPERT=y
 CONFIG_RCU_TRACE=y
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/refscale/NOPREEMPT b/tools/testing/selftests/rcutorture/configs/refscale/NOPREEMPT
index 67f9d2998afd3..cb3219cb98c78 100644
--- a/tools/testing/selftests/rcutorture/configs/refscale/NOPREEMPT
+++ b/tools/testing/selftests/rcutorture/configs/refscale/NOPREEMPT
@@ -1,5 +1,6 @@
 CONFIG_SMP=y
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
@@ -18,3 +19,4 @@ CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 CONFIG_RCU_EXPERT=y
 CONFIG_KPROBES=n
 CONFIG_FTRACE=n
+CONFIG_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/scf/NOPREEMPT b/tools/testing/selftests/rcutorture/configs/scf/NOPREEMPT
index 6133f54ce2a7d..241f28e965e57 100644
--- a/tools/testing/selftests/rcutorture/configs/scf/NOPREEMPT
+++ b/tools/testing/selftests/rcutorture/configs/scf/NOPREEMPT
@@ -1,5 +1,6 @@
 CONFIG_SMP=y
 CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_AUTO=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 CONFIG_PREEMPT_DYNAMIC=n
@@ -11,3 +12,4 @@ CONFIG_DEBUG_LOCK_ALLOC=n
 CONFIG_PROVE_LOCKING=n
 CONFIG_KPROBES=n
 CONFIG_FTRACE=n
+CONFIG_EXPERT=y