From: Christoph Hellwig > Sent: 13 August 2020 08:32 > > 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? That would copy out 8 bytes/fd instead of 2. So a slight change for 32bit kernels. However the 'user copy hardening' checks that copy_to_user() does probably make a measurable difference. > Also please don't pointlessly add overly long lines. Shorten the error label? David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
