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

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Message-ID: <Pine.LNX.4.64.0811141412290.12992@hs20-bc2-1.build.redhat.com>
Date:	Fri, 14 Nov 2008 14:34:11 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
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()



On Fri, 14 Nov 2008, Alan Cox wrote:

> > * driver unload --- check the count of outstanding requests and call 
> > yield() repeatedly until it goes to zero, then unload.
> 
> Use a wakeup when the request count hits zero
> 
> > * reduced size of data structures (and reduced cache footprint for the hot 
> > path that actually processes requests)
> 
> The CPU will predict the non-wakeup path if that is normal. You can even
> make the wakeup use something like
> 
> 	if (exiting & count == 0)
> 
> to get the prediction righ
> 
> > The downside of yield is slower unloading of the driver by few tens of 
> > miliseconds, but the user doesn't really care about fractions of a second 
> > when unloading drivers.
> 
> And more power usage, plus extremely rude behaviour when virtualising.

How these unlikely cases can be rude?

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)

Mikulas

> There are cases you have to use cpu_relax/spins or yield simply because
> the hardware doesn't feel like providing interrupts when you need them,
> but for the general case its best to use proper sleeping.
> 
> Remember also you can use a single wait queue for an entire driver for
> obscure happenings like unloads.
> 
> Alan
> 
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