On Sat, Dec 19, 2009 at 01:40:28AM +0100, Frederic Weisbecker wrote: > 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. Converting s390 to use hw breakpoints for ptrace was the third patch in the series, which I think would be a nice cleanup. Just wondering why using having hw breakpoints implicitly implies enabling perf events. > > 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 Yes, that looks nice. Unfortunately s390 traps only on write accesses and not on read accesses. But it still should give some nice numbers. Btw (haven't read the code) what is the default memory length that is being watched on if you specify an address like above? On s390 you would have to specify the start and end address in control registers, that's why I'm asking. -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
