Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] fs/select.c: batch user writes in do_sys_poll | Date | Thu, 13 Aug 2020 12:23:48 +0000 |
| |
From: Daniel Axtens > Sent: 13 August 2020 12:37 > > >> Seem like this could simply use a copy_to_user to further simplify > >> things? > > > > I'll benchmark it and find out. > > I tried this: > > for (walk = head; walk; walk = walk->next) { > - struct pollfd *fds = walk->entries; > - int j; > - > - for (j = 0; j < walk->len; j++, ufds++) > - if (__put_user(fds[j].revents, &ufds->revents)) > - goto out_fds; > + if (copy_to_user(ufds, walk->entries, > + sizeof(struct pollfd) * walk->len)) > + goto out_fds; > + ufds += walk->len; > } > > With that approach, the poll2 microbenchmark (which polls 128 fds) is > about as fast as v1. > > However, the poll1 microbenchmark, which polls just 1 fd, regresses a > touch (<1% - ~2%) compared to the current code, although it's largely > within the noise. Thoughts?
Is that with or without 'user copy hardening'? Or use __copy_to_user() to skip all that 'crap'.
David
- Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
| |