On Thu, Dec 18, 2008 at 11:00 PM, Corey J Ashford <cjash...@us.ibm.com> wrote:
> Thanks for your reply, Stephane,
>
> "stephane eranian" <eran...@googlemail.com> wrote on 12/18/2008 01:56:16
> AM:
>
>> Corey,
>>
>> I looked at the pfm_handle_work() code. Normally
>> this functions gets called only if the TIF_PERFMON_WORK
>> flag is set. On Power, it seems the logic is coded differently.
>> You call systematically and in pfm_handle_work() you check
>> the flag, and if not set you return.
>>
>> During boot, I don't expect TIF_PERFMON_WORK to be set.
>> So I don't understand why FTRACER would have a problem
>> with a function that does a simple test and returns.
>
> Yes, in fact it appears that the call to _mcount is made even before the
> test of TIF_PERFMON_WORK is made. Reading the ftrace.txt docs, it appears
> that these _mcount calls are inserted by the compiler at the beginning of
> every function, through the addition of gcc's -pg switch, if
> CONFIG_DYNAMIC_FTRACE is true. However, the docs are out of date, I
> think, and the -pg flag is dependent only upon CONFIG_FUNCTION_TRACER
> being true (see Makefile).
>
> But why this is causing a problem only on this call, I don't yet know. The
> _mcount code is in assembler, and might possibly make some incorrect
> assumptions about how the registers are initialized.
>
Yes, but I wonder what's so special in a function that does:
void pfm_handle_work(struct pt_regs *regs)
{
struct pfm_context *ctx;
unsigned long flags, dummy_flags;
int type, ret, info;
#ifdef CONFIG_PPC
/*
* This is just a temporary fix. Obviously we'd like to fix the powerpc
* code to make that check before calling __pfm_handle_work() to
* prevent the function call overhead, but the call is made from
* assembly code, so it will take a little while to figure out how to
* perform the check correctly.
*/
if (!test_thread_flag(TIF_PERFMON_WORK))
return;
#endif
And you should hit that test and return.
Maybe this is because it is always called from assembly.
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