Messages in this thread | | | From | Philippe Rétornaz <> | Subject | Re: ftrace performance impact with different configuration | Date | Fri, 30 Dec 2011 14:07:11 +0100 |
| |
Le jeudi 29 décembre 2011 11:21:25 Steven Rostedt a écrit : > On Thu, 2011-12-29 at 21:12 +0530, Rabin Vincent wrote: > > On Thu, Dec 29, 2011 at 14:08, Lei Wen <adrian.wenl@gmail.com> wrote: > > > 2. Seem dynamic ftrace also could involve some penalty for the > > > running > > > system, although it patching the running kernel with nop stub... > > > > > > For the second item, is there anyone done some research before that > > > could zero the cost for the running system when the tracing is not > > > enabled yet? > > > > One thing that needs to be fixed (for ARM) is that for the new-style > > mcounts, the nop that's currently being done is not really a nop -- it > > removes the function call, but there is still an unnecessary push/pop > > sequence. This should be modified to have the push {lr} removed too. > > (Two instructions replaced instead of one.) > > Unfortunately you can't do this, at least not when the kernel is > preemptible. > > Say we have: > > push lr > call mcount > > then we convert it to: > > nop > nop > > The conversion to nop should not be an issue, and this is what would be > done when the system boots up. But then we enable tracing, some low > priority task could have been preempted after executing the first nop, > and we call stop machine to do the conversions (if no stop machine, then > lets just say a higher prio task is running while we do the > conversions). Then we add both the push lr and call back. But when that > lower priority task gets scheduled in again, it would have looked like > it ran: > > nop > call mcount > > Since the call to mcount requires that the lr was pushed, this process > will crash when the return is done and we never saved the lr. > > If you don't like the push. the best thing you can do is convert to: > > jmp 1f > call mcount > 1: > > This may not be as cheap as two nops, but it may be better than a push.
Sorry about being a bit naive, but why it is not possible to do it in two steps ? call stop_machine to put the jmp which skip the call to mcount Then wait until all tasks hits schedule() (synchronize_sched() ?) Then modify both instructions to put in place the two nops since we know that nobody is calling mcount.
Thanks,
Philippe
-- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |