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The low level index is the index in the underlying hardware buffer of the most recently captured taken branch which is always saved in branch_entries[0]. It is very useful for reconstructing the call stack. For example, in Intel LBR call stack mode, the depth of reconstructed LBR call stack limits to the number of LBR registers. With the low level index information, perf tool may stitch the stacks of two samples. The reconstructed LBR call stack can break the HW limitation. Add a new branch sample type to retrieve low level index of raw branch records. The low level index is between -1 (unknown) and max depth which can be retrieved in /sys/devices/cpu/caps/branches. Only when the new branch sample type is set, the low level index information is dumped into the PERF_SAMPLE_BRANCH_STACK output. Perf tool should check the attr.branch_sample_type, and apply the corresponding format for PERF_SAMPLE_BRANCH_STACK samples. Otherwise, some user case may be broken. For example, users may parse a perf.data, which include the new branch sample type, with an old version perf tool (without the check). Users probably get incorrect information without any warning. Signed-off-by: Kan Liang <kan.liang@linux.intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Ingo Molnar <mingo@kernel.org> Link: https://lkml.kernel.org/r/20200127165355.27495-2-kan.liang@linux.intel.com |
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