Re: [patch v6 3/7] genirq/affinity: Add new callback for (re)calculating interrupt sets

[Ming Lei]

On Tue, Jun 15, 2021 at 02:57:07PM -0500, Bjorn Helgaas wrote:
> On Sat, Feb 16, 2019 at 06:13:09PM +0100, Thomas Gleixner wrote:
> > From: Ming Lei <ming.lei@xxxxxxxxxx>
> > 
> > The interrupt affinity spreading mechanism supports to spread out
> > affinities for one or more interrupt sets. A interrupt set contains one or
> > more interrupts. Each set is mapped to a specific functionality of a
> > device, e.g. general I/O queues and read I/O queus of multiqueue block
> > devices.
> > 
> > The number of interrupts per set is defined by the driver. It depends on
> > the total number of available interrupts for the device, which is
> > determined by the PCI capabilites and the availability of underlying CPU
> > resources, and the number of queues which the device provides and the
> > driver wants to instantiate.
> > 
> > The driver passes initial configuration for the interrupt allocation via a
> > pointer to struct irq_affinity.
> > 
> > Right now the allocation mechanism is complex as it requires to have a loop
> > in the driver to determine the maximum number of interrupts which are
> > provided by the PCI capabilities and the underlying CPU resources.  This
> > loop would have to be replicated in every driver which wants to utilize
> > this mechanism. That's unwanted code duplication and error prone.
> > 
> > In order to move this into generic facilities it is required to have a
> > mechanism, which allows the recalculation of the interrupt sets and their
> > size, in the core code. As the core code does not have any knowledge about the
> > underlying device, a driver specific callback is required in struct
> > irq_affinity, which can be invoked by the core code. The callback gets the
> > number of available interupts as an argument, so the driver can calculate the
> > corresponding number and size of interrupt sets.
> > 
> > At the moment the struct irq_affinity pointer which is handed in from the
> > driver and passed through to several core functions is marked 'const', but for
> > the callback to be able to modify the data in the struct it's required to
> > remove the 'const' qualifier.
> > 
> > Add the optional callback to struct irq_affinity, which allows drivers to
> > recalculate the number and size of interrupt sets and remove the 'const'
> > qualifier.
> > 
> > For simple invocations, which do not supply a callback, a default callback
> > is installed, which just sets nr_sets to 1 and transfers the number of
> > spreadable vectors to the set_size array at index 0.
> > 
> > This is for now guarded by a check for nr_sets != 0 to keep the NVME driver
> > working until it is converted to the callback mechanism.
> > 
> > To make sure that the driver configuration is correct under all circumstances
> > the callback is invoked even when there are no interrupts for queues left,
> > i.e. the pre/post requirements already exhaust the numner of available
> > interrupts.
> > 
> > At the PCI layer irq_create_affinity_masks() has to be invoked even for the
> > case where the legacy interrupt is used. That ensures that the callback is
> > invoked and the device driver can adjust to that situation.
> > 
> > [ tglx: Fixed the simple case (no sets required). Moved the sanity check
> >   	for nr_sets after the invocation of the callback so it catches
> >   	broken drivers. Fixed the kernel doc comments for struct
> >   	irq_affinity and de-'This patch'-ed the changelog ]
> > 
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> > @@ -1196,6 +1196,13 @@ int pci_alloc_irq_vectors_affinity(struc
> >  	/* use legacy irq if allowed */
> >  	if (flags & PCI_IRQ_LEGACY) {
> >  		if (min_vecs == 1 && dev->irq) {
> > +			/*
> > +			 * Invoke the affinity spreading logic to ensure that
> > +			 * the device driver can adjust queue configuration
> > +			 * for the single interrupt case.
> > +			 */
> > +			if (affd)
> > +				irq_create_affinity_masks(1, affd);
> 
> This looks like a leak because irq_create_affinity_masks() returns a
> pointer to kcalloc()ed space, but we throw away the pointer.
> 
> Or is there something very subtle going on here, like this special
> case doesn't allocate anything?  I do see the "Nothing to assign?"
> case that returns NULL with no alloc, but it's not completely trivial
> to verify that we take that case here.

The purpose is to provide chance to call ->calc_sets() for single
interrupt, maybe it can be improved by the following change:

diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
index 217dc9f0231f..025c647279f5 100644
--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c
@@ -1223,8 +1223,7 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
 			 * the device driver can adjust queue configuration
 			 * for the single interrupt case.
 			 */
-			if (affd)
-				irq_create_affinity_masks(1, affd);
+			irq_affinity_calc_sets_legacy(affd);
 			pci_intx(dev, 1);
 			return 1;
 		}
diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 4777850a6dc7..f21f93ce460b 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -368,6 +368,7 @@ irq_create_affinity_masks(unsigned int nvec, struct irq_affinity *affd);
 
 unsigned int irq_calc_affinity_vectors(unsigned int minvec, unsigned int maxvec,
 				       const struct irq_affinity *affd);
+void irq_affinity_calc_sets_legacy(struct irq_affinity *affd);
 
 #else /* CONFIG_SMP */
 
@@ -419,6 +420,10 @@ irq_calc_affinity_vectors(unsigned int minvec, unsigned int maxvec,
 	return maxvec;
 }
 
+static inline void irq_affinity_calc_sets_legacy(struct irq_affinity *affd)
+{
+}
+
 #endif /* CONFIG_SMP */
 
 /*
diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
index 4d89ad4fae3b..d01f7dfa5712 100644
--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c
@@ -405,6 +405,30 @@ static void default_calc_sets(struct irq_affinity *affd, unsigned int affvecs)
 	affd->set_size[0] = affvecs;
 }
 
+static void irq_affinity_calc_sets(unsigned int affvecs,
+		struct irq_affinity *affd)
+{
+	/*
+	 * Simple invocations do not provide a calc_sets() callback. Install
+	 * the generic one.
+	 */
+	if (!affd->calc_sets)
+		affd->calc_sets = default_calc_sets;
+
+	/* Recalculate the sets */
+	affd->calc_sets(affd, affvecs);
+
+	WARN_ON_ONCE(affd->nr_sets > IRQ_AFFINITY_MAX_SETS);
+}
+
+/* Provide a chance to call ->calc_sets for legacy */
+void irq_affinity_calc_sets_legacy(struct irq_affinity *affd)
+{
+	if (!affd)
+		return;
+	irq_affinity_calc_sets(0, affd);
+}
+
 /**
  * irq_create_affinity_masks - Create affinity masks for multiqueue spreading
  * @nvecs:	The total number of vectors
@@ -429,17 +453,8 @@ irq_create_affinity_masks(unsigned int nvecs, struct irq_affinity *affd)
 	else
 		affvecs = 0;
 
-	/*
-	 * Simple invocations do not provide a calc_sets() callback. Install
-	 * the generic one.
-	 */
-	if (!affd->calc_sets)
-		affd->calc_sets = default_calc_sets;
-
-	/* Recalculate the sets */
-	affd->calc_sets(affd, affvecs);
-
-	if (WARN_ON_ONCE(affd->nr_sets > IRQ_AFFINITY_MAX_SETS))
+	irq_affinity_calc_sets(affvecs, affd);
+	if (affd->nr_sets > IRQ_AFFINITY_MAX_SETS)
 		return NULL;
 
 	/* Nothing to assign? */


Thanks, 
Ming