On Sun, 25 Dec 2016 13:51:17 -0800 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Sat, Dec 24, 2016 at 7:00 PM, Nicholas Piggin <npiggin@xxxxxxxxx> wrote: > > Add a new page flag, PageWaiters, to indicate the page waitqueue has > > tasks waiting. This can be tested rather than testing waitqueue_active > > which requires another cacheline load. > > Ok, I applied this one too. I think there's room for improvement, but > I don't think it's going to help to just wait another release cycle > and hope something happens. > > Example room for improvement from a profile of unlock_page(): > > 46.44 │ lock andb $0xfe,(%rdi) > 34.22 │ mov (%rdi),%rax > > this has the old "do atomic op on a byte, then load the whole word" > issue that we used to have with the nasty zone lookup code too. And it > causes a horrible pipeline hickup because the load will not forward > the data from the (partial) store. > > Its' really a misfeature of our asm optimizations of the atomic bit > ops. Using "andb" is slightly smaller, but in this case in particular, > an "andq" would be a ton faster, and the mask still fits in an imm8, > so it's not even hugely larger. I did actually play around with that. I could not get my skylake to forward the result from a lock op to a subsequent load (the latency was the same whether you use lock ; andb or lock ; andl (32 cycles for my test loop) whereas with non-atomic versions I was getting about 15 cycles for andb vs 2 for andl. I guess the lock op drains the store queue to coherency and does not allow forwarding so as to provide the memory ordering semantics. > But it might also be a good idea to simply use a "cmpxchg" loop here. > That also gives atomicity guarantees that we don't have with the > "clear bit and then load the value". cmpxchg ends up at 19 cycles including the initial load, so it may be worthwhile. Powerpc has a similar problem with doing a clear_bit; test_bit (not the size mismatch, but forwarding from atomic ops being less capable). Thanks, Nick -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>