On Thu, Aug 13, 2020 at 05:11:20PM +1000, Daniel Axtens wrote: > When returning results to userspace, do_sys_poll repeatedly calls > put_user() - once per fd that it's watching. > > This means that on architectures that support some form of > kernel-to-userspace access protection, we end up enabling and disabling > access once for each file descripter we're watching. This is inefficent > and we can improve things by batching the accesses together. > > To make sure there's not too much happening in the window when user > accesses are permitted, we don't walk the linked list with accesses on. > This leads to some slightly messy code in the loop, unfortunately. > > Unscientific benchmarking with the poll2_threads microbenchmark from > will-it-scale, run as `./poll2_threads -t 1 -s 15`: > > - Bare-metal Power9 with KUAP: ~48.8% speed-up > - VM on amd64 laptop with SMAP: ~25.5% speed-up > > Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx> Seem like this could simply use a copy_to_user to further simplify things? Also please don't pointlessly add overly long lines.
