[RFC][PATCH 3/9] irq_work: Optimize irq_work_single()

Peter Zijlstra
2020-07-22T15:33:37Z

Trade one atomic op for a full memory barrier.

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

--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -88,14 +88,14 @@ 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)

--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -34,7 +34,7 @@ static bool irq_work_claim(struct irq_wo
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);
}