Messages in this thread | | | Date | Tue, 22 Jul 2014 18:47:07 +0200 | From | Oleg Nesterov <> | Subject | Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines |
| |
On 07/03, Steven Rostedt wrote: > > [ NOT READY FOR INCLUSION! ] > > Note, this is based off of my remove ftrace_start/stop() patch set.
So I simply pulled your tree. I can't really comment these changes simply because I do not understand this code. But I am hunting for RHEL bug in (probably) this area, so I decided to take a look in a hope that may be this can help me to understand the current code ;)
> The way the function callback mechanism works in ftrace is that if there's > only one function callback registered, it will set the mcount/fentry > trampoline to call that function directly. But as soon as you register > another callback, the mcount trampoline calls a loop function that iterates > over all the registered callbacks (ftrace_ops) checking their hash tables > to see if the called function matches the ops before calling its callback. > This happens even if the two registered functions are not even tracing > the same function! > > This really sucks if you are tracing all functions, and then add a kprobe > or perf event that traces a single function. That will cause all the > other functions being traced to perform the loop test.
But this is even worse or I missed something? I mean, currently even if you trace nothing and then add a KPROBE_FLAG_FTRACE kprobe, then kprobe_ftrace_handler() is called by ftrace_ops_list_func() ?
After these changes it seems that kprobe will use a trampoline.
And I can't understand even the basic code. Say, __ftrace_hash_rec_update:
if (inc) { rec->flags++; if (FTRACE_WARN_ON(ftrace_rec_count(rec) == FTRACE_REF_MAX)) return;
/* * If there's only a single callback registered to a * function, and the ops has a trampoline registered * for it, then we can call it directly. */ if (ftrace_rec_count(rec) == 1 && ops->trampoline) { rec->flags |= FTRACE_FL_TRAMP; ops->trampolines++; } else { /* * If we are adding another function callback * to this function, and the previous had a * trampoline used, then we need to go back to * the default trampoline. */ rec->flags &= ~FTRACE_FL_TRAMP;
/* remove trampolines from any ops for this rec */ ftrace_clear_tramps(rec); }
It seems that "else if (ftrace_rec_count(rec) == 2)" can avoid the unnecessary ftrace_clear_tramps() ? And not only unnecessary, ftrace_clear_tramps() decrements ->trampolines, can't this break the accounting?
} else { if (FTRACE_WARN_ON(ftrace_rec_count(rec) == 0)) return; rec->flags--;
if (ops->trampoline && !ftrace_rec_count(rec)) ftrace_remove_tramp(ops, rec);
I am wondering what should we do if ftrace_rec_count() becomes 1 again...
ftrace_save_ops_tramp_hash():
do_for_each_ftrace_rec(pg, rec) { if (ftrace_rec_count(rec) == 1 && ftrace_ops_test(ops, rec->ip, rec)) {
/* This record had better have a trampoline */ if (FTRACE_WARN_ON(!(rec->flags & FTRACE_FL_TRAMP_EN))) return -1;
Yes, but I can't understand how this can work.
This ops can have ->trampolines > 0, but FTRACE_FL_TRAMP_EN can be cleared by another ftrace_ops?
Suppose there is a single tracer of this function, rec->flags = TRAMP | TRAMP_EN. Suppose also that it traces more than 1 function, so ->trampolines > 1.
Another tracer comes, __ftrace_hash_rec_update() clears TRAMP. But it should also do ftrace_check_record() and this should clear TRAMP_EN?
And yes, I can trigger this bug if I simply do "echo function > current_tracer" and then add/remove a KPROBE_FLAG_FTRACE kprobe.
And you know, when I try to read this code I can't distinguish ->trampoline from ->trampolines ;)
Oleg.
| |