linux-kernel - Re: [PATCH v3 1/8] ftrace: Add DYNAMIC_FTRACE_WITH_CALL

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Message-ID: <Y+DIKQvfYE15QL3F@FVFF77S0Q05N>
Date:   Mon, 6 Feb 2023 09:28:09 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     "Wangshaobo (bobo)" <bobo.shaobowang@...wei.com>
Cc:     linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        lenb@...nel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        ndesaulniers@...gle.com, ojeda@...nel.org, peterz@...radead.org,
        rafael.j.wysocki@...el.com, revest@...omium.org,
        robert.moore@...el.com, rostedt@...dmis.org, will@...nel.org,
        "liwei (GF)" <liwei391@...wei.com>
Subject: Re: [PATCH v3 1/8] ftrace: Add DYNAMIC_FTRACE_WITH_CALL_OPS

On Tue, Jan 31, 2023 at 09:25:51AM +0800, Wangshaobo (bobo) wrote:
> 在 2023/1/30 18:25, Mark Rutland 写道:
> > On Sat, Jan 28, 2023 at 04:46:48PM +0800, Wangshaobo (bobo) wrote:
> > > 锟斤拷 2023/1/23 21:45, Mark Rutland 写锟斤拷:
> > > > +config DYNAMIC_FTRACE_WITH_CALL_OPS
> > > > +	def_bool y
> > > > +	depends on HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS
> > > > +
> > > Hi Mark,
> > 
> > Hi,
> > 
> > > I have test your patches and it looks fine with my sample module,
> > 
> > Thanks for testing!
> > 
> > > but here setting DYNAMIC_FTRACE_WITH_CALL_OPS to y immutably may increase the
> > > .text section size by 5% or more, how about making this to optional^^
> > 
> > We could consider making this optional. I had not made this optional so far as
> > in the future I'd like to make this the only implementation of ftrace on arm64
> > (once we can drop the old mcount version, and once we've sorted out the
> > incompatibility with CFI). In the mean time, it probably makes sense to have
> > the option at least to enable testing of each of the two forms.
> > 
> > Is your concern that the overall kernel image size is larger, or do you care
> > specifically about the size of the .text section for some reason?
> > 
> > Thanks,
> > Mark
> Embedded devices may pay more attention to Image size, and which may also
> indirectly affects performance, for more reason,

I appreciate those concerns, however:

a) For the Image size, the mcount_loc table and associated relocations already
   imposes a much greater penalty. So I'd expect that where the size truly
   matters, ftrace would be completely disabled anyway.

   I'm currently looking at shrinking the mcount_loc table (and removing the
   need for relocationgs), which should save much more space.

b) For performance, without data this is supposition. Everything so far
   indicates that there is not a measureable performance difference, and from
   other threads it's possible that the increased function alignment *aids*
   performance.

   If you have data to the contrary, I'm happy to investigate.

> I think making sense to have the option for testing is more important.

As above, I'm happy to add an option for functional testing of the ftrace
implementation, but I don't think that it's a good idea to use that as a size
or performance tweak.

Thanks,
Mark.