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

[Andi Kleen <ak@linux.intel.com>]

Peter Zijlstra <peterz@infradead.org> writes:

> On Mon, Nov 14, 2022 at 01:10:38PM +0200, Adrian Hunter wrote:
>> 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@vger.kernel.org
>> >> Signed-off-by: Adrian Hunter <adrian.hunter@intel.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
>
> Ah, ok. Not as bad then. Anyway, I'll go queue it for perf/urgent I
> suppose.

It would be better to only limit on the CPUs with the bug because
switching buffers causes some extra latencies. So this patch may regress
PT overhead or tail latencies.

-Andi