linux-kernel - [RFC][PATCH v2 06/10] irq_work: Provide irq_work_queue_remote

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Date:   Tue, 18 Aug 2020 12:51:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org, torvalds@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org, will@...nel.org, paulmck@...nel.org,
        hch@....de, axboe@...nel.dk, chris@...is-wilson.co.uk,
        davem@...emloft.net, kuba@...nel.org, fweisbec@...il.com,
        oleg@...hat.com, vincent.guittot@...aro.org, peterz@...radead.org
Subject: [RFC][PATCH v2 06/10] irq_work: Provide irq_work_queue_remote_static()

Provide the same horrible semantics provided by
smp_call_function_single_async(), doing so allows skiping a bunch of
atomic ops.

API wise this is horrible crap as it relies on external serialization.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 include/linux/irq_work.h |    3 ++-
 kernel/irq_work.c        |   21 ++++++++++++++++++++-
 2 files changed, 22 insertions(+), 2 deletions(-)

--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -19,7 +19,7 @@ struct irq_work {
 };
 
 #define __IRQ_WORK_INIT(_func, _flags) (struct irq_work){	\
-	.node = { .u_flags = (_flags), },			\
+	.node = { .u_flags = CSD_TYPE_IRQ_WORK | (_flags), },	\
 	.func = (_func),					\
 }
 
@@ -68,6 +68,7 @@ static inline bool irq_work_needs_cpu(vo
 
 #ifdef CONFIG_SMP
 extern int irq_work_queue_remote(int cpu, struct irq_work *work);
+extern int irq_work_queue_remote_static(int cpu, struct irq_work *work);
 extern void irq_work_single(void *arg);
 #endif
 
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -29,7 +29,7 @@ static bool irq_work_claim(struct irq_wo
 {
 	int oflags;
 
-	oflags = atomic_fetch_or(IRQ_WORK_CLAIMED | CSD_TYPE_IRQ_WORK, &work->node.a_flags);
+	oflags = atomic_fetch_or(IRQ_WORK_CLAIMED, &work->node.a_flags);
 	/*
 	 * If the work is already pending, no need to raise the IPI.
 	 * The pairing smp_mb() in irq_work_single() makes sure
@@ -63,6 +63,9 @@ void irq_work_single(void *arg)
 	work->func(work);
 	lockdep_irq_work_exit(flags);
 
+	if (!(flags & IRQ_WORK_BUSY))
+		return;
+
 	/*
 	 * Clear the BUSY bit, if set, and return to the free state if no-one
 	 * else claimed it meanwhile.
@@ -108,6 +111,22 @@ int irq_work_queue_remote(int cpu, struc
 
 	return 0;
 }
+
+int irq_work_queue_remote_static(int cpu, struct irq_work *work)
+{
+	/*
+	 * Ensures preemption is disabled in the caller.
+	 */
+	WARN_ON_ONCE(cpu == smp_processor_id());
+
+	if (work->node.u_flags & IRQ_WORK_PENDING)
+		return -EBUSY;
+
+	work->node.u_flags |= IRQ_WORK_PENDING;
+	__smp_call_single_queue(cpu, &work->node.llist);
+
+	return 0;
+}
 
 #endif /* CONFIG_SMP */