* Peter Zijlstra <pet...@infradead.org> wrote:
> On Mon, Sep 30, 2013 at 09:13:52AM -0700, Linus Torvalds wrote:
>
> > So unlike a lot of other "let's try to make our locking fancy" that I
> > dislike because it tends to hide the fundamental problem of
> > contention, the rwlock patches make me go "those actually _fix_ a
> > fundamental problem".
>
> So here I'm slightly disagreeing; fixing a fundamental problem would be
> coming up a better anon_vma management that doesn't create such immense
> chains.
So, I think the fundamental problem seems to be that when rwsems are
applied to this usecase, they still don't perform as well as a primitive
rwlock.
That means that when rwsems cause a context switch it is a loss, while an
rwlock_t burning CPU time by looping around is a win. I'm not sure it's
even about 'immense chains' - if those were true then context-switching
should actually improve performance by allowing other work to continue
while the heavy chains are processed.
Alas that's not what happens!
Or is AIM7 essentially triggering a single large lock? I doubt that's the
case though.
> Its still the same lock, spinlock or not.
>
> And regardless of if we keep anon_vma lock a rwsem or not; I think we
> should merge those rwsem patches as they do improve the lock
> implementation and the hard work has already been done.
That I mostly agree with, except that without a serious usecase do we have
a guarantee that bugs in fancies queueing in rwsems gets ironed out?
Thanks,
Ingo
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