linux-kernel - Re: [PATCH 5/7] powerpc/ftrace: Update ftrace

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Message-Id: <1560881840.vz9llflvnf.naveen@linux.ibm.com>
Date:   Tue, 18 Jun 2019 23:53:11 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 5/7] powerpc/ftrace: Update ftrace_location() for powerpc
 -mprofile-kernel

Naveen N. Rao wrote:
> Steven Rostedt wrote:
>> On Tue, 18 Jun 2019 20:17:04 +0530
>> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
>> 
>>> @@ -1551,7 +1551,7 @@ unsigned long ftrace_location_range(unsigned long start, unsigned long end)
>>>  	key.flags = end;	/* overload flags, as it is unsigned long */
>>>  
>>>  	for (pg = ftrace_pages_start; pg; pg = pg->next) {
>>> -		if (end < pg->records[0].ip ||
>>> +		if (end <= pg->records[0].ip ||
>> 
>> This breaks the algorithm. "end" is inclusive. That is, if you look for
>> a single byte, where "start" and "end" are the same, and it happens to
>> be the first ip on the pg page, it will be skipped, and not found.
> 
> Thanks. It looks like I should be over-riding ftrace_location() instead.  
> I will update this patch.

I think I will have ftrace own the two instruction range, regardless of 
whether the preceding instruction is a 'mflr r0' or not. This simplifies 
things and I don't see an issue with it as of now. I will do more 
testing to confirm.

- Naveen


--- a/arch/powerpc/kernel/trace/ftrace.c
+++ b/arch/powerpc/kernel/trace/ftrace.c
@@ -951,6 +951,16 @@ void arch_ftrace_update_code(int command)
 }
 
 #ifdef CONFIG_MPROFILE_KERNEL
+/*
+ * We consider two instructions -- 'mflr r0', 'bl _mcount' -- to be part
+ * of ftrace. When checking for the first instruction, we want to include
+ * the next instruction in the range check.
+ */
+unsigned long ftrace_location(unsigned long ip)
+{
+	return ftrace_location_range(ip, ip + MCOUNT_INSN_SIZE);
+}
+
 /* Returns 1 if we patched in the mflr */
 static int __ftrace_make_call_prep(struct dyn_ftrace *rec)
 {
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 21d8e201ee80..122e2bb4a739 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1573,7 +1573,7 @@ unsigned long ftrace_location_range(unsigned long start, unsigned long end)
  * the function tracer. It checks the ftrace internal tables to
  * determine if the address belongs or not.
  */
-unsigned long ftrace_location(unsigned long ip)
+unsigned long __weak ftrace_location(unsigned long ip)
 {
 	return ftrace_location_range(ip, ip);
 }