blk: honor isolcpus configuration#824
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The calculation of the upper limit for queues does not depend solely on the number of online CPUs; for example, the isolcpus kernel command-line option must also be considered. To account for this, the block layer provides a helper function to retrieve the maximum number of queues. Use it to set an appropriate upper queue number limit. This patch brings aacraid in line with the API migration initiated for other SCSI drivers in commit 94970cf ("scsi: use block layer helpers to calculate num of queues"). Signed-off-by: Daniel Wagner <wagi@kernel.org> Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com> Reviewed-by: Hannes Reinecke <hare@suse.de> [atomlin: Drop "Fixes:" tag; indicate alignment with other SCSI drivers] Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
The core scheduler recently transitioned to compiling SMP data structures unconditionally to reduce code complexity - see commit cac5cef ("sched/smp: Make SMP unconditional"). In alignment with this philosophy of reducing dual-path maintenance, this patch removes the #ifdef CONFIG_SMP guards and the dedicated !SMP fallback logic here. While the !SMP path provided a slightly simpler execution flow for uniprocessor kernels (avoiding SMP-specific overhead), maintaining these separate code paths adds unnecessary complexity and testing burden. Removing these guards simplifies the codebase by standardizing entirely on the SMP logic, which safely resolves to single-CPU operations on UP configurations. Signed-off-by: Daniel Wagner <wagi@kernel.org> Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com> Reviewed-by: Hannes Reinecke <hare@suse.de> [atomlin: Updated commit message to clarify !SMP removal context] Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
This commit introduces group_mask_cpus_evenly(), which allows callers to
distribute a specific CPU mask evenly across groups. It serves as a bounded
version of group_cpus_evenly().
While group_cpus_evenly() operates on the global cpu_possible_mask,
group_mask_cpus_evenly() confines the distribution strictly within the
boundaries of the caller-provided mask. It preserves the kernel's native
two-stage spreading logic-first prioritising CPUs that are physically
present (cpu_present_mask) to prevent I/O starvation, and then distributing
any remaining vectors to non-present CPUs to maintain hotplug safety.
Signed-off-by: Daniel Wagner <wagi@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
[atomlin:
- Added check for numgrps == 0
- Updated commit message to resolve typo
- Removed unused <linux/sched/isolation.h>
- Fix TOCTOU race by caching the provided mask
- Implemented two-stage grouping logic to prioritise physically
present CPUs, mirroring group_cpus_evenly()]
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Multiqueue drivers spread I/O queues across all CPUs for optimal performance. However, these drivers are not aware of CPU isolation requirements and will distribute queues without considering the isolcpus configuration. Introduce a new isolcpus mask that allows users to define which CPUs should have I/O queues assigned. This is similar to managed_irq, but intended for drivers that do not use the managed IRQ infrastructure Signed-off-by: Daniel Wagner <wagi@kernel.org> Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com> Reviewed-by: Hannes Reinecke <hare@suse.de> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Extend the capabilities of the generic CPU to hardware queue (hctx)
mapping code, so it maps houskeeping CPUs and isolated CPUs to the
hardware queues evenly.
A hctx is only operational when there is at least one online
housekeeping CPU assigned (aka active_hctx). Thus, check the final
mapping that there is no hctx which has only offline housekeeing CPU and
online isolated CPUs.
Example mapping result:
16 online CPUs
isolcpus=io_queue,2-3,6-7,12-13
Queue mapping:
hctx0: default 0 2
hctx1: default 1 3
hctx2: default 4 6
hctx3: default 5 7
hctx4: default 8 12
hctx5: default 9 13
hctx6: default 10
hctx7: default 11
hctx8: default 14
hctx9: default 15
IRQ mapping:
irq 42 affinity 0 effective 0 nvme0q0
irq 43 affinity 0 effective 0 nvme0q1
irq 44 affinity 1 effective 1 nvme0q2
irq 45 affinity 4 effective 4 nvme0q3
irq 46 affinity 5 effective 5 nvme0q4
irq 47 affinity 8 effective 8 nvme0q5
irq 48 affinity 9 effective 9 nvme0q6
irq 49 affinity 10 effective 10 nvme0q7
irq 50 affinity 11 effective 11 nvme0q8
irq 51 affinity 14 effective 14 nvme0q9
irq 52 affinity 15 effective 15 nvme0q10
A corner case is when the number of online CPUs and present CPUs
differ and the driver asks for less queues than online CPUs, e.g.
8 online CPUs, 16 possible CPUs
isolcpus=io_queue,2-3,6-7,12-13
virtio_blk.num_request_queues=2
Queue mapping:
hctx0: default 0 1 2 3 4 5 6 7 8 12 13
hctx1: default 9 10 11 14 15
IRQ mapping
irq 27 affinity 0 effective 0 virtio0-config
irq 28 affinity 0-1,4-5,8 effective 5 virtio0-req.0
irq 29 affinity 9-11,14-15 effective 0 virtio0-req.1
Noteworthy is that for the normal/default configuration (!isoclpus) the
mapping will change for systems which have non hyperthreading CPUs. The
main assignment loop will completely rely that group_mask_cpus_evenly to
do the right thing. The old code would distribute the CPUs linearly over
the hardware context:
queue mapping for /dev/nvme0n1
hctx0: default 0 8
hctx1: default 1 9
hctx2: default 2 10
hctx3: default 3 11
hctx4: default 4 12
hctx5: default 5 13
hctx6: default 6 14
hctx7: default 7 15
The assign each hardware context the map generated by the
group_mask_cpus_evenly function:
queue mapping for /dev/nvme0n1
hctx0: default 0 1
hctx1: default 2 3
hctx2: default 4 5
hctx3: default 6 7
hctx4: default 8 9
hctx5: default 10 11
hctx6: default 12 13
hctx7: default 14 15
In case of hyperthreading CPUs, the resulting map stays the same.
Signed-off-by: Daniel Wagner <wagi@kernel.org>
[atomlin:
- Updated blk_mq_validate() to use test_bit() for the new bitmap
- Replaced __free cleanups with traditional goto unwinding to align
with subsystem styling
- Updated blk_mq_map_fallback() to use qmap->queue_offset ensuring
secondary maps do not incorrectly route to the primary default map
- Added a bitmap_empty() check to prevent out-of-bounds CPU routing
when all mapped CPUs are offline
- Migrated active_hctx to a dynamically sized bitmap to fix an
out-of-bounds write when hardware queues exceed the system CPU
count
- Fixed absolute vs. relative hardware queue index mix-up in
blk_mq_map_queues() and validation checks
- Fixed typographical errors
- Reduced stack frame size of blk_mq_num_queues()
- Resolved a TOCTOU race against CPU hotplug events by snapshotting
cpu_online_mask to ensure mapping and validation phases agree
- Corrected a loop overwrite bug in blk_mq_map_queues() by iterating
directly over masks to prevent orphaned queues from being activated
- Restored topology-aware multi-queue fallback in
blk_mq_map_hw_queues() for devices lacking IRQ affinity hints
- Hardened isolation logic in blk_mq_map_hw_queues() to require online
housekeeping CPUs before marking a hardware queue as active
- Optimised active queue evaluations by short-circuiting redundant
checks once a valid CPU is found]
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
When isolcpus=io_queue is enabled and the last housekeeping CPU
for a given hctx goes offline, no CPU would be left to handle I/O.
To prevent I/O stalls, disallow offlining housekeeping CPUs that are
still serving isolated CPUs.
Signed-off-by: Daniel Wagner <wagi@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
[atomlin:
- Removed duplicate paragraph from commit message
- Allow offlining of non-housekeeping CPUs
- Fix logic flaw that prematurely rejected valid offline requests
- Iterated over cpu_online_mask and manually reverse-mapped CPUs to
correctly detect isolated CPUs, as blk_mq_map_swqueue()
intentionally prunes them from hctx->cpumask
- Prevented a TOCTOU NULL pointer dereference race against
concurrent device teardown by using READ_ONCE() to fetch the disk
pointer]
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
At present, the managed interrupt spreading algorithm distributes vectors
across all available CPUs within a given node or system. On systems
employing CPU isolation (e.g., "isolcpus=io_queue"), this behaviour
defeats the primary purpose of isolation by routing hardware interrupts
(such as NVMe completion queues) directly to isolated cores.
Update irq_create_affinity_masks() to respect the housekeeping CPU mask.
By passing the HK_TYPE_IO_QUEUE mask directly to the topological
distribution function (group_mask_cpus_evenly()), we ensure that managed
interrupts are kept strictly off isolated CPUs.
This patch additionally addresses the architectural constraints of
restricted vector distribution:
1. Vector Limits: Updated irq_calc_affinity_vectors() to bound the
maximum number of allocated vectors to the weight of the
housekeeping mask. This prevents drivers from wasting memory on
dead hardware queues that cannot be routed to isolated CPUs.
2. Multi-set Alignment: When isolation constraints result in fewer
available masks than requested vectors for a given set, the
remaining vector slots are padded with irq_default_affinity. The
loop correctly advances by the requested vector count (this_vecs)
to prevent shifting and corrupting the 1:1 hardware queue mapping
for subsequent sets.
3. Zero Overhead: The housekeeping mask is conditionally assigned via
a direct pointer, completely avoiding temporary mask allocations
(e.g., alloc_cpumask_var) and bitwise operations when CPU
isolation is disabled. This guarantees zero performance or memory
overhead for standard configurations.
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Author
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Upstream branch: 70eda68 |
The io_queue flag informs multiqueue device drivers where to place hardware queues. Document this new flag in the isolcpus command-line argument description. Signed-off-by: Daniel Wagner <wagi@kernel.org> Reviewed-by: Hannes Reinecke <hare@suse.de> [atomlin: Refined io_queue kernel parameter documentation] Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
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Pull request for series with
subject: blk: honor isolcpus configuration
version: 13
url: https://patchwork.kernel.org/project/linux-block/list/?series=1093842