perf/aux: Allocate non-contiguous AUX pages by default

perf always allocates contiguous AUX pages based on aux_watermark.
However, this contiguous allocation doesn't benefit all PMUs. For
instance, ARM SPE and TRBE operate with virtual pages, and Coresight
ETR allocates a separate buffer. For these PMUs, allocating contiguous
AUX pages unnecessarily exacerbates memory fragmentation. This
fragmentation can prevent their use on long-running devices.

This patch modifies the perf driver to be memory-friendly by default,
by allocating non-contiguous AUX pages. For PMUs requiring contiguous
pages (Intel BTS and some Intel PT), the existing
PERF_PMU_CAP_AUX_NO_SG capability can be used. For PMUs that don't
require but can benefit from contiguous pages (some Intel PT), a new
capability, PERF_PMU_CAP_AUX_PREFER_LARGE, is added to maintain their
existing behavior.

Signed-off-by: Yabin Cui <yabinc@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: James Clark <james.clark@linaro.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20250508232642.148767-1-yabinc@google.com

3 files changed, 23 insertions, 9 deletions

diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
index fa37565f6418..25ead919fc48 100644
--- a/arch/x86/events/intel/pt.c
+++ b/arch/x86/events/intel/pt.c
@@ -1863,6 +1863,8 @@ 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 947ad12dfdbe..a96c00e2ceca 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -303,6 +303,7 @@ 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
 
 /**
  * pmu::scope
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 5130b119d0ae..d2aef87c7e9f 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -679,7 +679,15 @@ 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);
-	int ret = -ENOMEM, max_order;
+	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;
 
 	if (!has_aux(event))
 		return -EOPNOTSUPP;
@@ -689,8 +697,8 @@ int rb_alloc_aux(struct perf_buffer *rb, struct perf_event *event,
 
 	if (!overwrite) {
 		/*
-		 * Watermark defaults to half the buffer, and so does the
-		 * max_order, to aid PMU drivers in double buffering.
+		 * Watermark defaults to half the buffer, to aid PMU drivers
+		 * in double buffering.
 		 */
 		if (!watermark)
 			watermark = min_t(unsigned long,
@@ -698,16 +706,19 @@ int rb_alloc_aux(struct perf_buffer *rb, struct perf_event *event,
 					  (unsigned long)nr_pages << (PAGE_SHIFT - 1));
 
 		/*
-		 * Use aux_watermark as the basis for chunking to
-		 * help PMU drivers honor the watermark.
+		 * If using contiguous pages, use aux_watermark as the basis
+		 * for chunking to help PMU drivers honor the watermark.
 		 */
-		max_order = get_order(watermark);
+		if (use_contiguous_pages)
+			max_order = get_order(watermark);
 	} else {
 		/*
-		 * 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 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.
 		 */
-		max_order = ilog2(nr_pages);
+		if (use_contiguous_pages)
+			max_order = ilog2(nr_pages);
 		watermark = 0;
 	}