Re: [PATCH 1/2] sched/wait: Break up long wake list walk

[Linus Torvalds]

On Tue, Aug 22, 2017 at 11:56 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Won't we now prematurely terminate the wait when we get a spurious
> wakeup?

I think there's two answers to that:

 (a) do we even care?

 (b) what spurious wakeup?

The "do we even care" quesiton is because wait_on_page_bit by
definition isn't really serializing. And I'm not even talking about
memory ordering, altough that is true too - I'm talking just
fundamentally, that by definition when we're not locking, by the time
wait_on_page_bit() returns to the caller, it could obviously have
changed again.

So I think wait_on_page_bit() is by definition not really guaranteeing
that the bit really is clear. And I don't think we have really have
cases that matter.

But if we do - say, 'fsync()' waiting for a page to wait for
writeback, where would you get spurious wakeups from? They normally
happen either  when we have nested waiting (eg a page fault happens
while we have other wait queues active), and I'm not seeing that being
an issue here.

That said, I do think we might want to perhaps make a "careful" vs
"just wait a bit" version of this if the patch works out.

The patch is primarily for testing this particular case. I actually
think it's probably ok in general, but maybe there really is some
special case that could have multiple wakeup sources and it needs to
see *this* particular one.

(We could perhaps handle that case by checking "is the wait-queue
empty now" instead, and just get rid of the re-arming, not break out
of the loop immediately after the io_schedule()).

                 Linus