Re: [PATCH] perf script: Fix LBR skid dump problems in brstackinsn

[Adrian Hunter]

On 6/12/18 7:01 PM, Arnaldo Carvalho de Melo wrote:
> Em Mon, Nov 19, 2018 at 09:06:17PM -0800, Andi Kleen escreveu:
>> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>>
>> This is a fix for another instance of the skid problem Milian
>> recently found [1]
> 
> Milian, have you tested this?
> 
> Adrian, can I have your Reviewed-by or Acked-by?

Acked-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>

> 
> Thanks,
> 
> - Arnaldo
>  
>> The LBRs don't freeze at the exact same time as the PMI is triggered.
>> The perf script brstackinsn code that dumps LBR assembler
>> assumes that the last branch in the LBR leads to the sample point.
>> But with skid it's possible that the CPU executes one or more branches
>> before the sample, but which do not appear in the LBR.
>>
>> What happens then is either that the sample point is before
>> the last LBR branch. In this case the dumper sees a negative
>> length and ignores it. Or it the sample point is long after
>> the last branch. Then the dumper sees a very long block and dumps
>> it upto its block limit (16k bytes), which is noise in the output.
>>
>> On typical sample session this can happen regularly.
>>
>> This patch tries to detect and handle the situation. On the last
>> block that is dumped by the LBR dumper we always stop on the first
>> branch. If the block length is negative just scan forward to the
>> first branch. Otherwise scan until a branch is found.
>>
>> The PT decoder already has a function that uses the instruction
>> decoder to detect branches, so we can just reuse it here.
>>
>> Then when a terminating branch is found print an indication
>> and stop dumping. This might miss a few instructions, but at least
>> shows no runaway blocks.
>>
>> Cc: milian.wolff@xxxxxxxx
>> Cc: adrian.hunter@xxxxxxxxx
>> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>> ---
>>  tools/perf/builtin-script.c                    | 18 +++++++++++++++++-
>>  tools/perf/util/dump-insn.c                    |  8 ++++++++
>>  tools/perf/util/dump-insn.h                    |  2 ++
>>  .../intel-pt-decoder/intel-pt-insn-decoder.c   |  8 ++++++++
>>  4 files changed, 35 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/perf/builtin-script.c b/tools/perf/builtin-script.c
>> index 4da5e32b9e03..11868bf39e66 100644
>> --- a/tools/perf/builtin-script.c
>> +++ b/tools/perf/builtin-script.c
>> @@ -1049,9 +1049,18 @@ static int perf_sample__fprintf_brstackinsn(struct perf_sample *sample,
>>  
>>  	/*
>>  	 * Print final block upto sample
>> +	 *
>> +	 * Due to pipeline delays the LBRs might be missing a branch
>> +	 * or two, which can result in very large or negative blocks
>> +	 * between final branch and sample. When this happens just
>> +	 * continue walking after the last TO until we hit a branch.
>>  	 */
>>  	start = br->entries[0].to;
>>  	end = sample->ip;
>> +	if (end < start) {
>> +		/* Missing jump. Scan 128 bytes for the next branch */
>> +		end = start + 128;
>> +	}
>>  	len = grab_bb(buffer, start, end, machine, thread, &x.is64bit, &x.cpumode, true);
>>  	printed += ip__fprintf_sym(start, thread, x.cpumode, x.cpu, &lastsym, attr, fp);
>>  	if (len <= 0) {
>> @@ -1060,7 +1069,6 @@ static int perf_sample__fprintf_brstackinsn(struct perf_sample *sample,
>>  			      machine, thread, &x.is64bit, &x.cpumode, false);
>>  		if (len <= 0)
>>  			goto out;
>> -
>>  		printed += fprintf(fp, "\t%016" PRIx64 "\t%s\n", sample->ip,
>>  			dump_insn(&x, sample->ip, buffer, len, NULL));
>>  		goto out;
>> @@ -1070,6 +1078,14 @@ static int perf_sample__fprintf_brstackinsn(struct perf_sample *sample,
>>  				   dump_insn(&x, start + off, buffer + off, len - off, &ilen));
>>  		if (ilen == 0)
>>  			break;
>> +		if (arch_is_branch(buffer + off, len - off, x.is64bit) &&
>> +			start + off != sample->ip) {
>> +			/*
>> +			 * Hit a missing branch. Just stop.
>> +			 */
>> +			printed += fprintf(fp, "\t... not reaching sample ...\n");
>> +			break;
>> +		}
>>  	}
>>  out:
>>  	return printed;
>> diff --git a/tools/perf/util/dump-insn.c b/tools/perf/util/dump-insn.c
>> index 10988d3de7ce..2bd8585db93c 100644
>> --- a/tools/perf/util/dump-insn.c
>> +++ b/tools/perf/util/dump-insn.c
>> @@ -13,3 +13,11 @@ const char *dump_insn(struct perf_insn *x __maybe_unused,
>>  		*lenp = 0;
>>  	return "?";
>>  }
>> +
>> +__weak
>> +int arch_is_branch(const unsigned char *buf __maybe_unused,
>> +		   size_t len __maybe_unused,
>> +		   int x86_64 __maybe_unused)
>> +{
>> +	return 0;
>> +}
>> diff --git a/tools/perf/util/dump-insn.h b/tools/perf/util/dump-insn.h
>> index 0e06280a8860..650125061530 100644
>> --- a/tools/perf/util/dump-insn.h
>> +++ b/tools/perf/util/dump-insn.h
>> @@ -20,4 +20,6 @@ struct perf_insn {
>>  
>>  const char *dump_insn(struct perf_insn *x, u64 ip,
>>  		      u8 *inbuf, int inlen, int *lenp);
>> +int arch_is_branch(const unsigned char *buf, size_t len, int x86_64);
>> +
>>  #endif
>> diff --git a/tools/perf/util/intel-pt-decoder/intel-pt-insn-decoder.c b/tools/perf/util/intel-pt-decoder/intel-pt-insn-decoder.c
>> index 54818828023b..1c0e289f01e6 100644
>> --- a/tools/perf/util/intel-pt-decoder/intel-pt-insn-decoder.c
>> +++ b/tools/perf/util/intel-pt-decoder/intel-pt-insn-decoder.c
>> @@ -180,6 +180,14 @@ int intel_pt_get_insn(const unsigned char *buf, size_t len, int x86_64,
>>  	return 0;
>>  }
>>  
>> +int arch_is_branch(const unsigned char *buf, size_t len, int x86_64)
>> +{
>> +	struct intel_pt_insn in;
>> +	if (intel_pt_get_insn(buf, len, x86_64, &in) < 0)
>> +		return -1;
>> +	return in.branch != INTEL_PT_BR_NO_BRANCH;
>> +}
>> +
>>  const char *dump_insn(struct perf_insn *x, uint64_t ip __maybe_unused,
>>  		      u8 *inbuf, int inlen, int *lenp)
>>  {
>> -- 
>> 2.17.2
>