linux-kernel - Re: [PATCH] perf/x86/intel/pt: Fix sampling using single range output

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Date:   Mon, 14 Nov 2022 13:10:38 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] perf/x86/intel/pt: Fix sampling using single range output

On 14/11/22 12:51, Peter Zijlstra wrote:
> On Sat, Nov 12, 2022 at 05:15:08PM +0200, Adrian Hunter wrote:
>> Deal with errata TGL052, ADL037 and RPL017 "Trace May Contain Incorrect
>> Data When Configured With Single Range Output Larger Than 4KB" by
>> disabling single range output whenever larger than 4KB.
>>
>> Fixes: 670638477aed ("perf/x86/intel/pt: Opportunistically use single range output mode")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
>> ---
>>  arch/x86/events/intel/pt.c | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
>> index 82ef87e9a897..42a55794004a 100644
>> --- a/arch/x86/events/intel/pt.c
>> +++ b/arch/x86/events/intel/pt.c
>> @@ -1263,6 +1263,15 @@ static int pt_buffer_try_single(struct pt_buffer *buf, int nr_pages)
>>  	if (1 << order != nr_pages)
>>  		goto out;
>>  
>> +	/*
>> +	 * Some processors cannot always support single range for more than
>> +	 * 4KB - refer errata TGL052, ADL037 and RPL017. Future processors might
>> +	 * also be affected, so for now rather than trying to keep track of
>> +	 * which ones, just disable it for all.
>> +	 */
>> +	if (nr_pages > 1)
>> +		goto out;
> 
> This effectively declares single-output-mode dead? Because I don't think
> anybody uses PT with a single 4K buffer.

4K is the default size for "sample mode" i.e. stuffing 4KB of Intel PT trace
data into a PERF_RECORD_SAMPLE record that has sample_type bit PERF_SAMPLE_AUX

e.g.

$ perf record -vv --aux-sample -e '{intel_pt//u,cycles:u}' uname 2>err.txt
Linux
$ grep aux_sample_size err.txt
  aux_sample_size                  4096
$