Re: ftrace performance impact with different configuration

From: Steven Rostedt
Date: Fri Dec 30 2011 - 17:25:49 EST

On Fri, 2011-12-30 at 14:07 +0100, Philippe Rétornaz wrote:

> 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() ?)

Here's the problem. With a preemptible kernel, hitting schedule() does
not mean that you can guarantee that all tasks have not been

nop (use to be push lr)
------------> interrupt
end of interrupt

[ another processes is now running ]
call stop_machine()
put in push lr
call stop_machine()
put in call to mcount

everything seems to be running fine.

the low priority task gets scheduled again...

schedule() finishes
preempt_schedule() finishes
back at tail of return_from_intr in entry_64.S
call mcount
pop lr (which was never pushed)
jmp lr

Now we jump back to some random stuff and the stack is corrupted.

There's no way to safely modify two instructions that depend on each
other in a preemptible kernel, with the exception of waiting for all
CPUs to hit idle (which may never happen on a busy system). And even
that may be racy.

-- Steve

> Then modify both instructions to put in place the two nops since we know that
> nobody is calling mcount.

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