On Fri, Dec 18, 2009 at 11:34:31PM +0100, Heiko Carstens wrote:
> This selects PERF_EVENTS unconditionally. Just be saying "we support
> HW_BREAKPOINT" we will get additional cpu overhead by perf events?
> Not good.
Yep, if s390 doesn't have breakpoints over ptrace support (which I
don't know) then I guess that can be an option.
> The big question is: what will this feature buy us? I currently can't
> see a strong reason why we want to have HW breakpoint support on s390.
There is a cross-arch reason: you can profile memory accesses.
Say you want to profile tasklist_lock accesses:
$ grep tasklist_lock /proc/kallsyms
ffffffff81c09000 D tasklist_lock
$ perf record -f -g -a -e mem:0xffffffff81c09000:rw sleep 5
$ perf report
# Overhead Command Shared Object Symbol
# ........ ............... ................. ......
#
47.73% hal-acl-tool [kernel] [k] do_raw_read_lock
|
--- do_raw_read_lock
_raw_read_lock
do_wait
sys_wait4
system_call_fastpath
__waitpid
|
|--50.00%-- 0x1b432a000000030
|
|--41.67%-- 0x1b40f4000000030
|
--8.33%-- 0x1b40f4000000000
16.14% hal-acl-tool [kernel] [k] do_raw_write_unlock
|
--- do_raw_write_unlock
_raw_write_unlock_irq
|
|--80.00%-- copy_process
| do_fork
| sys_clone
| stub_clone
| __libc_fork
| 0x7f44d2209a7f
|
--20.00%-- exit_ptrace
etc....
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