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Subject: Re: svn commit: r308424 - head/sys/arm/broadcom/bcm2835
To: John Baldwin <jhb@freebsd.org>
References: <201611071738.uA7HceYu045944@repo.freebsd.org>
 <3214848.geWV8qu7rM@ralph.baldwin.cx>
 <5c170b6b-1189-4712-ffdd-653cc6b8c59e@selasky.org>
 <2064206.nvhhXHc9vW@ralph.baldwin.cx>
Cc: Oleksandr Tymoshenko <gonzo@freebsd.org>, src-committers@freebsd.org,
 svn-src-all@freebsd.org, svn-src-head@freebsd.org,
 Marshall Kirk McKusick <mckusick@mckusick.com>
From: Hans Petter Selasky <hps@selasky.org>
Message-ID: <6bce6a4b-d782-77af-8515-7243eab38cf3@selasky.org>
Date: Thu, 10 Nov 2016 08:52:07 +0100
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Hi,

On 11/09/16 17:32, John Baldwin wrote:
> It isn't predicted and avoided by turnstiles.  In particular, the cv_signal has
> no idea what lock is associated, so it can't try to do something "smart" and
> move the thread from the sleepq to the lockq.  You will pay the double context
> switch.

Maybe this is an idea for a turnstile optimisation? In another embedded 
OS based on FreeBSD I did exactly this. If someone is sleeping through 
cv_wait() the lock it is sleeping on, if different from NULL, is stored 
in the sleepqueue. When cv_signal() is invoked we simply check if the 
saved lock is owned by the caller and then don't do the pre-emption, 
because we predict it will be locked by cv_wait() before returning.

--HPS