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

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Date:   Mon, 14 Nov 2022 13:59:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
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 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@...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

Ah, ok. Not as bad then. Anyway, I'll go queue it for perf/urgent I
suppose.