[PATCH] Revert "perf/aux: Allocate non-contiguous AUX pages by default"

[Leo Yan <leo.yan@arm.com>]

This reverts commit 18049c8cff9cc89daadc4df6975f7d9069638926.

The original commit claimed that contiguous AUX pages unnecessarily
exacerbate fragmentation for SPE, TRBE and ETR. This is not generally
valid: larger allocations can keep pages with the same lifetime
together, while order-0 allocations can scatter them across large
blocks and prevent free pages from coalescing [1]. The follow-up
discussion [2] has not established how forcing order-0 allocations
addresses the reported problem.

TRBE and SPE write to virtually addressed buffers. The TRBE L1 TLB
has one entry on Neoverse V2 [3] and two on Neoverse V3 [4]. Small
mappings increase translation pressure, and TTW stalls can contribute
to FIFO overflow and trace loss on TRBE.

Larger allocations provide contiguous, aligned chunks for PMD or
contiguous-PTE mappings with the proposed vmap() support [5].
With the proposed vmap() support already applied, the tests in [2]
showed that preferring larger AUX allocations reduced L2 data TLB
refills by 99.68% in the TRBE test and 34.74% in the SPE test.

Restore the previous policy of trying higher-order allocations first.
The allocator already uses __GFP_NORETRY and falls back to smaller
orders, including order 0, when larger allocations fail.

Remove PERF_PMU_CAP_AUX_PREFER_LARGE, since the restored default
provides that preference. Intel PT retains its existing allocation
behaviour, with no functional change.

[1] https://lore.kernel.org/all/aPjrRkjiIt6HmXmT@casper.infradead.org/
[2] https://lore.kernel.org/linux-arm-kernel/20260810-perf_aux_trace_large_granule-v1-0-03306c9339e3@arm.com/
[3] https://documentation-service.arm.com/static/633fe1ec4c59b30b517730b8#page=52
[4] https://documentation-service.arm.com/static/67361bd2c7fc0d1f211da382#page=54
[5] https://lore.kernel.org/linux-mm/20260715120813.3609949-1-jiangwen6@xiaomi.com/

Signed-off-by: Leo Yan <leo.yan@arm.com>
---
 arch/x86/events/intel/pt.c  |  2 --
 include/linux/perf_event.h  |  5 ++---
 kernel/events/ring_buffer.c | 29 +++++++++--------------------
 3 files changed, 11 insertions(+), 25 deletions(-)

diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
index 49349afee6119fb6fad815c847cb3cd9daa97604..3782af40f0832a874a7f0b60961d6652513de241 100644
--- a/arch/x86/events/intel/pt.c
+++ b/arch/x86/events/intel/pt.c
@@ -1896,8 +1896,6 @@ static __init int pt_init(void)
 
 	if (!intel_pt_validate_hw_cap(PT_CAP_topa_multiple_entries))
 		pt_pmu.pmu.capabilities = PERF_PMU_CAP_AUX_NO_SG;
-	else
-		pt_pmu.pmu.capabilities = PERF_PMU_CAP_AUX_PREFER_LARGE;
 
 	pt_pmu.pmu.capabilities		|= PERF_PMU_CAP_EXCLUSIVE |
 					   PERF_PMU_CAP_ITRACE |
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index eb3f7bab498ca9bee9ce7193744943b24ab2e0c5..b8d8b005f5c8a7c1def7cad02230bf9ffb876bd6 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -304,9 +304,8 @@ struct perf_event_pmu_context;
 #define PERF_PMU_CAP_AUX_OUTPUT		0x0080
 #define PERF_PMU_CAP_EXTENDED_HW_TYPE	0x0100
 #define PERF_PMU_CAP_AUX_PAUSE		0x0200
-#define PERF_PMU_CAP_AUX_PREFER_LARGE	0x0400
-#define PERF_PMU_CAP_MEDIATED_VPMU	0x0800
-#define PERF_PMU_CAP_SIMD_REGS		0x1000
+#define PERF_PMU_CAP_MEDIATED_VPMU	0x0400
+#define PERF_PMU_CAP_SIMD_REGS		0x0800
 
 /**
  * pmu::scope
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 64d4dbdc6c071b72b670dd044616f52981206ae8..95f4fbdfd2764de5b72d901d778c46302f525fad 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -684,15 +684,7 @@ int rb_alloc_aux(struct perf_buffer *rb, struct perf_event *event,
 {
 	bool overwrite = !(flags & RING_BUFFER_WRITABLE);
 	int node = (event->cpu == -1) ? -1 : cpu_to_node(event->cpu);
-	bool use_contiguous_pages = event->pmu->capabilities & (
-		PERF_PMU_CAP_AUX_NO_SG | PERF_PMU_CAP_AUX_PREFER_LARGE);
-	/*
-	 * Initialize max_order to 0 for page allocation. This allocates single
-	 * pages to minimize memory fragmentation. This is overridden if the
-	 * PMU needs or prefers contiguous pages (use_contiguous_pages = true).
-	 */
-	int max_order = 0;
-	int ret = -ENOMEM;
+	int ret = -ENOMEM, max_order;
 
 	if (!has_aux(event))
 		return -EOPNOTSUPP;
@@ -702,8 +694,8 @@ int rb_alloc_aux(struct perf_buffer *rb, struct perf_event *event,
 
 	if (!overwrite) {
 		/*
-		 * Watermark defaults to half the buffer, to aid PMU drivers
-		 * in double buffering.
+		 * Watermark defaults to half the buffer, and so does the
+		 * max_order, to aid PMU drivers in double buffering.
 		 */
 		if (!watermark)
 			watermark = min_t(unsigned long,
@@ -711,19 +703,16 @@ int rb_alloc_aux(struct perf_buffer *rb, struct perf_event *event,
 					  (unsigned long)nr_pages << (PAGE_SHIFT - 1));
 
 		/*
-		 * If using contiguous pages, use aux_watermark as the basis
-		 * for chunking to help PMU drivers honor the watermark.
+		 * Use aux_watermark as the basis for chunking to
+		 * help PMU drivers honor the watermark.
 		 */
-		if (use_contiguous_pages)
-			max_order = get_order(watermark);
+		max_order = get_order(watermark);
 	} else {
 		/*
-		 * If using contiguous pages, we need to start with the
-		 * max_order that fits in nr_pages, not the other way around,
-		 * hence ilog2() and not get_order.
+		 * We need to start with the max_order that fits in nr_pages,
+		 * not the other way around, hence ilog2() and not get_order.
 		 */
-		if (use_contiguous_pages)
-			max_order = ilog2(nr_pages);
+		max_order = ilog2(nr_pages);
 		watermark = 0;
 	}
 

---
base-commit: 21a7f44fd0765fcc5194130b28767786eba1d1e7
change-id: 20261025-perf_revert_prefer_large_alloc-a0177d6379f6

Best regards,
-- 
Leo Yan <leo.yan@arm.com>