On 2018-03-01 00:19, Rasmus Villemoes wrote: > I noticed that offsetof(struct filename, iname) is actually 28 on 64 > bit platforms, so we always pass an unaligned pointer to > strncpy_from_user. This is mostly a problem for those 64 bit platforms > without HAVE_EFFICIENT_UNALIGNED_ACCESS, but even on x86_64, unaligned > accesses carry a penalty. > > A user-space microbenchmark doing nothing but strncpy_from_user from the > same (aligned) source string runs about 5% faster when the destination > is aligned. That number increases to 20% when the string is long > enough (~32 bytes) that we cross a cache line boundary - that's for > example the case for about half the files a "git status" in a kernel > tree ends up stat'ing. > > This won't make any real-life workloads 5%, or even 1%, faster, but path > lookup is common enough that cutting even a few cycles should be > worthwhile. So ensure we always pass an aligned destination pointer to > strncpy_from_user. Instead of explicit padding, simply swap the refcnt > and aname members, as suggested by Al Viro. polite ping...
