Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Sep 18, 2015 at 7:23 AM, Jens Axboe <axboe@fb.com> wrote:
>
> It makes no sense for preemption schedule to NOT unplug, the fact that it
> doesn't is news to me as well. It was never the intent of the
> unplug-on-schedule to NOT unplug for certain schedule out events, that seems
> like very odd behavior.

Actually, even a *full* schedule doesn't unplug, unless the process is
going to sleep. See sched_submit_work(), which  will only call the
unplugging if the process is actually going to sleep (ok, so it's a
bit subtle if you don't know the state rules, but it's the
"!tsk->state" check there)

So preemption and cond_resched() isn't _that_ odd. We've basically
treated a non-sleeping schedule as a no-op for the task work.

The thinking was probably that it might be better to delay starting
the IO in case we get scheduled back quickly, and we're obviously not
actually _sleeping_, so it's likely not too bad.

Now, that's probably bogus, and I think that we should perhaps just
make the rule be that "if we actually switch to another task, we run
blk_schedule_flush_plug()".

But it should be noted that that really *does* introduce a lot of new
potential races. Traditionally, our block layer plugging has been
entirely thread-synchronous, and would never happen asynchronously.
But with preemption, that "switch to another thread" really *does*
happen asynchronously.

So making things always happen on task switch is actually fairly
dangerous, and potentially adds the need for much more synchronization
for the IO submission.

What we possibly *could* make the scheduler rule be:

 - if it's not an actual PREEMPT_ACTIVE (ie in a random place)

 - _and_ we actually switch to another thread

 - _then_ do the whole blk_schedule_flush_plug(tsk) thing.

adding some scheduler people to the explicit cc list.

That said, the "cond_resched[_lock]()" functions currently always set
PREEMPT_ACTIVE (indirectly - they use preempt_schedule_common()), so
even though those are synchronous, right now they *look* asynchronous
to the scheduler, so we'd still have to sort that out.

Ingo/Peter/Frederic? Comments?

                        Linus