On Wed, Jun 15, 2016 at 03:31:51PM -0400, Waiman Long wrote:

On 06/15/2016 01:43 PM, Peter Zijlstra wrote:

quoted

On Tue, Jun 14, 2016 at 06:48:08PM -0400, Waiman Long wrote:

quoted

even the reduced maximum of about 16k (32-bit) or 1G (64-bit) should
be more than enough for the foreseeable future.

So what happens if I manage to create 16k+ threads on my 32bit kernel
and get them all to do mmap() or so at the same time.

That doesn't seem too far fetched.

Then again, with double that (the current limit) that doesn't seem
impossible either.

To hit the limit, we need to have all the threads calling down_write() at
exactly the same instance in time which, I think, is pretty hard to do.
Also, I don't believe you will ever see a 16k-cpu massive SMP system running
on 32-bit kernel. I can imagine such a system running on 64-bit kernel, but
certainly not 32-bit.

Ah, so I thought we kept the WRITE_BIAS while blocking, which we don't.

But if they all get preempted before we undo the WRITE_BIAS then 1 CPU
will be able to trigger this. However utterly unlikely.