callchain sampling bug in perf?

[Christoph Hellwig <hch@infradead.org>]

I'm trying to play with perf record -g, that is sampling the callchains.
Doing this with simple workloads works fine, but when I try to do this
with compilebench a lot of samples seem to get lost.

I'm doing the following:

perf probe --add xlog_sync
perf record -g -e probe:xlog_sync -- ./compilebench

Trying to report it I do not get any callchains at all:

[root@virtlab106 compilebench-0.6]# perf report -g flat -n
# Events: 9K cycles
#
# Overhead  Samples         Command      Shared Object     Symbol
# ........ ..........  ............  .................  .........
#
    70.41%       6757  compilebench  [kernel.kallsyms]  [k] xlog_sync
    22.61%       2170          sync  [kernel.kallsyms]  [k] xlog_sync
     3.89%        373            sh  [kernel.kallsyms]  [k] xlog_sync
     2.50%        240        python  [kernel.kallsyms]  [k] xlog_sync
     0.33%         32         :3881  [kernel.kallsyms]  [k] xlog_sync
     0.13%         12         :3971  [kernel.kallsyms]  [k] xlog_sync
     0.11%         11         :3956  [kernel.kallsyms]  [k] xlog_sync
     0.01%          1         :3972  [kernel.kallsyms]  [k] xlog_sync

Doing a perf report -g flat,0.0 -n shows lots of callgraph, but the
percentag for them doesn't add up at all.