Re: [PATCH 2/2] coresight: etm-perf: Add AUX_NON_CONTIGUOUS_PAGES to cs_etm PMU

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/24/25 01:31, Yabin Cui wrote:
> On Tue, Apr 22, 2025 at 7:21 AM Leo Yan <leo.yan@arm.com> wrote:
>>
>> On Mon, Apr 21, 2025 at 02:58:18PM -0700, Yabin Cui wrote:
>>> The cs_etm PMU, regardless of the underlying trace sink (ETF, ETR or
>>> TRBE), doesn't require contiguous pages for its AUX buffer.
>>
>> Though contiguous pages are not mandatory for TRBE, I would set the
>> PERF_PMU_CAP_AUX_NO_SG flag for it.  This can potentially benefit
>> performance.
> 
> As explained in the patch 1/2, my use case periodically collects ETM data
> from the field (using both TRBE and ETR), and needs to reduce memory
> fragmentation. If the performance impact is big, we can make it user
> configurable. Otherwise, shall we default it to non-contiguous pages?

But is not that already happening ? cs_etm does not set the PMU cap
PERF_PMU_CAP_AUX_NO_SG that means it can allocate non-contig memory
chunk. Where am I missing ?

> 
>>
>> For non per CPU sinks, it is fine to allocate non-contiguous pages.
>>
>> Thanks,
>> Leo
>>
>>> This patch adds the PERF_PMU_CAP_AUX_NON_CONTIGUOUS_PAGES capability
>>> to the cs_etm PMU. This allows the kernel to allocate non-contiguous
>>> pages for the AUX buffer, reducing memory fragmentation when using
>>> cs_etm.
>>>
>>> Signed-off-by: Yabin Cui <yabinc@google.com>
>>> ---
>>>  drivers/hwtracing/coresight/coresight-etm-perf.c | 3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
>>> index f4cccd68e625..c98646eca7f8 100644
>>> --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
>>> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
>>> @@ -899,7 +899,8 @@ int __init etm_perf_init(void)
>>>       int ret;
>>>
>>>       etm_pmu.capabilities            = (PERF_PMU_CAP_EXCLUSIVE |
>>> -                                        PERF_PMU_CAP_ITRACE);
>>> +                                        PERF_PMU_CAP_ITRACE |
>>> +                                        PERF_PMU_CAP_AUX_NON_CONTIGUOUS_PAGES);
>>>
>>>       etm_pmu.attr_groups             = etm_pmu_attr_groups;
>>>       etm_pmu.task_ctx_nr             = perf_sw_context;
>>> --
>>> 2.49.0.805.g082f7c87e0-goog
>>>
>>>
>