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

[Dev Jain]

On 25/10/26 9:54 pm, Leo Yan wrote:
> 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@xxxxxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/linux-arm-kernel/20260810-perf_aux_trace_large_granule-v1-0-03306c9339e3@xxxxxxx/
> [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@xxxxxxxxxx/
> 
> Signed-off-by: Leo Yan <leo.yan@xxxxxxx>
> ---

Reviewed-by: Dev Jain <dev.jain@xxxxxxx>


>  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,