[PATCH 4.19 03/78] genirq/matrix: Improve target CPU selection for managed interrupts.

From: Greg Kroah-Hartman
Date: Mon Mar 04 2019 - 03:28:30 EST

4.19-stable review patch. If anyone has any objections, please let me know.


[ Upstream commit e8da8794a7fd9eef1ec9a07f0d4897c68581c72b ]

On large systems with multiple devices of the same class (e.g. NVMe disks,
using managed interrupts), the kernel can affinitize these interrupts to a
small subset of CPUs instead of spreading them out evenly.

irq_matrix_alloc_managed() tries to select the CPU in the supplied cpumask
of possible target CPUs which has the lowest number of interrupt vectors

This is done by searching the CPU with the highest number of available
vectors. While this is correct for non-managed CPUs it can select the wrong
CPU for managed interrupts. Under certain constellations this results in
affinitizing the managed interrupts of several devices to a single CPU in
a set.

The book keeping of available vectors works the following way:

1) Non-managed interrupts:

available is decremented when the interrupt is actually requested by
the device driver and a vector is assigned. It's incremented when the
interrupt and the vector are freed.

2) Managed interrupts:

Managed interrupts guarantee vector reservation when the MSI/MSI-X
functionality of a device is enabled, which is achieved by reserving
vectors in the bitmaps of the possible target CPUs. This reservation
decrements the available count on each possible target CPU.

When the interrupt is requested by the device driver then a vector is
allocated from the reserved region. The operation is reversed when the
interrupt is freed by the device driver. Neither of these operations
affect the available count.

The reservation persist up to the point where the MSI/MSI-X
functionality is disabled and only this operation increments the
available count again.

For non-managed interrupts the available count is the correct selection
criterion because the guaranteed reservations need to be taken into
account. Using the allocated counter could lead to a failing allocation in
the following situation (total vector space of 10 assumed):

available: 2 0
allocated: 5 3 <--- CPU1 is selected, but available space = 0
managed reserved: 3 7

while available yields the correct result.

For managed interrupts the available count is not the appropriate
selection criterion because as explained above the available count is not
affected by the actual vector allocation.

The following example illustrates that. Total vector space of 10
assumed. The starting point is:

available: 5 4
allocated: 2 3
managed reserved: 3 3

Allocating vectors for three non-managed interrupts will result in
affinitizing the first two to CPU0 and the third one to CPU1 because the
available count is adjusted with each allocation:

available: 5 4 <- Select CPU0 for 1st allocation
--> allocated: 3 3

available: 4 4 <- Select CPU0 for 2nd allocation
--> allocated: 4 3

available: 3 4 <- Select CPU1 for 3rd allocation
--> allocated: 4 4

But the allocation of three managed interrupts starting from the same
point will affinitize all of them to CPU0 because the available count is
not affected by the allocation (see above). So the end result is:

available: 5 4
allocated: 5 3

Introduce a "managed_allocated" field in struct cpumap to track the vector
allocation for managed interrupts separately. Use this information to
select the target CPU when a vector is allocated for a managed interrupt,
which results in more evenly distributed vector assignments. The above
example results in the following allocations:

managed_allocated: 0 0 <- Select CPU0 for 1st allocation
--> allocated: 3 3

managed_allocated: 1 0 <- Select CPU1 for 2nd allocation
--> allocated: 3 4

managed_allocated: 1 1 <- Select CPU0 for 3rd allocation
--> allocated: 4 4

The allocation of non-managed interrupts is not affected by this change and
is still evaluating the available count.

The overall distribution of interrupt vectors for both types of interrupts
might still not be perfectly even depending on the number of non-managed
and managed interrupts in a system, but due to the reservation guarantee
for managed interrupts this cannot be avoided.

Expose the new field in debugfs as well.

[ tglx: Clarified the background of the problem in the changelog and
described it independent of NVME ]

Signed-off-by: Long Li <longli@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Michael Kelley <mikelley@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20181106040000.27316-1-longli@xxxxxxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
kernel/irq/matrix.c | 34 ++++++++++++++++++++++++++++++----
1 file changed, 30 insertions(+), 4 deletions(-)

--- a/kernel/irq/matrix.c
+++ b/kernel/irq/matrix.c
@@ -14,6 +14,7 @@ struct cpumap {
unsigned int available;
unsigned int allocated;
unsigned int managed;
+ unsigned int managed_allocated;
bool initialized;
bool online;
unsigned long alloc_map[IRQ_MATRIX_SIZE];
@@ -145,6 +146,27 @@ static unsigned int matrix_find_best_cpu
return best_cpu;

+/* Find the best CPU which has the lowest number of managed IRQs allocated */
+static unsigned int matrix_find_best_cpu_managed(struct irq_matrix *m,
+ const struct cpumask *msk)
+ unsigned int cpu, best_cpu, allocated = UINT_MAX;
+ struct cpumap *cm;
+ best_cpu = UINT_MAX;
+ for_each_cpu(cpu, msk) {
+ cm = per_cpu_ptr(m->maps, cpu);
+ if (!cm->online || cm->managed_allocated > allocated)
+ continue;
+ best_cpu = cpu;
+ allocated = cm->managed_allocated;
+ }
+ return best_cpu;
* irq_matrix_assign_system - Assign system wide entry in the matrix
* @m: Matrix pointer
@@ -269,7 +291,7 @@ int irq_matrix_alloc_managed(struct irq_
if (cpumask_empty(msk))
return -EINVAL;

- cpu = matrix_find_best_cpu(m, msk);
+ cpu = matrix_find_best_cpu_managed(m, msk);
if (cpu == UINT_MAX)
return -ENOSPC;

@@ -282,6 +304,7 @@ int irq_matrix_alloc_managed(struct irq_
return -ENOSPC;
set_bit(bit, cm->alloc_map);
+ cm->managed_allocated++;
*mapped_cpu = cpu;
trace_irq_matrix_alloc_managed(bit, cpu, m, cm);
@@ -395,6 +418,8 @@ void irq_matrix_free(struct irq_matrix *

clear_bit(bit, cm->alloc_map);
+ if(managed)
+ cm->managed_allocated--;

if (cm->online)
@@ -464,13 +489,14 @@ void irq_matrix_debug_show(struct seq_fi
seq_printf(sf, "Total allocated: %6u\n", m->total_allocated);
seq_printf(sf, "System: %u: %*pbl\n", nsys, m->matrix_bits,
- seq_printf(sf, "%*s| CPU | avl | man | act | vectors\n", ind, " ");
+ seq_printf(sf, "%*s| CPU | avl | man | mac | act | vectors\n", ind, " ");
for_each_online_cpu(cpu) {
struct cpumap *cm = per_cpu_ptr(m->maps, cpu);

- seq_printf(sf, "%*s %4d %4u %4u %4u %*pbl\n", ind, " ",
- cpu, cm->available, cm->managed, cm->allocated,
+ seq_printf(sf, "%*s %4d %4u %4u %4u %4u %*pbl\n", ind, " ",
+ cpu, cm->available, cm->managed,
+ cm->managed_allocated, cm->allocated,
m->matrix_bits, cm->alloc_map);