linux-kernel - RE: [PATCH v2 3/4] tools headers UAPI: add cpu_relax() implementation for x86 and arm64

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Message-ID: <5db490c6f264431e91bcdbb62fcf3be5@AcuMS.aculab.com>
Date:   Wed, 21 Jul 2021 20:53:29 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Yunsheng Lin' <linyunsheng@...wei.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "mst@...hat.com" <mst@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>
CC:     "nickhu@...estech.com" <nickhu@...estech.com>,
        "green.hu@...il.com" <green.hu@...il.com>,
        "deanbo422@...il.com" <deanbo422@...il.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "yury.norov@...il.com" <yury.norov@...il.com>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "ojeda@...nel.org" <ojeda@...nel.org>,
        "ndesaulniers@...ogle.com" <ndesaulniers@...ogle.com>,
        "joe@...ches.com" <joe@...ches.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>
Subject: RE: [PATCH v2 3/4] tools headers UAPI: add cpu_relax() implementation
 for x86 and arm64

From: Yunsheng Lin
> Sent: 20 July 2021 03:22
> 
> As x86 and arm64 is the two available systems that I can build
> and test the cpu_relax() implementation, so only add cpu_relax()
> implementation for x86 and arm64, other arches can be added easily
> when needed.
> 
...
> +#if defined(__i386__) || defined(__x86_64__)
> +/* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */
> +static __always_inline void rep_nop(void)
> +{
> +	asm volatile("rep; nop" ::: "memory");
> +}

Beware, Intel increased the stall for 'rep nop' in some recent
cpu to IIRC about 200 cycles.

They even document that this might have a detrimental effect.
It is basically far too long for the sort of thing it makes
sense to busy-wait for.

	David

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