On 1/30/23 13:57, Jens Axboe wrote:
This does cause about a 2.7% regression for me, using O_DIRECT on a raw block device. Looking at a perf diff, here's the top: +2.71% [kernel.vmlinux] [k] mod_node_page_state +2.22% [kernel.vmlinux] [k] iov_iter_extract_pages and these two are gone: 2.14% [kernel.vmlinux] [k] __iov_iter_get_pages_alloc 1.53% [kernel.vmlinux] [k] iov_iter_get_pages rest is mostly in the noise, but mod_node_page_state() sticks out like a sore thumb. They seem to be caused by the node stat accounting done in gup.c for FOLL_PIN.Confirmed just disabling the node_stat bits in mm/gup.c and now the performance is back to the same levels as before. An almost 3% regression is a bit hard to swallow...
This is something that we say when adding pin_user_pages_fast(), yes. I doubt that I can quickly find the email thread, but we measured it and weren't immediately able to come up with a way to make it faster. At this point, it's a good time to consider if there is any way to speed it up. But I wanted to confirm that you're absolutely right: the measurement sounds about right, and that's also the hotspot that we say, too.
thanks, -- John Hubbard NVIDIA
