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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Nitesh Lal <nilal@redhat.com>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Marcelo Tosatti <mtosatti@redhat.com>
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Subject: [PATCH] genirq: Provide new interfaces for affinity hints
In-Reply-To: <87zgwo9u79.ffs@nanos.tec.linutronix.de>
References: <20210504092340.00006c61@intel.com> <87pmxpdr32.ffs@nanos.tec.linutronix.de> <CAFki+Lkjn2VCBcLSAfQZ2PEkx-TR0Ts_jPnK9b-5ne3PUX37TQ@mail.gmail.com> <87im3gewlu.ffs@nanos.tec.linutronix.de> <CAFki+L=gp10W1ygv7zdsee=BUGpx9yPAckKr7pyo=tkFJPciEg@mail.gmail.com> <CAFki+L=eQoMq+mWhw_jVT-biyuDXpxbXY5nO+F6HvCtpbG9V2w@mail.gmail.com> <CAFki+LkB1sk3mOv4dd1D-SoPWHOs28ZwN-PqL_6xBk=Qkm40Lw@mail.gmail.com> <87zgwo9u79.ffs@nanos.tec.linutronix.de>
Date:   Fri, 21 May 2021 14:03:06 +0200
Message-ID: <87wnrs9tvp.ffs@nanos.tec.linutronix.de>
MIME-Version: 1.0
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Precedence: bulk

The discussion about removing the side effect of irq_set_affinity_hint() of
actually applying the cpumask (if not NULL) as affinity to the interrupt,
unearthed a few unpleasantries:

  1) The modular perf drivers rely on the current behaviour for the very
     wrong reasons.

  2) While none of the other drivers prevents user space from changing
     the affinity, a cursorily inspection shows that there are at least
     expectations in some drivers.

#1 needs to be cleaned up anyway, so that's not a problem

#2 might result in subtle regressions especially when irqbalanced (which
   nowadays ignores the affinity hint) is disabled.

Provide new interfaces:

  irq_update_affinity_hint() - Only sets the affinity hint pointer
  irq_apply_affinity_hint()  - Set the pointer and apply the affinity to
  			       the interrupt

Make irq_set_affinity_hint() a wrapper around irq_apply_affinity_hint() and
document it to be phased out.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210501021832.743094-1-jesse.brandeburg@intel.com
---
Applies on:
   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git irq/core
---
 include/linux/interrupt.h |   41 ++++++++++++++++++++++++++++++++++++++++-
 kernel/irq/manage.c       |    8 ++++----
 2 files changed, 44 insertions(+), 5 deletions(-)

--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -328,7 +328,46 @@ extern int irq_force_affinity(unsigned i
 extern int irq_can_set_affinity(unsigned int irq);
 extern int irq_select_affinity(unsigned int irq);
 
-extern int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m);
+extern int __irq_apply_affinity_hint(unsigned int irq, const struct cpumask *m,
+				     bool setaffinity);
+
+/**
+ * irq_update_affinity_hint - Update the affinity hint
+ * @irq:	Interrupt to update
+ * @cpumask:	cpumask pointer (NULL to clear the hint)
+ *
+ * Updates the affinity hint, but does not change the affinity of the interrupt.
+ */
+static inline int
+irq_update_affinity_hint(unsigned int irq, const struct cpumask *m)
+{
+	return __irq_apply_affinity_hint(irq, m, true);
+}
+
+/**
+ * irq_apply_affinity_hint - Update the affinity hint and apply the provided
+ *			     cpumask to the interrupt
+ * @irq:	Interrupt to update
+ * @cpumask:	cpumask pointer (NULL to clear the hint)
+ *
+ * Updates the affinity hint and if @cpumask is not NULL it applies it as
+ * the affinity of that interrupt.
+ */
+static inline int
+irq_apply_affinity_hint(unsigned int irq, const struct cpumask *m)
+{
+	return __irq_apply_affinity_hint(irq, m, true);
+}
+
+/*
+ * Deprecated. Use irq_update_affinity_hint() or irq_apply_affinity_hint()
+ * instead.
+ */
+static inline int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m)
+{
+	return irq_apply_affinity_hint(irq, cpumask);
+}
+
 extern int irq_update_affinity_desc(unsigned int irq,
 				    struct irq_affinity_desc *affinity);
 
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -487,7 +487,8 @@ int irq_force_affinity(unsigned int irq,
 }
 EXPORT_SYMBOL_GPL(irq_force_affinity);
 
-int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m)
+int __irq_apply_affinity_hint(unsigned int irq, const struct cpumask *m,
+			      bool setaffinity)
 {
 	unsigned long flags;
 	struct irq_desc *desc = irq_get_desc_lock(irq, &flags, IRQ_GET_DESC_CHECK_GLOBAL);
@@ -496,12 +497,11 @@ int irq_set_affinity_hint(unsigned int i
 		return -EINVAL;
 	desc->affinity_hint = m;
 	irq_put_desc_unlock(desc, flags);
-	/* set the initial affinity to prevent every interrupt being on CPU0 */
-	if (m)
+	if (m && setaffinity)
 		__irq_set_affinity(irq, m, false);
 	return 0;
 }
-EXPORT_SYMBOL_GPL(irq_set_affinity_hint);
+EXPORT_SYMBOL_GPL(__irq_apply_affinity_hint);
 
 static void irq_affinity_notify(struct work_struct *work)
 {