From 874fc35cdd55e2d46161901de43ec58ca2efc5fe Mon Sep 17 00:00:00 2001 From: Alexander Shishkin Date: Wed, 14 Apr 2021 18:49:55 +0300 Subject: perf intel-pt: Use aux_watermark Turns out, the default setting of attr.aux_watermark to half of the total buffer size is not very useful, especially with smaller buffers. The problem is that, after half of the buffer is filled up, the kernel updates ->aux_head and sets up the next "transaction", while observing that ->aux_tail is still zero (as userspace haven't had the chance to update it), meaning that the trace will have to stop at the end of this second "transaction". This means, for example, that the second PERF_RECORD_AUX in every trace comes with TRUNCATED flag set. Setting attr.aux_watermark to quarter of the buffer gives enough space for the ->aux_tail update to be observed and prevents the data loss. The obligatory before/after showcase: > # perf_before record -e intel_pt//u -m,8 uname > Linux > [ perf record: Woken up 6 times to write data ] > Warning: > AUX data lost 4 times out of 10! > > [ perf record: Captured and wrote 0.099 MB perf.data ] > # perf record -e intel_pt//u -m,8 uname > Linux > [ perf record: Woken up 4 times to write data ] > [ perf record: Captured and wrote 0.039 MB perf.data ] The effect is still visible with large workloads and large buffers, although less pronounced. Signed-off-by: Alexander Shishkin Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20210414154955.49603-3-alexander.shishkin@linux.intel.com --- tools/perf/arch/x86/util/intel-pt.c | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'tools/perf') diff --git a/tools/perf/arch/x86/util/intel-pt.c b/tools/perf/arch/x86/util/intel-pt.c index a6420c647959..6df0dc00d73a 100644 --- a/tools/perf/arch/x86/util/intel-pt.c +++ b/tools/perf/arch/x86/util/intel-pt.c @@ -776,6 +776,12 @@ static int intel_pt_recording_options(struct auxtrace_record *itr, } } + if (!opts->auxtrace_snapshot_mode && !opts->auxtrace_sample_mode) { + u32 aux_watermark = opts->auxtrace_mmap_pages * page_size / 4; + + intel_pt_evsel->core.attr.aux_watermark = aux_watermark; + } + intel_pt_parse_terms(intel_pt_pmu->name, &intel_pt_pmu->format, "tsc", &tsc_bit); -- cgit v1.2.3