linux-kernel - Re: Active waiting with yield()

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Message-ID: <20081114212102.2313a8a3@lxorguk.ukuu.org.uk>
Date:	Fri, 14 Nov 2008 21:21:02 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, rml@...h9.net,
	Alasdair G Kergon <agk@...hat.com>,
	Milan Broz <mbroz@...hat.com>
Subject: Re: Active waiting with yield()

> If I have a race condition that gets triggered just for one user in the 
> world when repeatedly loading & unloading a driver for an hour, and I use 
> yield() to solve it, what's wrong with it? A wait queue increases cache 
> footprint for every user. (even if I use preallocated hashed wait queue, 
> it still eats a cacheline to access it and find out that it's empty)

Reread what I wrote.

You don't need to use a hashed queue, you don't need to even reference
the queue in the normal case. You cost is one variable, which you can
probably sensibly locate, and a predicted jump.

For that you get proper sleeping behaviour, CPU used by other tasks or
guests and you don't hang on hard real time tasks.
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