From 5a486be49ffc4dfe3fd54d6a4fe288a4d04d062c Mon Sep 17 00:00:00 2001
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Date: Tue, 18 May 2021 19:17:16 -0700
Subject: [PATCH] rcu: Remove special bit at the bottom of the ->dynticks
counter
References: bnc#1189998 (PREEMPT_RT prerequisite backports)
Patch-mainline: v5.15-rc1
Git-commit: a86baa69c2b7b85bab41692fa3ec188a5aae1d27
Commit b8c17e6664c4 ("rcu: Maintain special bits at bottom of ->dynticks
counter") reserved a bit at the bottom of the ->dynticks counter to defer
flushing of TLBs, but this facility never has been used. This commit
therefore removes this capability along with the rcu_eqs_special_set()
function used to trigger it.
Link: https://lore.kernel.org/linux-doc/CALCETrWNPOOdTrFabTDd=H7+wc6xJ9rJceg6OL1S0rTV5pfSsA@mail.gmail.com/
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: "Joel Fernandes (Google)" <joel@joelfernandes.org>
[ paulmck: Forward-port to v5.13-rc1. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
include/linux/rcutiny.h | 3 --
kernel/rcu/tree.c | 77 +++++++++----------------------------------------
2 files changed, 14 insertions(+), 66 deletions(-)
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 953e70fafe38..9be015305f9f 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -14,9 +14,6 @@
#include <asm/param.h> /* for HZ */
-/* Never flag non-existent other CPUs! */
-static inline bool rcu_eqs_special_set(int cpu) { return false; }
-
unsigned long get_state_synchronize_rcu(void);
unsigned long start_poll_synchronize_rcu(void);
bool poll_state_synchronize_rcu(unsigned long oldstate);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f116ce58f4e6..66a8d5c887a3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -74,17 +74,10 @@
/* Data structures. */
-/*
- * Steal a bit from the bottom of ->dynticks for idle entry/exit
- * control. Initially this is for TLB flushing.
- */
-#define RCU_DYNTICK_CTRL_MASK 0x1
-#define RCU_DYNTICK_CTRL_CTR (RCU_DYNTICK_CTRL_MASK + 1)
-
static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
.dynticks_nesting = 1,
.dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
- .dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR),
+ .dynticks = ATOMIC_INIT(1),
#ifdef CONFIG_RCU_NOCB_CPU
.cblist.flags = SEGCBLIST_SOFTIRQ_ONLY,
#endif
@@ -266,7 +259,6 @@ void rcu_softirq_qs(void)
*/
static noinstr void rcu_dynticks_eqs_enter(void)
{
- struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
int seq;
/*
@@ -275,13 +267,9 @@ static noinstr void rcu_dynticks_eqs_enter(void)
* next idle sojourn.
*/
rcu_dynticks_task_trace_enter(); // Before ->dynticks update!
- seq = arch_atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
+ seq = arch_atomic_inc_return(&this_cpu_ptr(&rcu_data)->dynticks);
// RCU is no longer watching. Better be in extended quiescent state!
- WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
- (seq & RCU_DYNTICK_CTRL_CTR));
- /* Better not have special action (TLB flush) pending! */
- WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
- (seq & RCU_DYNTICK_CTRL_MASK));
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & 0x1));
}
/*
@@ -291,7 +279,6 @@ static noinstr void rcu_dynticks_eqs_enter(void)
*/
static noinstr void rcu_dynticks_eqs_exit(void)
{
- struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
int seq;
/*
@@ -299,15 +286,10 @@ static noinstr void rcu_dynticks_eqs_exit(void)
* and we also must force ordering with the next RCU read-side
* critical section.
*/
- seq = arch_atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
+ seq = arch_atomic_inc_return(&this_cpu_ptr(&rcu_data)->dynticks);
// RCU is now watching. Better not be in an extended quiescent state!
rcu_dynticks_task_trace_exit(); // After ->dynticks update!
- WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
- !(seq & RCU_DYNTICK_CTRL_CTR));
- if (seq & RCU_DYNTICK_CTRL_MASK) {
- arch_atomic_andnot(RCU_DYNTICK_CTRL_MASK, &rdp->dynticks);
- smp_mb__after_atomic(); /* _exit after clearing mask. */
- }
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & 0x1));
}
/*
@@ -324,9 +306,9 @@ static void rcu_dynticks_eqs_online(void)
{
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
- if (atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR)
+ if (atomic_read(&rdp->dynticks) & 0x1)
return;
- atomic_add(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
+ atomic_inc(&rdp->dynticks);
}
/*
@@ -336,9 +318,7 @@ static void rcu_dynticks_eqs_online(void)
*/
static __always_inline bool rcu_dynticks_curr_cpu_in_eqs(void)
{
- struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
-
- return !(arch_atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);
+ return !(arch_atomic_read(&this_cpu_ptr(&rcu_data)->dynticks) & 0x1);
}
/*
@@ -347,9 +327,7 @@ static __always_inline bool rcu_dynticks_curr_cpu_in_eqs(void)
*/
static int rcu_dynticks_snap(struct rcu_data *rdp)
{
- int snap = atomic_add_return(0, &rdp->dynticks);
-
- return snap & ~RCU_DYNTICK_CTRL_MASK;
+ return atomic_add_return(0, &rdp->dynticks);
}
/*
@@ -358,7 +336,7 @@ static int rcu_dynticks_snap(struct rcu_data *rdp)
*/
static bool rcu_dynticks_in_eqs(int snap)
{
- return !(snap & RCU_DYNTICK_CTRL_CTR);
+ return !(snap & 0x1);
}
/* Return true if the specified CPU is currently idle from an RCU viewpoint. */
@@ -389,8 +367,7 @@ bool rcu_dynticks_zero_in_eqs(int cpu, int *vp)
int snap;
// If not quiescent, force back to earlier extended quiescent state.
- snap = atomic_read(&rdp->dynticks) & ~(RCU_DYNTICK_CTRL_MASK |
- RCU_DYNTICK_CTRL_CTR);
+ snap = atomic_read(&rdp->dynticks) & ~0x1;
smp_rmb(); // Order ->dynticks and *vp reads.
if (READ_ONCE(*vp))
@@ -398,32 +375,7 @@ bool rcu_dynticks_zero_in_eqs(int cpu, int *vp)
smp_rmb(); // Order *vp read and ->dynticks re-read.
// If still in the same extended quiescent state, we are good!
- return snap == (atomic_read(&rdp->dynticks) & ~RCU_DYNTICK_CTRL_MASK);
-}
-
-/*
- * Set the special (bottom) bit of the specified CPU so that it
- * will take special action (such as flushing its TLB) on the
- * next exit from an extended quiescent state. Returns true if
- * the bit was successfully set, or false if the CPU was not in
- * an extended quiescent state.
- */
-bool rcu_eqs_special_set(int cpu)
-{
- int old;
- int new;
- int new_old;
- struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
-
- new_old = atomic_read(&rdp->dynticks);
- do {
- old = new_old;
- if (old & RCU_DYNTICK_CTRL_CTR)
- return false;
- new = old | RCU_DYNTICK_CTRL_MASK;
- new_old = atomic_cmpxchg(&rdp->dynticks, old, new);
- } while (new_old != old);
- return true;
+ return snap == atomic_read(&rdp->dynticks);
}
/*
@@ -442,10 +394,9 @@ notrace void rcu_momentary_dyntick_idle(void)
int special;
raw_cpu_write(rcu_data.rcu_need_heavy_qs, false);
- special = atomic_add_return(2 * RCU_DYNTICK_CTRL_CTR,
- &this_cpu_ptr(&rcu_data)->dynticks);
+ special = atomic_add_return(2, &this_cpu_ptr(&rcu_data)->dynticks);
/* It is illegal to call this from idle state. */
- WARN_ON_ONCE(!(special & RCU_DYNTICK_CTRL_CTR));
+ WARN_ON_ONCE(!(special & 0x1));
rcu_preempt_deferred_qs(current);
}
EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);