[PATCH v3] perf: Allocate non-contiguous AUX pages by default

[Yabin Cui]

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@xxxxxxxxxx>
---
Changes since v2:
Let NO_SG imply PREFER_LARGE. So PMUs don't need to set both flags.
Then the only place needing PREFER_LARGE is intel/pt.c.

Changes since v1:
In v1, default is preferring contiguous pages, and add a flag to
allocate non-contiguous pages. In v2, default is allocating
non-contiguous pages, and add a flag to prefer contiguous pages.

v1 patchset:
perf,coresight: Reduce fragmentation with non-contiguous AUX pages for
cs_etm

 arch/x86/events/intel/pt.c  |  2 ++
 include/linux/perf_event.h  |  1 +
 kernel/events/ring_buffer.c | 20 +++++++++++++-------
 3 files changed, 16 insertions(+), 7 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 0069ba6866a4..56d77348c511 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -301,6 +301,7 @@ 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..4d2f1c95673e 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -679,7 +679,7 @@ 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);
-	int ret = -ENOMEM, max_order;
+	int ret = -ENOMEM, max_order = 0;
 
 	if (!has_aux(event))
 		return -EOPNOTSUPP;
@@ -689,8 +689,8 @@ 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 +698,22 @@ 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
+		 * For PMUs that need or prefer large contiguous buffers,
+		 * use aux_watermark as the basis for chunking to
 		 * help PMU drivers honor the watermark.
 		 */
-		max_order = get_order(watermark);
+		if (event->pmu->capabilities & (PERF_PMU_CAP_AUX_NO_SG |
+						PERF_PMU_CAP_AUX_PREFER_LARGE))
+			max_order = get_order(watermark);
 	} else {
 		/*
-		 * We need to start with the max_order that fits in nr_pages,
+		 * For PMUs that need or prefer large contiguous buffers,
+		 * 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 (event->pmu->capabilities & (PERF_PMU_CAP_AUX_NO_SG |
+						PERF_PMU_CAP_AUX_PREFER_LARGE))
+			max_order = ilog2(nr_pages);
 		watermark = 0;
 	}
 
-- 
2.49.0.967.g6a0df3ecc3-goog