linux-kernel - Re: [External] Re: [PATCH] arm64/ftrace: Make function graph use ftrace directly

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Date:   Wed, 23 Feb 2022 16:00:27 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mingo@...hat.com, catalin.marinas@....com, will@...nel.org,
        mark.rutland@....com, broonie@...nel.org, songmuchun@...edance.com,
        qirui.001@...edance.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH] arm64/ftrace: Make function graph use
 ftrace directly

On 2022/2/22 11:52 下午, Steven Rostedt wrote:
> On Tue, 22 Feb 2022 21:00:49 +0800
> Chengming Zhou <zhouchengming@...edance.com> wrote:
> 
>> As we do in commit 0c0593b45c9b ("x86/ftrace: Make function graph
>> use ftrace directly"), we don't need special hook for graph tracer,
>> but instead we use graph_ops:func function to install return_hooker.
>>
>> Since commit 3b23e4991fb6 ("arm64: implement ftrace with regs") add
>> implementation for FTRACE_WITH_REGS on arm64, we can easily adopt
>> the same optimization on arm64.
> 
> Note. Ideally we want it to hook with DYNAMIC_FTARCE_WITH_ARGS, and not
> FTRACE_WITH_REGS. If arm64 is like x86_64, saving all regs at every
> function call has a bit more overhead than saving the minimum. The
> DYNAMIC_FTRACE_WITH_ARGS, means that the minimum is still saved, but now
> exposes the arguments and the stack pointer, which function_graph_tracer
> needs.

Yes, it would be better to implement DYNAMIC_FTRACE_WITH_ARGS on arm64 too,
and this patch just use DYNAMIC_FTRACE_WITH_REGS to install return_hooker
for graph tracer, so it's a code cleanup, no performance optimization.

Thanks.

> 
> -- Steve
> 
> 
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>> ---