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

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Date:   Thu, 27 Jun 2019 20:20:56 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 4/7] powerpc/ftrace: Additionally nop out the preceding
 mflr with -mprofile-kernel

Naveen N. Rao wrote:
> 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.
> 
> Suggested-by: Nicholas Piggin <npiggin@...il.com>
> Reviewed-by: Nicholas Piggin <npiggin@...il.com>
> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> ---
>  arch/powerpc/kernel/trace/ftrace.c | 258 ++++++++++++++++++++++++++---
>  1 file changed, 236 insertions(+), 22 deletions(-)
> 

I missed adding a comment here to explain the changes. As discussed in 
the previous series, I think we are ok with this patch from a CMODX 
perspective. For smp_call_function(), I decided to have it included in 
this patch since we know that we need it here for sure. I am not 
entirely sure we want to do that in patch_instruction() since ftrace 
doesn't seem to need it elsewhere. As Nick Piggin pointed out, we may 
want to have users of patch_instruction() (kprobes) add the necessary 
synchronization.


Thanks,
Naveen