On Tue, Nov 03 2015, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, Nov 3, 2015 at 1:41 AM, Rasmus Villemoes > <linux@xxxxxxxxxxxxxxxxxx> wrote: >> >> I'm sure I've missed something, hence the RFC. But if not, there's >> probably also a few memsets which become redundant. And the >> __set_close_on_exec part should probably be its own patch... > > The patch looks fine to me. I'm not sure the __set_close_on_exec part > even makes sense, because if you set that bit, it usually really *is* > clear before, so testing it beforehand is just pointless. And if > somebody really keeps setting the bit, they are doing something stupid > anyway.. So that's true for the lifetime of a single fd where no-one of course does fcntl(fd, FD_CLOEXEC) more than once. But the scenario I was thinking of was when fds get recycled. open(, O_CLOEXEC) => 5, close(5), open(, O_CLOEXEC) => 5; in that case, letting the close_on_exec bit keep its value avoids dirtying the cache line on all subsequent allocations of fd 5 (for example, had Eric's app been using *_CLOEXEC for all its open's, socket's etc. there wouldn't have been any gain by adding the conditional to __clear_close_on_exec, but I'd expect to see a similar gain by doing the symmetric thing). Again, this is assuming that almost all fd allocations either do or do not apply CLOEXEC - after a while, ->close_on_exec would reach a steady-state where no bits get flipped anymore. The "usually really *is* clear" only holds when we do "bother clearing close_on_exec bit for unused fds", which is what I suggest we don't :-) I don't think either state of the bit in close_on_exec is more or less 'up-to-date' when its buddy in open_fds is not set. Rasmus -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
