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Date: Fri, 7 Dec 2018 11:21:43 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Arnaldo Carvalho de Melo <acme@...nel.org>,
Milian Wolff <milian.wolff@...b.com>
Cc: Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
jolsa@...nel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] perf script: Fix LBR skid dump problems in brstackinsn
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@...ux.intel.com>
>>
>> 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@...el.com>
>
> 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@...b.com
>> Cc: adrian.hunter@...el.com
>> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
>> ---
>> 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
>
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