Re: [PATCH v2 4/7] powerpc/ftrace: Additionally nop out the preceding mflr with -mprofile-kernel

[Nicholas Piggin]

Naveen N. Rao's on June 27, 2019 9:23 pm:
> With -mprofile-kernel, gcc emits 'mflr r0', followed by 'bl _mcount' to
> enable function tracing and profiling. So far, with dynamic ftrace, we
> used to only patch out the branch to _mcount(). However, mflr is
> executed by the branch unit that can only execute one per cycle on
> POWER9 and shared with branches, so it would be nice to avoid it where
> possible.
> 
> We cannot simply nop out the mflr either. When enabling function
> tracing, there can be a race if tracing is enabled when some thread was
> interrupted after executing a nop'ed out mflr. In this case, the thread
> would execute the now-patched-in branch to _mcount() without having
> executed the preceding mflr.
> 
> To solve this, we now enable function tracing in 2 steps: patch in the
> mflr instruction, use 'smp_call_function(isync);
> synchronize_rcu_tasks()' to ensure all existing threads make progress,
> and then patch in the branch to _mcount(). We override
> ftrace_replace_code() with a powerpc64 variant for this purpose.

I think this seems like a reasonable sequence that will work on our
hardware, although technically outside the ISA as specified maybe
we should add a feature bit or at least comment for it.

It would be kind of nice to not put this stuff directly in the 
ftrace code, but rather in the function patching subsystem.

I think it would be too expensive to just make a runtime variant of
patch_instruction that always does the SMP isync, but possibly a
patch_instruction_sync() or something that we say ensures no
processor is running code that has been patched away.

Thanks,
Nick