linux-kernel - Re: [PATCHv7 18/33] lib/vdso: Add unlikely() hint into vdso_read

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Message-ID: <100f6921-9081-7eb0-7acc-f10cfb647c21@arm.com>
Date:   Wed, 16 Oct 2019 12:24:14 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     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>,
        Thomas Gleixner <tglx@...utronix.de>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCHv7 18/33] lib/vdso: Add unlikely() hint into
 vdso_read_begin()

On 10/11/19 2:23 AM, Dmitry Safonov wrote:
> From: Andrei Vagin <avagin@...il.com>
> 
> Place the branch with no concurrent write before contended case.
> 
> Performance numbers for Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz
> (more clock_gettime() cycles - the better):
>         | before    | after
> -----------------------------------
>         | 150252214 | 153242367
>         | 150301112 | 153324800
>         | 150392773 | 153125401
>         | 150373957 | 153399355
>         | 150303157 | 153489417
>         | 150365237 | 153494270
> -----------------------------------
> avg     | 150331408 | 153345935
> diff %  | 2	    | 0
> -----------------------------------
> stdev % | 0.3	    | 0.1
> 
> Signed-off-by: Andrei Vagin <avagin@...il.com>
> Co-developed-by: Dmitry Safonov <dima@...sta.com>
> Signed-off-by: Dmitry Safonov <dima@...sta.com>

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@....com>

> ---
>  include/vdso/helpers.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/vdso/helpers.h b/include/vdso/helpers.h
> index 01641dbb68ef..9a2af9fca45e 100644
> --- a/include/vdso/helpers.h
> +++ b/include/vdso/helpers.h
> @@ -10,7 +10,7 @@ static __always_inline u32 vdso_read_begin(const struct vdso_data *vd)
>  {
>  	u32 seq;
>  
> -	while ((seq = READ_ONCE(vd->seq)) & 1)
> +	while (unlikely((seq = READ_ONCE(vd->seq)) & 1))
>  		cpu_relax();
>  
>  	smp_rmb();
> 

-- 
Regards,
Vincenzo