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Thu, 08 May 2025 02:02:14 -0700 (PDT) Message-ID: Date: Thu, 8 May 2025 10:02:12 +0100 Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] perf: Allocate non-contiguous AUX pages by default To: Yabin Cui Cc: coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Suzuki K Poulose , Mike Leach , Alexander Shishkin , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Jiri Olsa , Ian Rogers , Adrian Hunter , Liang Kan , Thomas Gleixner , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Anshuman Khandual References: <20250507181346.1892103-1-yabinc@google.com> Content-Language: en-US From: James Clark In-Reply-To: <20250507181346.1892103-1-yabinc@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 > --- > 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 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. > */ > if (!watermark) > watermark = min_t(unsigned long, > @@ -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); > } 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; > } >