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From: David Laight <David.Laight@ACULAB.COM>
To: 'Yunsheng Lin' <linyunsheng@huawei.com>,
	"davem@davemloft.net" <davem@davemloft.net>,
	"kuba@kernel.org" <kuba@kernel.org>,
	"mst@redhat.com" <mst@redhat.com>,
	"jasowang@redhat.com" <jasowang@redhat.com>
Cc: "nickhu@andestech.com" <nickhu@andestech.com>,
	"green.hu@gmail.com" <green.hu@gmail.com>,
	"deanbo422@gmail.com" <deanbo422@gmail.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"yury.norov@gmail.com" <yury.norov@gmail.com>,
	"andriy.shevchenko@linux.intel.com" 
	<andriy.shevchenko@linux.intel.com>,
	"ojeda@kernel.org" <ojeda@kernel.org>,
	"ndesaulniers@gooogle.com" <ndesaulniers@gooogle.com>,
	"joe@perches.com" <joe@perches.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"virtualization@lists.linux-foundation.org" 
	<virtualization@lists.linux-foundation.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"linuxarm@openeuler.org" <linuxarm@openeuler.org>
Subject: RE: [PATCH v2 3/4] tools headers UAPI: add cpu_relax() implementation for x86 and arm64
Date: Wed, 21 Jul 2021 20:53:29 +0000	[thread overview]
Message-ID: <5db490c6f264431e91bcdbb62fcf3be5@AcuMS.aculab.com> (raw)
In-Reply-To: <1626747709-34013-4-git-send-email-linyunsheng@huawei.com>

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

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


  reply	other threads:[~2021-07-21 20:53 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-20  2:21 [PATCH v2 0/4] refactor the ringtest testing for ptr_ring Yunsheng Lin
2021-07-20  2:21 ` [PATCH v2 1/4] tools headers UAPI: add cache aligning related macro Yunsheng Lin
2021-07-20  2:21 ` [PATCH v2 2/4] tools headers UAPI: add kmalloc/vmalloc related interface Yunsheng Lin
2021-07-20  2:21 ` [PATCH v2 3/4] tools headers UAPI: add cpu_relax() implementation for x86 and arm64 Yunsheng Lin
2021-07-21 20:53   ` David Laight [this message]
2021-07-22  8:18     ` Yunsheng Lin
2021-07-22  8:45       ` David Laight
2021-07-20  2:21 ` [PATCH v2 4/4] tools/virtio: use common infrastructure to build ptr_ring.h Yunsheng Lin

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