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    SubjectRe: Active waiting with yield()
    On Fri, 14 Nov 2008, Peter Zijlstra wrote:

    > On Fri, 2008-11-14 at 14:34 -0500, Mikulas Patocka wrote:
    > >
    > > 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)
    >
    > Depending on the situation, yield() might be a NOP and therefore not
    > wait at all and possibly lock up the machine.
    >
    > Consider the task in question to be the highest priority RT task on the
    > system, you doing: while (!condition) yield(); will lock up the system,
    > because whatever is to make condition true will never get a chance to
    > run (not considering SMP).
    >
    > Clearly you don't understand it, please refrain from using it. Use
    > regular condition variables (waitqueues).

    So, use msleep(1) instead of yield() ?

    Mikulas

    > The rules about yield are:
    >
    > - You're likely wrong, don't use it.
    > - Seriously, you don't need it.
    > - If you still think you do, goto 1.
    >
    > In all of the kernel there is 1 valid use (and it might only be in the
    > -rt kernel - didn't check mainline recently).
    >
    > The _ONLY_ valid use case of yield(), is if you have two equal priority
    > FIFO threads that co-depend. And that situation is almost always
    > avoidable.
    >


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