|author||Peter Xu <email@example.com>||2020-03-12 16:58:30 -0400|
|committer||Thomas Gleixner <firstname.lastname@example.org>||2020-03-13 15:29:26 +0100|
x86/vector: Remove warning on managed interrupt migration
The vector management code assumes that managed interrupts cannot be migrated away from an online CPU. free_moved_vector() has a WARN_ON_ONCE() which triggers when a managed interrupt vector association on a online CPU is cleared. The CPU offline code uses a different mechanism which cannot trigger this. This assumption is not longer correct because the new CPU isolation feature which affects the placement of managed interrupts must be able to move a managed interrupt away from an online CPU. There are two reasons why this can happen: 1) When the interrupt is activated the affinity mask which was established in irq_create_affinity_masks() is handed in to the vector allocation code. This mask contains all CPUs to which the interrupt can be made affine to, but this does not take the CPU isolation 'managed_irq' mask into account. When the interrupt is finally requested by the device driver then the affinity is checked again and the CPU isolation 'managed_irq' mask is taken into account, which moves the interrupt to a non-isolated CPU if possible. 2) The interrupt can be affine to an isolated CPU because the non-isolated CPUs in the calculated affinity mask are not online. Once a non-isolated CPU which is in the mask comes online the interrupt is migrated to this non-isolated CPU In both cases the regular online migration mechanism is used which triggers the WARN_ON_ONCE() in free_moved_vector(). Case #1 could have been addressed by taking the isolation mask into account, but that would require a massive code change in the activation logic and the eventual migration event was accepted as a reasonable tradeoff when the isolation feature was developed. But even if #1 would be addressed, #2 would still trigger it. Of course the warning in free_moved_vector() was overlooked at that time and the above two cases which have been discussed during patch review have obviously never been tested before the final submission. So keep it simple and remove the warning. [ tglx: Rewrote changelog and added a comment to free_moved_vector() ] Fixes: 11ea68f553e2 ("genirq, sched/isolation: Isolate from handling managed interrupts") Signed-off-by: Peter Xu <email@example.com> Signed-off-by: Thomas Gleixner <firstname.lastname@example.org> Reviewed-by: Ming Lei <email@example.com> Link: https://firstname.lastname@example.org
1 files changed, 8 insertions, 6 deletions
diff --git a/arch/x86/kernel/apic/vector.c b/arch/x86/kernel/apic/vector.c
index 2c5676b0a6e7..48293d15f1e1 100644
@@ -838,13 +838,15 @@ static void free_moved_vector(struct apic_chip_data *apicd)
bool managed = apicd->is_managed;
- * This should never happen. Managed interrupts are not
- * migrated except on CPU down, which does not involve the
- * cleanup vector. But try to keep the accounting correct
- * nevertheless.
+ * Managed interrupts are usually not migrated away
+ * from an online CPU, but CPU isolation 'managed_irq'
+ * can make that happen.
+ * 1) Activation does not take the isolation into account
+ * to keep the code simple
+ * 2) Migration away from an isolated CPU can happen when
+ * a non-isolated CPU which is in the calculated
+ * affinity mask comes online.
trace_vector_free_moved(apicd->irq, cpu, vector, managed);
irq_matrix_free(vector_matrix, cpu, vector, managed);
per_cpu(vector_irq, cpu)[vector] = VECTOR_UNUSED;