Re: [benchmark] 1% performance overhead of paravirt_ops on native kernels
From: Nick Piggin
Date: Wed Jun 10 2009 - 01:53:31 EST
On Tue, Jun 09, 2009 at 10:08:53AM -0700, Linus Torvalds wrote:
> On Tue, 9 Jun 2009, Nick Piggin wrote:
> > On Tue, Jun 09, 2009 at 09:26:47AM -0700, Linus Torvalds wrote:
> > >
> > >
> > > On Tue, 9 Jun 2009, Nick Piggin wrote:
> > > >
> > > > The idea seems nice but isn't the problem that kmap gives back a
> > > > basically 1st class kernel virtual memory? (ie. it can then be used
> > > > by any other CPU at any point without it having to use kmap?).
> > >
> > > No, everybody has to use kmap()/kunmap().
> > So it is strictly a bug to expose a pointer returned by kmap to
> > another CPU?
> No, not at all. The pointers are all global. They have to be, since the
> original kmap() user may well be scheduled away.
Sorry, I meant another task.
> > > The "problem" is that you could in theory run out of kmap frames, since if
> > > everybody does a kmap() in an interruptible context and you have lots and
> > > lots of threads doing different pages, you'd run out. But that has nothing
> > > to do with kmap_atomic(), which is basically limited to just the number of
> > > CPU's and a (very small) level of nesting.
> > This could be avoided with an anti-deadlock pool. If a task
> > attempts a nested kmap and already holds a kmap, then give it
> > exclusive access to this pool until it releases its last
> > nested kmap.
> We just sleep, waiting for somebody to release their. Again, that
> obviously won't work in atomic context, but it's easy enough to just have
> a "we need to have a few entries free" for the atomic case, and make it
> busy-loop if it runs out (which is not going to happen in practice
The really theoretical one (which Andrew likes complaining about) is
when *everybody* is holding a kmap and asking for another one ;)
But I think it isn't too hard to make a pool for that. And yes we'd
also need a pool for atomic kmaps as you point out.
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