linux-kernel - Re: [PATCH -next V7 0/7] riscv: Optimize function trace

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Message-ID: <CAJF2gTQuEiBMg=2KQXb3UZNaH_AdwCHNyNe07dbPd7JDGGExzg@mail.gmail.com>
Date:   Thu, 9 Feb 2023 17:11:24 +0800
From:   Guo Ren <guoren@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Evgenii Shatokhin <e.shatokhin@...ro.com>,
        "suagrfillet@...il.com" <suagrfillet@...il.com>,
        "andy.chiu@...ive.com" <andy.chiu@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        "anup@...infault.org" <anup@...infault.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "palmer@...belt.com" <palmer@...belt.com>,
        "conor.dooley@...rochip.com" <conor.dooley@...rochip.com>,
        "heiko@...ech.de" <heiko@...ech.de>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "jolsa@...hat.com" <jolsa@...hat.com>, "bp@...e.de" <bp@...e.de>,
        "jpoimboe@...nel.org" <jpoimboe@...nel.org>,
        "linux@...ro.com" <linux@...ro.com>
Subject: Re: [PATCH -next V7 0/7] riscv: Optimize function trace

On Thu, Feb 9, 2023 at 5:00 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Guo Ren
> > Sent: 09 February 2023 01:51
> ...
> > Yours cost one more instruction, right?
> >          addr-12  auipc
> >          addr-8    jalr
> >          addr-4    // Literal (32-bits)
> >          addr+0   nop or jmp addr-n // one more?
> >          addr+4   function_code
>
> Yes, it is 4 bytes larger but there is one less
> instruction executed (only one nop) when ftrace is disabled.
> That probably matters more than anything in the ftrace
> 'prologue' code.
I've got your point, thx. I would consider your advice and make the tradeoff.

>
> I also suspect that you can use a 32bit integer as
> a table index in 64bit mode to save a word there.
Yes, good idea. I've asked Mark Rutland. Let's wait for his feedback.
It also helps arm64.

>
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)



-- 
Best Regards
 Guo Ren