Re: [kvm-devel] [PATCH 08 of 11] anon-vma-rwsem

[Christoph L. <cla...@sg...>]

On Wed, 14 May 2008, Linus Torvalds wrote:

> One thing to realize is that most of the time (read: pretty much *always*) 
> when we have the problem of wanting to sleep inside a spinlock, the 
> solution is actually to just move the sleeping to outside the lock, and 
> then have something else that serializes things.

The problem is that the code in rmap.c try_to_umap() and friends loops 
over reverse maps after taking a spinlock. The mm_struct is only known 
after the rmap has been acccessed. This means *inside* the spinlock.

That is why I tried to convert the locks to scan the revese maps to 
semaphores. If that is done then one can indeed do the callouts outside of 
atomic contexts.

> Can it be done? I don't know. But I do know that I'm unlikely to accept a 
> noticeable slowdown in some very core code for a case that affects about 
> 0.00001% of the population. In other words, I think you *have* to do it.

With larger number of processor semaphores make a lot of sense since the 
holdoff times on spinlocks will increase. If we go to sleep then the 
processor can do something useful instead of hogging a cacheline.

A rw lock there can also increase concurrency during reclaim espcially if 
the anon_vma chains and the number of address spaces mapping a page is 
high.