Re: GFP_REPEAT usage in vhost_net_open resp. vhost_vsock_dev_open

On Wed, Jan 04, 2017 at 04:08:00PM +0100, Michal Hocko wrote:

Hi Michael,
I am currently cleaning up opencoded kmalloc with vmalloc fallback users
[1] and my current kvmalloc_node helper doesn't support GFP_REPEAT
because there are no users which would need it. At least that's what I
thought until I've encountered vhost_vsock_dev_open resp.
vhost_vsock_dev_open which are trying to use GFP_REPEAT for kmalloc.
23cc5a991c7a ("vhost-net: extend device allocation to vmalloc") explains
the motivation as follows:
"
As vmalloc() adds overhead on a critical network path, add __GFP_REPEAT
to kzalloc() flags to do this fallback only when really needed.
"

I am wondering whether vmalloc adds more overhead than GFP_REPEAT

Yes but the GFP_REPEAT overhead is during allocation time.
Using vmalloc means all accesses are slowed down.
Allocation is not on data path, accesses are.

which
can get pretty costly for order-4 allocation which will be used here as
struct vhost_net seems to be 36104 (at least in with my config). Have
you ever measured the difference?

I think it was measureable.

So I am just trying to understand whether we should teach kvmalloc_node
to understand GFP_REPEAT or there is no strong reason to keep the repeat
flag.

[1] http://lkml.kernel.org/r/20170102133700.1734-1-mhocko@kernel.org
-- 
Michal Hocko
SUSE Labs