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SubjectRe: [RFC][PATCH 16/16] writeback: prevent unnecessary bdi threads wakeups
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On Sun, 2010-07-18 at 03:45 -0400, Christoph Hellwig wrote:
> > + if (wb_has_dirty_io(wb) && dirty_writeback_interval) {
> > + unsigned long wait;
> >
> > - wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
> > - schedule_timeout(wait_jiffies);
> > + wait = msecs_to_jiffies(dirty_writeback_interval * 10);
> > + schedule_timeout(wait);
>
> No need for a local variable. If you want to shorten things a bit a
> schedule_timeout_msecs helper in generic code would be nice, as there
> are lots of patterns like this in various kernel threads.

OK, do you want me to ignore the 80-lines limitation or you want me
to add schedule_timeout_msecs() as a part of this patch series?

> > void __mark_inode_dirty(struct inode *inode, int flags)
> > {
> > + bool wakeup_bdi;
> > struct super_block *sb = inode->i_sb;
> > + struct backing_dev_info *uninitialized_var(bdi);
>
> Just initialize wakeup_bdi and bdi here - a smart compiler will defer
> them until we need them, and it makes the code a lot easier to read, as
> well as getting rid of the uninitialized_var hack.

OK.

> > + /*
> > + * If this is the first dirty inode for this bdi, we
> > + * have to wake-up the corresponding bdi thread to make
> > + * sure background write-back happens later.
> > + */
> > + if (!wb_has_dirty_io(&bdi->wb) &&
> > + bdi_cap_writeback_dirty(bdi))
> > + wakeup_bdi = true;
>
> How about redoing this as:
>
> if (bdi_cap_writeback_dirty(bdi)) {
> WARN(!test_bit(BDI_registered, &bdi->state),
> "bdi-%s not registered\n", bdi->name);
>
> /*
> * If this is the first dirty inode for this
> * bdi, we have to wake-up the corresponding
> * flusher thread to make sure background
> * writeback happens later.
> */
> if (!wb_has_dirty_io(&bdi->wb))
> wakeup_bdi = true;
> }

OK.

> > + if (wakeup_bdi) {
> > + bool wakeup_default = false;
> > +
> > + spin_lock(&bdi->wb_lock);
> > + if (unlikely(!bdi->wb.task))
> > + wakeup_default = true;
> > + else
> > + wake_up_process(bdi->wb.task);
> > + spin_unlock(&bdi->wb_lock);
> > +
> > + if (wakeup_default)
> > + wake_up_process(default_backing_dev_info.wb.task);
>
> Same comment about just keeping wb_lock over the
> default_backing_dev_info wakup as for one of the earlier patches applies
> here.

I just figured that I have to add 'trace_writeback_nothread(bdi, work)'
here, just like in 'bdi_queue_work()'. I'd feel safer to call tracer
outside the spinlock. What do you think?

> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -326,7 +326,7 @@ static unsigned long bdi_longest_inactive(void)
> > unsigned long interval;
> >
> > interval = msecs_to_jiffies(dirty_writeback_interval * 10);
> > - return max(5UL * 60 * HZ, wait_jiffies);
> > + return max(5UL * 60 * HZ, interval);
>
> So previously we just ignored interval here?

Yes, my fault, thanks for catching.

--
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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