linux-kernel - Re: [PATCHv8 02/34] lib/vdso: make do_hres and do_coarse as __always

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Date:   Fri, 10 Jan 2020 12:18:15 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCHv8 02/34] lib/vdso: make do_hres and do_coarse as
 __always_inline

On 1/10/20 12:02 PM, Thomas Gleixner wrote:
> Vincenzo Frascino <vincenzo.frascino@....com> writes:
>> On 1/10/20 11:42 AM, Thomas Gleixner wrote:
>>>>> +» » /*·goto·allows·to·avoid·extra·inlining·of·do_hres.·*/
>>>>> +» » goto·out_hres;
>>>>
>>>> What is the performance impact of "goto out_hres"?
>>>
>>> On x86 it's invisible at least in my limited testing.
>>
>> On arm64 as well based on mine as well. Shall we keep the code more readable
>> here (without goto)?
> 
> The delta patch below makes it readable again and also avoids the double
> inlining. Quick testing shows no difference.
>

I tested it on arm64 and it does not show any difference as well.
I vote for it :)

> Thanks,
> 
>         tglx
> 
> 8<---------------
> --- a/lib/vdso/gettimeofday.c
> +++ b/lib/vdso/gettimeofday.c
> @@ -98,17 +98,15 @@ static __maybe_unused int
>  	msk = 1U << clock;
>  	if (likely(msk & VDSO_HRES)) {
>  		vd = &vd[CS_HRES_COARSE];
> -out_hres:
> -		return do_hres(vd, clock, ts);
>  	} else if (msk & VDSO_COARSE) {
>  		do_coarse(&vd[CS_HRES_COARSE], clock, ts);
>  		return 0;
>  	} else if (msk & VDSO_RAW) {
>  		vd = &vd[CS_RAW];
> -		/* This goto avoids extra inlining of do_hres. */
> -		goto out_hres;
> +	} else {
> +		return -1;
>  	}
> -	return -1;
> +	return do_hres(vd, clock, ts);
>  }
>  
>  static __maybe_unused int
> 

-- 
Regards,
Vincenzo