linux-kernel - Re: [PATCH v6] arm64: implement ftrace with regs

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Message-ID: <20190114122616.GD10258@lakrids.cambridge.arm.com>
Date:   Mon, 14 Jan 2019 12:26:16 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     Torsten Duwe <duwe@....de>, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Julien Thierry <julien.thierry@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Amit Daniel Kachhap <amit.kachhap@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v6] arm64: implement ftrace with regs

On Mon, Jan 14, 2019 at 11:13:59PM +1100, Balbir Singh wrote:
> On Fri, Jan 04, 2019 at 05:50:18PM +0000, Mark Rutland wrote:
> > Hi Torsten,
> > 
> > On Fri, Jan 04, 2019 at 03:10:53PM +0100, Torsten Duwe wrote:
> > > Use -fpatchable-function-entry (gcc8) to add 2 NOPs at the beginning
> > > of each function. Replace the first NOP thus generated with a quick LR
> > > saver (move it to scratch reg x9), so the 2nd replacement insn, the call
> > > to ftrace, does not clobber the value. Ftrace will then generate the
> > > standard stack frames.
> 
> Do we know what the overhead would be, if this was a link time change
> for the first instruction?

No, but it should be possible to benchamrk that for a given workload,
which is what I'd like to see.

> Also, I was under the impression that some arch's do ftrace_call_replace
> under stop_machine(), is that a possibility here?

Something like that is a possibility.

I think we need numbers either way.

Thanks,
Mark.