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

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Message-ID: <875zhja59q.fsf@nanos.tec.linutronix.de>
Date:   Fri, 10 Jan 2020 13:02:25 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vincenzo Frascino <vincenzo.frascino@....com>,
        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

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.

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