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@xxxxxxxxxx
> --
> Michal Hocko
> SUSE Labs
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