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iommu/arm-smmu-v3: Shrink command/event/PRI queues in kdump kernel

PATCH Superseded Kiryl Shutsemau (Meta) 4 participants 6 messages Jul 2

Subsystems (2)
IOMMU Arm SMMU drivers

v2 of 3 Superseded Latest is v3 →

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Kiryl Shutsemau (Meta)

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#0 lore raw

PATCH v2 1/1 iommu/arm-smmu-v3: Shrink command/event/PRI queues in kdump kernel

The command, event and PRI queues are sized from the maxima the hardware advertises in IDR1, which can be several megabytes each. On systems with many SMMUv3 instances that cost is paid per instance and adds up to tens of megabytes of coherent DMA in the capture kernel.

A kdump capture kernel runs from a small crashkernel reservation and only has to drive the few devices used to save the dump, so deep queues serve no purpose. The queues carry invalidation commands and fault records, not DMA data, so dump throughput is unaffected; a shallower queue only bounds how many commands may be in flight before a sync, which does not matter for the capture kernel's small device count and modest I/O.

Clamp every queue to a single page when is_kdump_kernel() is true. Doing it in arm_smmu_init_one_queue() covers the command, event and PRI queues in one place. The command queue still holds at least one batch plus a sync (256 entries on a 4K-page kernel, well above CMDQ_BATCH_ENTRIES), so command batching keeps working.

Suggested-by Kyle McMartin <[email protected]>
Signed-off-by Kiryl Shutsemau (Meta) <[email protected]>
Reviewed-by Breno Leitao <[email protected]>

---
v2:
- Use min() instead of min_t(); both operands are u32 so the cast was redundant (Jason Gunthorpe, Breno Leitao).
- Add Reviewed-by from Breno.

 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c +14 -0
index e8d7dbe495f0..a4ec4a59e527 100644
@@ -4414,6 +4414,20 @@ int arm_smmu_init_one_queue(struct arm_smmu_device *smmu,
 {
 	size_t qsz;
 
+	/*
+	 * A kdump capture kernel runs from a small crashkernel reservation and
+	 * only has to drive the few devices used to save the dump, so there is
+	 * no point sizing the queues for the (multi-megabyte) maxima the
+	 * hardware advertises. Clamp each queue to a single page. ent_sz_shift
+	 * is the log2 of the entry size in bytes (dwords * 8).
+	 */
+	if (is_kdump_kernel()) {
+		u32 ent_sz_shift = ilog2(dwords) + 3;
+
+		q->llq.max_n_shift = min(q->llq.max_n_shift,
+					 PAGE_SHIFT - ent_sz_shift);
+	}
+
 	do {
 		qsz = ((1 << q->llq.max_n_shift) * dwords) << 3;
 		q->base = dmam_alloc_coherent(smmu->dev, qsz, &q->base_dma,
Signature · 1 line
2.54.0
Reviewed-by on v2 1/1
#1 lore raw
Quoted 1 line · Kiryl Shutsemau (Meta)

The command, event and PRI queues are sized from the maxima the hardware

A minor note here is PRI & EVT queues are disabled for the kdump kernel (see arm_smmu_device_reset). We could just mention all SMMU queues are sized [...] in the commit message.

Quoted 17 lines

advertises in IDR1, which can be several megabytes each. On systems with many SMMUv3 instances that cost is paid per instance and adds up to tens of megabytes of coherent DMA in the capture kernel.

A kdump capture kernel runs from a small crashkernel reservation and only has to drive the few devices used to save the dump, so deep queues serve no purpose. The queues carry invalidation commands and fault records, not DMA data, so dump throughput is unaffected; a shallower queue only bounds how many commands may be in flight before a sync, which does not matter for the capture kernel's small device count and modest I/O.

Clamp every queue to a single page when is_kdump_kernel() is true. Doing it in arm_smmu_init_one_queue() covers the command, event and PRI queues in one place. The command queue still holds at least one batch plus a sync (256 entries on a 4K-page kernel, well above CMDQ_BATCH_ENTRIES), so command batching keeps working.

Quoted 4 lines
Suggested-by Kyle McMartin <[email protected]>
Signed-off-by Kiryl Shutsemau (Meta) <[email protected]>
Reviewed-by Breno Leitao <[email protected]>

---

Apart from that.

Reviewed-by Pranjal Shrivastava <[email protected]>

Thanks,
Praan

author
#2 lore raw
Quoted 6 lines · Pranjal Shrivastava
Quoted 1 line · Kiryl Shutsemau (Meta)

The command, event and PRI queues are sized from the maxima the hardware

A minor note here is PRI & EVT queues are disabled for the kdump kernel (see arm_smmu_device_reset). We could just mention all SMMU queues are sized [...] in the commit message.

Fair enough.

Here's updated commit message (I will send v3 in few days, if no new feedback):

Subject: [PATCH v3] iommu/arm-smmu-v3: Shrink command/event/PRI queues in
kdump kernel

All SMMU queues are sized from the maxima the hardware advertises in IDR1, which can be several megabytes each, and are allocated at probe. The kdump kernel already disables the event and PRI queues (arm_smmu_device_reset() drops CR0_EVTQEN/CR0_PRIQEN) but still allocates them at full size. On systems with many SMMUv3 instances that cost is paid per instance and adds up to tens of megabytes of coherent DMA in the capture kernel.

A kdump capture kernel runs from a small crashkernel reservation and only has to drive the few devices used to save the dump, so deep queues serve no purpose. The queues are not on the DMA data path, so dump throughput is unaffected; a shallower command queue only bounds how many commands may be in flight before a sync, which does not matter for the capture kernel's small device count and modest I/O.

Clamp every queue to a single page when is_kdump_kernel() is true. Doing it in arm_smmu_init_one_queue() covers the command, event and PRI queues in one place. The command queue still holds at least one batch plus a sync (256 entries on a 4K-page kernel, well above CMDQ_BATCH_ENTRIES), so command batching keeps working.

Signature · 1 line
  Kiryl Shutsemau / Kirill A. Shutemov
#3 lore raw
Quoted 30 lines · Kiryl Shutsemau
Quoted 6 lines · Pranjal Shrivastava
Quoted 1 line · Kiryl Shutsemau (Meta)

The command, event and PRI queues are sized from the maxima the hardware

A minor note here is PRI & EVT queues are disabled for the kdump kernel (see arm_smmu_device_reset). We could just mention all SMMU queues are sized [...] in the commit message.

Fair enough.

Here's updated commit message (I will send v3 in few days, if no new feedback):

Subject: [PATCH v3] iommu/arm-smmu-v3: Shrink command/event/PRI queues in
kdump kernel

All SMMU queues are sized from the maxima the hardware advertises in IDR1, which can be several megabytes each, and are allocated at probe. The kdump kernel already disables the event and PRI queues (arm_smmu_device_reset() drops CR0_EVTQEN/CR0_PRIQEN) but still allocates them at full size. On systems with many SMMUv3 instances that cost is paid per instance and adds up to tens of megabytes of coherent DMA in the capture kernel.

A kdump capture kernel runs from a small crashkernel reservation and only has to drive the few devices used to save the dump, so deep queues serve no purpose. The queues are not on the DMA data path, so dump throughput is unaffected; a shallower command queue only bounds how many commands may be in flight before a sync, which does not matter for the capture kernel's small device count and modest I/O.

Quoted 6 lines

Clamp every queue to a single page when is_kdump_kernel() is true. Doing it in arm_smmu_init_one_queue() covers the command, event and PRI queues in one place. The command queue still holds at least one batch plus a sync (256 entries on a 4K-page kernel, well above CMDQ_BATCH_ENTRIES), so command batching keeps working.

Looks good. Thanks!

Praan

Reviewed-by on v2 1/1
#4 lore raw
In reply to Kiryl Shutsemau (Meta)
Quoted 21 lines · Kiryl Shutsemau (Meta)

The command, event and PRI queues are sized from the maxima the hardware advertises in IDR1, which can be several megabytes each. On systems with many SMMUv3 instances that cost is paid per instance and adds up to tens of megabytes of coherent DMA in the capture kernel.

A kdump capture kernel runs from a small crashkernel reservation and only has to drive the few devices used to save the dump, so deep queues serve no purpose. The queues carry invalidation commands and fault records, not DMA data, so dump throughput is unaffected; a shallower queue only bounds how many commands may be in flight before a sync, which does not matter for the capture kernel's small device count and modest I/O.

Clamp every queue to a single page when is_kdump_kernel() is true. Doing it in arm_smmu_init_one_queue() covers the command, event and PRI queues in one place. The command queue still holds at least one batch plus a sync (256 entries on a 4K-page kernel, well above CMDQ_BATCH_ENTRIES), so command batching keeps working.

Suggested-by Kyle McMartin <[email protected]>
Signed-off-by Kiryl Shutsemau (Meta) <[email protected]>
Reviewed-by Breno Leitao <[email protected]>
Quoted 8 lines

---
v2:
- Use min() instead of min_t(); both operands are u32 so the cast was redundant (Jason Gunthorpe, Breno Leitao).
- Add Reviewed-by from Breno.

 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
Reviewed-by Jason Gunthorpe <[email protected]>

Jason

Reviewed-by Tested-by on v2 1/1
#5 lore raw
In reply to Kiryl Shutsemau (Meta)
Quoted 18 lines · Kiryl Shutsemau (Meta)

The command, event and PRI queues are sized from the maxima the hardware advertises in IDR1, which can be several megabytes each. On systems with many SMMUv3 instances that cost is paid per instance and adds up to tens of megabytes of coherent DMA in the capture kernel.

A kdump capture kernel runs from a small crashkernel reservation and only has to drive the few devices used to save the dump, so deep queues serve no purpose. The queues carry invalidation commands and fault records, not DMA data, so dump throughput is unaffected; a shallower queue only bounds how many commands may be in flight before a sync, which does not matter for the capture kernel's small device count and modest I/O.

Clamp every queue to a single page when is_kdump_kernel() is true. Doing it in arm_smmu_init_one_queue() covers the command, event and PRI queues in one place. The command queue still holds at least one batch plus a sync (256 entries on a 4K-page kernel, well above CMDQ_BATCH_ENTRIES), so command batching keeps working.

Quoted 3 lines
Suggested-by Kyle McMartin <[email protected]>
Signed-off-by Kiryl Shutsemau (Meta) <[email protected]>
Reviewed-by Breno Leitao <[email protected]>
Reviewed-by Nicolin Chen <[email protected]>
Tested-by Nicolin Chen <[email protected]>