On Sat, Sep 12, 2020 at 1:59 PM James Bottomley
<James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Sat, 2020-09-12 at 10:59 -0700, Linus Torvalds wrote:
> [...]
> > Any other suggestions than those (a)-(d) ones above?
>
> What about revert and try to fix the outliers? Say by having a timer
> set when a process gets put to sleep waiting on the page lock.
No timer needed, I suspect.
I tried to code something like this up yesterday (hjmm. Thursday?) as
a "hybrid" scheme, where we'd start out with the old behavior and let
people unfairly get the lock while there were waiters, but when a
waiter woke up and noticed that it still couldn't get the lock, _then_
it would stat using the new scheme.
So still be unfair for a bit, but limit the unfairness so that a
waiter won't lose the lock more than once (but obviously while the
waiter initially slept, _many_ other lockers could have come through).
I ended up with a code mess and gave up on it (it seemed to just get
all the complications from the old _and_ the new model), but maybe I
should try again now that I know what went wrong last time. I think I
tried too hard to actually mix the old and the new code.
(If I tried again, I'd not try to mix the new and the old code, I'd
make the new one start out with a non-exclusive wait - which the code
already supports for that whole "wait for PG_writeback to end" as
opposed to "wait to take PG_lock" - and then turn it into an exclusive
wait if it fails.. That might work out better and not mix entirely
different approaches).
Linus
