On 26/07/2023 10:40, Ming Lei wrote:
Hi Ming,
blk_mq_alloc_tag_set() may override set->nr_hw_queues as 1 in case of kdump
kernel. This way causes trouble for driver, because blk-mq and driver see
different queue mapping. Especially the only online CPU may not be 1 for
kdump kernel, in which 'maxcpus=1' is passed from kernel command line,
"the only online CPU may not be 1 for kdump kernel, in which 'maxcpus=1'
..." - this seems inconsistent with the cover letter, where we have
"'maxcpus=1' just bring up one single cpu core during booting."
then driver may map hctx0 into one inactive real hw queue which cpu
affinity is 0(offline).
The issue exists on all drivers which use managed irq and support
multiple hw queue.
Prepare for fixing this kind of issue by applying the added helper, so
driver can take blk-mq max nr_hw_queues knowledge into account when
calculating io queues.
Could you alternatively solve in blk_mq_pci_map_queues() by fixing the
mappings in case of kdump to have all per-CPU mappings point at the HW
queue associated with cpu0 (which I assume would be active)? It ain't
pretty ...
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
block/blk-mq.c | 16 ++++++++++++++++
include/linux/blk-mq.h | 1 +
2 files changed, 17 insertions(+)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index b04ff6f56926..617d6f849a7b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -140,6 +140,22 @@ void blk_mq_freeze_queue_wait(struct request_queue *q)
}
EXPORT_SYMBOL_GPL(blk_mq_freeze_queue_wait);
+/*
+ * Return the max supported nr_hw_queues for each hw queue type
+ *
+ * blk_mq_alloc_tag_set() may change nr_hw_queues for kdump kernel, so
+ * driver has to take blk-mq max supported nr_hw_queues into account
+ * when figuring out nr_hw_queues from hardware info, for avoiding
+ * inconsistency between driver and blk-mq.
+ */
+unsigned int blk_mq_max_nr_hw_queues(void)
+{
+ if (is_kdump_kernel())
+ return 1;
+ return nr_cpu_ids;
+}
We rely on the driver having set->nr_hw_queues == 1 for kdump, right?
If so, how about enforcing it then, like:
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4426,7 +4426,8 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
* 64 tags to prevent using too much memory.
*/
if (is_kdump_kernel()) {
- set->nr_hw_queues = 1;
+ if (set->nr_hw_queues != 1)
+ return -EINVAL;
set->nr_maps = 1;
set->queue_depth = min(64U, set->queue_depth);
}
Thanks,
John
