linux-kernel - Re: [PATCH/RFC 0/5] cpu_relax: introduce yield, remove lowlatency

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Message-Id: <20161021.105727.140184460493941551.davem@davemloft.net>
Date:   Fri, 21 Oct 2016 10:57:27 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     borntraeger@...ibm.com
Cc:     peterz@...radead.org, npiggin@...il.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        heiko.carstens@...ibm.com, schwidefsky@...ibm.com, noamc@...hip.com
Subject: Re: [PATCH/RFC 0/5] cpu_relax: introduce yield, remove lowlatency

From: Christian Borntraeger <borntraeger@...ibm.com>
Date: Fri, 21 Oct 2016 13:58:53 +0200

> For spinning loops people did often use barrier() or cpu_relax().
> For most architectures cpu_relax and barrier are the same, but on
> some architectures cpu_relax can add some latency. For example on s390
> cpu_relax gives up the time slice to the hypervisor. On power cpu_relax
> tries to give some of the CPU to the neighbor threads. To reduce the
> latency another variant cpu_relax_lowlatency was introduced. Before this
> is used in more and more places, lets revert the logic of provide a new
> function cpu_relax_yield that can spend some time and for s390 yields
> the guest CPU.

Sparc64, fwiw, behaves similarly to powerpc.