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SubjectRe: scheduling problem?
On Tue, Jan 02, 2001 at 01:02:30PM -0800, Linus Torvalds wrote:
>
>
> On Tue, 2 Jan 2001, Andrea Arcangeli wrote:
>
> > On Tue, Jan 02, 2001 at 11:02:41AM -0800, Linus Torvalds wrote:
> > > What does the system feel like if you just change the "sleep for bdflush"
> > > logic in wakeup_bdflush() to something like
> > >
> > > wake_up_process(bdflush_tsk);
> > > __set_current_state(TASK_RUNNING);
> > > current->policy |= SCHED_YIELD;
> > > schedule();
> > >
> > > instead of trying to wait for bdflush to wake us up?
> >
> > My bet is a `VM: killing' message.
>
> Maybe in 2.2.x, yes.
>
> > Waiting bdflush back-wakeup is mandatory to do write throttling correctly. The
> > above will break write throttling at least unless something foundamental is
> > changed recently and that doesn't seem the case.
>
> page_launder() should wait for the dirty pages, and that's not something
> 2.2.x ever did.

In late 2.2.x we have sync_page_buffers too but I'm not sure how well it
behaves when the whole MM is costantly kept totally dirty and we don't have
swap. Infact also the 2.4.x implementation:

static void sync_page_buffers(struct buffer_head *bh, int wait)
{
struct buffer_head * tmp = bh;

do {
struct buffer_head *p = tmp;
tmp = tmp->b_this_page;
if (buffer_locked(p)) {
if (wait > 1)
__wait_on_buffer(p);
} else if (buffer_dirty(p))
ll_rw_block(WRITE, 1, &p);
} while (tmp != bh);
}

won't cope with the memory totally dirty. It will make the buffer from dirty to
locked then it will wait I/O completion at the second pass, but it
won't try again to free the page for the third time (when the page is finally
freeable):

if (wait) {
sync_page_buffers(bh, wait);
/* We waited synchronously, so we can free the buffers. */
if (wait > 1 && !loop) {
loop = 1;
goto cleaned_buffers_try_again;
}

Probably not a big deal.

The real point is that even if try_to_free_buffers will deal perfectly with the
VM totally dirty we'll end waiting I/O completion in the wrong place.
setiathome will end waiting I/O completion instead of `cp`. It's not setiathome
but `cp` that should do write throttling. And `cp` will block again very soon
even if setiathome blocks too. The whole point is that the write throttling
must happen in balance_dirty(), _not_ in sync_page_buffers().

Infact from 2.2.19pre2 there's a wait_io per-bh bitflag that remembers when a
dirty bh is very old and it doesn't get flushed away automatically (from
either kupdate or kflushd). So we don't block in sync_page_buffers until it's
necessary to avoid hurting non-IO apps when I/O is going on.

> NOTE! I think that throttling writers is fine and good, but as it stands
> now, the dirty buffer balancing will throttle anybody, not just the
> writer. That's partly because of the 2.4.x mis-feature of doing the

How can it throttle everybody and not only the writers? _Only_ the
writers calls balance_dirty.

> balance_dirty call even for previously dirty buffers (fixed in my tree,
> btw).

Yes I seen, people overwriting dirty data was blocking too, that was
not necessary, but they were still writers.

> It's _really_ bad to wait for bdflush to finish if we hold on to things
> like the superblock lock - which _does_ happen right now. That's why I'm
> pretty convinced that we should NOT blindly do the dirty balance in
> "mark_buffer_dirty()", but instead at more well-defined points (in places
> like "generic_file_write()", for example).

Right way to avoid blocking with lock helds is to replace mark_buffer_dirty
with __mark_buffer_dirty() and to call balance_dirty() later when the locks are
released. That's why it's exported to modules. Everybody is always been
allowed to optimize away the mark_buffer_dirty(), it's just that nobody did
that yet. I think it's useful to keep providing an interface that does the
write throttling automatically.

Andrea
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