On Sun, 25 Nov 2018, Linus Torvalds wrote: > On Sat, Nov 24, 2018 at 7:21 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > > > Linus, I'm addressing this patch to you because I see from Tim Chen's > > thread that it would interest you, and you were disappointed not to > > root cause the issue back then. I'm not pushing for you to fast-track > > this into 4.20-rc, but I expect Andrew will pick it up for mmotm, and > > thence linux-next. Or you may spot a terrible defect, but I hope not. > > The only terrible defect I spot is that I wish the change to the > 'lock' argument in wait_on_page_bit_common() came with a comment > explaining the new semantics.o Thanks a lot for looking through it. > > The old semantics were somewhat obvious (even if not documented): if > 'lock' was set, we'd make the wait exclusive, and we'd lock the page > before returning. That kind of matches the intuitive meaning for the > function prototype, and it's pretty obvious in the callers too. > > The new semantics don't have the same kind of really intuitive > meaning, I feel. That "-1" doesn't mean "unlock", it means "drop page > reference", so there is no longer a fairly intuitive and direct > mapping between the argument name and type and the behavior of the > function. > > So I don't hate the concept of the patch at all, but I do ask to: > > - better documentation. > > This might not be "documentation" at all, maybe that "lock" > variable should just be renamed (because it's not about just locking > any more), and would be better off as a tristate enum called > "behavior" that has "LOCK, DROP, WAIT" values? Agreed, an enum should be best. I'll try it out now, and see what naming fits - I'm not all that keen on "LOCK", since (like many of the existing comments) it forgets that PG_locked is only one of the flags that comes here. Admittedly, the only other is PG_writeback, and nobody wants exclusive behavior on that one, but... > > - while it sounds likely that this is indeed the same issue that > plagues us with the insanely long wait-queues, it would be *really* > nice to have that actually confirmed. I echo your words: it would be *really* nice. We do already know that this patch is good for many problem loads, but it would be very satisfying if it could also wrap that discussion from last year. > > Does somebody still have access to the customer load that triggered > the horrible scaling issues before? Kan? Tim? > > In particular, on that second issue: the "fixes" that went in for the > wait-queues didn't really fix any real scalability problem, it really > just fixed the excessive irq latency issues due to the long traversal > holding a lock. > > If this really fixes the fundamental issue, that should show up as an > actual performance difference, I'd expect.. I guess so, though it might be more convincing to add a hack to suppress the bookmarking (e.g. #define WAITQUEUE_WALK_BREAK_CNT (INT_MAX - 1)) when trying out the put_and_wait patch - if they can persuade the customer to go back in time on this, which is asking a lot. Not that I have any ambitions to do away with the bookmarking myself; though I do have several reservations about the way it works out (that I'd rather go into some other time). > > End result: I like and approve of the patch, but I'd like it a lot > more if the code behavior was clarified a bit, and I'd really like to > close the loop on that old nasty page wait queue issue... Thanks! Hugh
