linux-kernel - Re: [PATCH 3/5] metag: smp: don't spin waiting for CPU to start

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAAG0J98-mWk6p5hph14ttzBXbZmtDsQtoh8FF2DgoUA1MXv2VA@mail.gmail.com>
Date:	Mon, 1 Jul 2013 23:02:01 +0100
From:	James Hogan <james.hogan@...tec.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/5] metag: smp: don't spin waiting for CPU to start

On 1 July 2013 22:46, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Mon, 1 Jul 2013, James Hogan wrote:
>> Use a completion to block until a secondary CPU has started up, like ARM
>> do, instead of a loop of udelays.
>>
>> On Meta, SMP is really SMT, with each "CPU" being a different hardware
>> thread on the same Meta processor core, so as well as being more
>> efficient and latency friendly, using a completion prevents the bogomips
>> of the secondary CPU from being drastically skewed every time by the
>> execution of the tight in-cache udelay loop on the other CPU.
>
> The least of our worries is the correctness of bogomips. But if that
> patch makes bogomips less bogus while fixing a real issue, please add
> my
>
> Acked-by: Thomas Gleixner <tglx@...utronix.de>

Yes, it's not so much the absolute value, but that secondary cpus will
have a bogomips around 25% higher than the boot cpu, even though they
should be about equal (ignoring the effects of other cpus). If I
understand correctly, all else being equal this would make udelay
block for a different amount of time on different cpus (although I
haven't tried measuring it).

James
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/