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SubjectRe: [patch(?)] question wrt context switching during disk i/o
On Sat, 21 Oct 2000, Mark Hahn wrote:

> > } > bdflush is broken in current kernels. I posted to linux-mm about this,
> > } > but Rik et al haven't shown any interest. I normally see bursts of
> > } > up to around 40K cs/second when doing writes; I hacked a little
> > } > premption counter into the kernel and verified that they're practially
> > } > all bdflush...
> > }
> > There's some strangness in bdflush(). The comment says:
> >
> > /*
> > * If there are still a lot of dirty buffers around,
> > * skip the sleep and flush some more. Otherwise, we
> > * go to sleep waiting a wakeup.
> > */
> > if (!flushed || balance_dirty_state(NODEV) < 0) {
> > run_task_queue(&tq_disk);
> > schedule();
> > }
>
> to me, that says: we haven't succeeded in flushing anything,
> and our balance looks OK, so unplug the disks and sleep. this logic
> must have got accidentally inverted at some point.
>
> I think we want to unplug if we've flushed and are low on memory:

To me, whether we suceeded in flushing something is meaningless.
balance_dirty_state() tells us everything we need to know.. that
we still have too many dirty buffers despite having tried to flush.
We should then unplug to accelerate io completion. I don't see why
bdflush() even calls page_launder().. that calls wakeup_bdflush()
when it hasn't been able to free enough.

Something else that looks strange to me is wakeup_bdflush(1) usage.
In those spots, we add a SCHED_YIELD and schedule() after returning
from wakeup_bdflush().. where we've already scheduled. I moved the
SCHED_YIELD addition into the wakeup_bdflush() blocking portion and
killed the extra schedule, seemingly without doing harm.

> if (flushed && balance_dirty_state(NODEV) >= 0)
>
> > Which leads me to believe that the `<' should be either `==' or `<='. I
> > tried it with the `<=' and it doesn't seem to be so bad...Here's a patch
> > to see if it helps you?
>
> definitely. actually, I think there's some major code rot in the tuning
> logic of the VM. specifically, free_shortage() and inactive_shortage()
> both return an int that tells you how many pages we're short. negative
> returns mean we're not short. but all calls to these functions treat the
> return as a boolean! so for example, the following is wrong:
>
> if (can_get_io_locks && !launder_loop && free_shortage()) {
>
> (vmscan.c:page_launder) should be "free_shortage > 0". there are
> about a dozen other similar places, for which I'll shortly post a patch.

Looking forward to trying your patch.

-Mike

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