linux-kernel - Re: [PATCH V6 01/16] perf/core: Add PERF_SAMPLE_DATA_PAGE

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Date:   Mon, 10 Aug 2020 18:36:03 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...nel.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, eranian@...gle.com, ak@...ux.intel.com,
        dave.hansen@...el.com, kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH V6 01/16] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE



On 8/10/2020 5:39 PM, Peter Zijlstra wrote:
> On Mon, Aug 10, 2020 at 02:24:21PM -0700, Kan Liang wrote:
>> Current perf can report both virtual addresses and physical addresses,
>> but not the page size. Without the page size information of the utilized
>> page, users cannot decide whether to promote/demote large pages to
>> optimize memory usage.
>>
>> Add a new sample type for the data page size.
>>
>> Current perf already has a facility to collect data virtual addresses.
>> A page walker is required to walk the pages tables and calculate the
>> page size from a given virtual address.
>>
>> On some platforms, e.g., X86, the page walker is invoked in an NMI
>> handler. So the page walker must be IRQ-safe and low overhead. Besides,
>> the page walker should work for both user and kernel virtual address.
>> The existing generic page walker, e.g., walk_page_range_novma(), is a
>> little bit complex and doesn't guarantee the IRQ-safe. The follow_page()
>> is only for user-virtual address.
>>
>> Add a new function perf_get_page_size() to walk the page tables and
>> calculate the page size. In the function:
>> - Interrupts have to be disabled to prevent any teardown of the page
>>    tables.
>> - The size of a normal page is from the pre-defined page size macros.
>> - The size of a compound page is retrieved from the helper function,
>>    page_size().
>>
>> Suggested-by: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
> 
>>   /* default value for data source */
>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>> index 52ca2093831c..32484accc7a3 100644
>> --- a/include/uapi/linux/perf_event.h
>> +++ b/include/uapi/linux/perf_event.h
>> @@ -143,8 +143,9 @@ enum perf_event_sample_format {
>>   	PERF_SAMPLE_PHYS_ADDR			= 1U << 19,
>>   	PERF_SAMPLE_AUX				= 1U << 20,
>>   	PERF_SAMPLE_CGROUP			= 1U << 21,
>> +	PERF_SAMPLE_DATA_PAGE_SIZE		= 1U << 22,
>>   
>> -	PERF_SAMPLE_MAX = 1U << 22,		/* non-ABI */
>> +	PERF_SAMPLE_MAX = 1U << 23,		/* non-ABI */
>>   
>>   	__PERF_SAMPLE_CALLCHAIN_EARLY		= 1ULL << 63, /* non-ABI; internal use */
>>   };
> 
>> @@ -7151,6 +7269,9 @@ void perf_prepare_sample(struct perf_event_header *header,
>>   	}
>>   #endif
>>   
>> +	if (sample_type & PERF_SAMPLE_DATA_PAGE_SIZE)
>> +		data->data_page_size = perf_get_page_size(data->addr);
>> +
> 
> We could just require SAMPLE_DATA_PAGE requires SAMPLE_ADDR.
> 

If we only require the SAMPLE_DATA_PAGE_SIZE and no SAMPLE_ADDR, the 
data->addr will be updated implicitly, but the value will not dump to 
userspace tool. I will add a comment here.

Thanks,
Kan