Messages in this thread | | | From | Linus Torvalds <> | Date | Sun, 25 Dec 2016 13:51:17 -0800 | Subject | Re: [PATCH 2/2] mm: add PageWaiters indicating tasks are waiting for a page bit |
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On Sat, Dec 24, 2016 at 7:00 PM, Nicholas Piggin <npiggin@gmail.com> 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.
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".
Regardless, I think this is worth more people looking at and testing. And merging it is probably the best way for that to happen.
Linus
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