Messages in this thread | | | Date | Wed, 16 Nov 2022 08:26:47 +0200 | Subject | Re: [PATCH] perf/x86/intel/pt: Fix sampling using single range output | From | Adrian Hunter <> |
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On 15/11/22 21:46, Andi Kleen wrote: > 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.
I could whitelist CPUs that do not have the issue, because a blacklist would keep expanding, which would be a bit of a pain to maintain.
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