Re: [RFC PATCH 07/18] kthread: Make iterant kthreads freezable by default

[Petr Mladek <pmladek@suse.cz>]

On Sat 2015-06-13 18:22:22, Tejun Heo wrote:
> > I try to better understand why freezer is considered to be a blunt
> > tool. Is it because it is a generic API, try_to_freeze() is put on
> > "random" locations, so that it does not define the safe point
> > precisely enough?
>
> Not that.  I don't know how to explain it better.  Hmmm... okay, let's
> say there's a shared queue Q and N users o fit.  If you wanna make Q
> empty and keep it that way for a while, the right thing to do is
> blocking new queueing and then wait till Q drains - you choke the
> entity that you wanna control.
>
> Instead of that, freezer is trying to block the "N" users part.  In
> majority of cases, it blocks enough but it's pretty difficult to be
> sure whether you actually got all N of them (as some of them may not
> involve kthreads at all or unfreezable kthreads might end up doing
> those operations too on corner cases) and it's also not that clear
> whether blocking the N users actually make Q empty.  Maybe there are
> things which can be in flight asynchronously on Q even all its N users
> are blocked.  This is inherently finicky.

I feel convinced that it does not make sense to make kthreads
freezable by default and that we should not use it when not
necessary.

Thanks a lot for patience and so detailed explanation.

Best Regards,
Petr