[tip: sched/core] irq_work: Optimize irq_work_single()

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     2914b0ba61a9d253535e51af16c7122a8148995d
Gitweb:        https://git.kernel.org/tip/2914b0ba61a9d253535e51af16c7122a8148995d
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 18 Jun 2020 22:28:37 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 24 Nov 2020 16:47:49 +01:00

irq_work: Optimize irq_work_single()

Trade one atomic op for a full memory barrier.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 include/linux/irqflags.h |  8 ++++----
 kernel/irq_work.c        | 29 +++++++++++++++++------------
 2 files changed, 21 insertions(+), 16 deletions(-)

diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index fef2d43..8de0e13 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -107,14 +107,14 @@ do {						\
 		  current->irq_config = 0;			\
 	  } while (0)
 
-# define lockdep_irq_work_enter(__work)					\
+# define lockdep_irq_work_enter(_flags)					\
 	  do {								\
-		  if (!(atomic_read(&__work->node.a_flags) & IRQ_WORK_HARD_IRQ))\
+		  if (!((_flags) & IRQ_WORK_HARD_IRQ))			\
 			current->irq_config = 1;			\
 	  } while (0)
-# define lockdep_irq_work_exit(__work)					\
+# define lockdep_irq_work_exit(_flags)					\
 	  do {								\
-		  if (!(atomic_read(&__work->node.a_flags) & IRQ_WORK_HARD_IRQ))\
+		  if (!((_flags) & IRQ_WORK_HARD_IRQ))			\
 			current->irq_config = 0;			\
 	  } while (0)
 
diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index fbff25a..e8da1e7 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -34,7 +34,7 @@ static bool irq_work_claim(struct irq_work *work)
 	oflags = atomic_fetch_or(IRQ_WORK_CLAIMED | CSD_TYPE_IRQ_WORK, &work->node.a_flags);
 	/*
 	 * If the work is already pending, no need to raise the IPI.
-	 * The pairing atomic_fetch_andnot() in irq_work_run() makes sure
+	 * The pairing smp_mb() in irq_work_single() makes sure
 	 * everything we did before is visible.
 	 */
 	if (oflags & IRQ_WORK_PENDING)
@@ -136,22 +136,27 @@ void irq_work_single(void *arg)
 	int flags;
 
 	/*
-	 * Clear the PENDING bit, after this point the @work
-	 * can be re-used.
-	 * Make it immediately visible so that other CPUs trying
-	 * to claim that work don't rely on us to handle their data
-	 * while we are in the middle of the func.
+	 * Clear the PENDING bit, after this point the @work can be re-used.
+	 * The PENDING bit acts as a lock, and we own it, so we can clear it
+	 * without atomic ops.
 	 */
-	flags = atomic_fetch_andnot(IRQ_WORK_PENDING, &work->node.a_flags);
+	flags = atomic_read(&work->node.a_flags);
+	flags &= ~IRQ_WORK_PENDING;
+	atomic_set(&work->node.a_flags, flags);
+
+	/*
+	 * See irq_work_claim().
+	 */
+	smp_mb();
 
-	lockdep_irq_work_enter(work);
+	lockdep_irq_work_enter(flags);
 	work->func(work);
-	lockdep_irq_work_exit(work);
+	lockdep_irq_work_exit(flags);
+
 	/*
-	 * Clear the BUSY bit and return to the free state if
-	 * no-one else claimed it meanwhile.
+	 * Clear the BUSY bit, if set, and return to the free state if no-one
+	 * else claimed it meanwhile.
 	 */
-	flags &= ~IRQ_WORK_PENDING;
 	(void)atomic_cmpxchg(&work->node.a_flags, flags, flags & ~IRQ_WORK_BUSY);
 }