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SubjectRe: 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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