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The kernel says here http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/kernel/events/ring_buffer.c?id=7a1dcf6ad#n61 : * Since the mmap() consumer (userspace) can run on a different CPU: * * kernel user * * if (LOAD ->data_tail) { LOAD ->data_head * (A) smp_rmb() (C) * STORE $data LOAD $data * smp_wmb() (B) smp_mb() (D) * STORE ->data_head STORE ->data_tail * } * * Where A pairs with D, and B pairs with C. * * In our case (A) is a control dependency that separates the load of * the ->data_tail and the stores of $data. In case ->data_tail * indicates there is no room in the buffer to store $data we do not. * * D needs to be a full barrier since it separates the data READ * from the tail WRITE. * * For B a WMB is sufficient since it separates two WRITEs, and for C * an RMB is sufficient since it separates two READs. * * See perf_output_begin(). So I'm pretty sure we need a full barrier before writing out data_tail.
Sysprof is a sampling profiler that uses a kernel module to generate
stacktraces which are then interpreted by the userspace program
"sysprof".
See the Sysprof homepage:
http://sysprof.com/
for more information
Questions, patches and bug reports should be sent to the sysprof
mailing list:
sysprof-list@gnome.org
The list is archived here:
https://mail.gnome.org/archives/sysprof-list/
Debugging symbols
- The programs and libraries you want to profile should be compiled
with -fno-omit-frame-pointer and have debugging symbols available,
or you won't get much usable information.