On Thu, 20 Dec 2018 14:11:35 -0800 "Jonathan Lemon" <jonathan.le...@gmail.com> wrote: > On 20 Dec 2018, at 5:03, Jesper Dangaard Brouer wrote: > [...] > > I don't like adding this in the hot-path. Instead we could move this > > to the page alloc slow-path, and reject allocating pages with > > pgmemalloc in the first place. > > No real objection to that - but then why bother with pfmemalloc? If the > driver can't obtain pages for emergency use, then they might as well > not exist.
I've changed my mind. There is an interesting opportunity in allowing pfmemalloc-pages to be used by the driver. (So, I'm saying I'm okay with adding this to the hot-path. And this hopefully doesn't affect performance (too much), as page_is_pfmemalloc() is reading from the same cache-line). The opportunity is that XDP can handle/operate at wirespeed. We could allow XDP to get this info (simply via helper call, so we don't affect users not using this). When seeing PFMEMALLOC, which indicate a bad situation is occurring really soon, then we can react at a earlier stage (spending less cycles on reacting). One idea is to reduce-size of XDP frame, and use XDP_TX to send-back the frame to the sender as a congestion/drop notification, which inform sender to slowdown. If this is incast happening within the same data-center then the XDP_TX-feedback can reach the sender really fast. One example of such an approach: https://youtu.be/BO0QhaxBRr0 This is one example of how XDP can allow us to do stuff that was not possible before... -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
