Re: [PATCH 06/33] autonuma: teach gup_fast about pmd_numa

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Oct 11, 2012 at 01:22:55PM +0100, Mel Gorman wrote:
> On Thu, Oct 04, 2012 at 01:50:48AM +0200, Andrea Arcangeli wrote:
> > In the special "pmd" mode of knuma_scand
> > (/sys/kernel/mm/autonuma/knuma_scand/pmd == 1), the pmd may be of numa
> > type (_PAGE_PRESENT not set), however the pte might be
> > present. Therefore, gup_pmd_range() must return 0 in this case to
> > avoid losing a NUMA hinting page fault during gup_fast.
> > 
> 
> So if gup_fast fails, presumably we fall back to taking the mmap_sem and
> calling get_user_pages(). This is a heavier operation and I wonder if the
> cost is justified. i.e. Is the performance loss from using get_user_pages()
> offset by improved NUMA placement? I ask because we always incur the cost of
> taking mmap_sem but only sometimes get it back from improved NUMA placement.
> How bad would it be if gup_fast lost some of the NUMA hinting information?

Good question indeed. Now, I agree it wouldn't be bad to skip NUMA
hinting page faults in gup_fast for no-virt usage like
O_DIRECT/ptrace, but the only problem is that we'd lose AutoNUMA on
the memory touched by the KVM vcpus.

I've been also asked if the vhost-net kernel thread (KVM in kernel
virtio backend) will be controlled by autonuma in between
use_mm/unuse_mm and answer is yes, but to do that, it also needs
this. (see also the flush to task_autonuma_nid and mm/task statistics in
unuse_mm to reset it back to regular kernel thread status,
uncontrolled by autonuma)

$ git grep get_user_pages
tcm_vhost.c:            ret = get_user_pages_fast((unsigned long)ptr, 1, write, &page);
vhost.c:        r = get_user_pages_fast(log, 1, 1, &page);