On 07/05/2025 7:13 pm, Yabin Cui wrote:
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
Changes since v3: Add comments and a local variable to explain max_order value changes in rb_alloc_aux().
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 | 33 ++++++++++++++++++++++++--------- 3 files changed, 27 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 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..69c90ea1b79a 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -679,7 +679,19 @@ 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;
- /*
* True if the PMU needs a contiguous AUX buffer (CAP_AUX_NO_SG) or* prefers large contiguous pages (CAP_AUX_PREFER_LARGE).*/- bool use_contiguous_pages = event->pmu->capabilities & (> +
PERF_PMU_CAP_AUX_NO_SG | PERF_PMU_CAP_AUX_PREFER_LARGE);
Reviewed-by: James Clark james.clark@linaro.org
Minor nit: this comment is a bit verbose IMO, and it's only describing what rather than why. But the other one is ok.
- /*
* Initialize max_order to 0 for page allocation. This allocates single* pages to minimize memory fragmentation. This is overriden if* use_contiguous_pages is true.*/- int max_order = 0;
- int ret = -ENOMEM;
if (!has_aux(event)) return -EOPNOTSUPP; @@ -689,8 +701,8 @@ 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.@@ -698,16 +710,19 @@ 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);
* 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);
-
James Clark