| Message ID | 20230726094027.535126-2-ming.lei@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | blk-mq: fix wrong queue mapping for kdump kernel | expand |
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@redhat.com> > --- > 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
On Wed, Jul 26, 2023 at 05:36:34PM +0100, John Garry wrote: > 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." OK, looks I should have mentioned "the only online CPU may not be 1 for maxcpus=1" in cover letter. > > 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 ... cpu0 mapping isn't correct, as I mentioned that 'maxcpus=1' doesn't mean the only online cpu is cpu0, such as, on ppc64, the only online cpu could be selected at random during booting. > > > > > Signed-off-by: Ming Lei <ming.lei@redhat.com> > > --- > > 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); > } In theory, this way should be ideal approach, but it needs all MQ drivers to get converted with blk_mq_max_nr_hw_queues(). However, it shouldn't be one issue for non-managed irq given non-managed irq can be migrated to the only online cpu. Thanks, Ming
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; +} +EXPORT_SYMBOL_GPL(blk_mq_max_nr_hw_queues); + int blk_mq_freeze_queue_wait_timeout(struct request_queue *q, unsigned long timeout) { diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h index 495ca198775f..4c0cfd1f9e52 100644 --- a/include/linux/blk-mq.h +++ b/include/linux/blk-mq.h @@ -711,6 +711,7 @@ int blk_mq_alloc_sq_tag_set(struct blk_mq_tag_set *set, const struct blk_mq_ops *ops, unsigned int queue_depth, unsigned int set_flags); void blk_mq_free_tag_set(struct blk_mq_tag_set *set); +unsigned int blk_mq_max_nr_hw_queues(void); void blk_mq_free_request(struct request *rq); int blk_rq_poll(struct request *rq, struct io_comp_batch *iob,
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, 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. Signed-off-by: Ming Lei <ming.lei@redhat.com> --- block/blk-mq.c | 16 ++++++++++++++++ include/linux/blk-mq.h | 1 + 2 files changed, 17 insertions(+)