On Wed, Nov 24, 2010 at 09:14:37PM +0800, Wu Fengguang wrote:
> On Wed, Nov 24, 2010 at 08:50:47PM +0800, Peter Zijlstra wrote:
> > On Wed, 2010-11-24 at 20:10 +0800, Wu Fengguang wrote:
> > > > > + /*
> > > > > + * When there lots of tasks throttled in balance_dirty_pages(), they
> > > > > + * will each try to update the bandwidth for the same period, making
> > > > > + * the bandwidth drift much faster than the desired rate (as in the
> > > > > + * single dirtier case). So do some rate limiting.
> > > > > + */
> > > > > + if (jiffies - bdi->write_bandwidth_update_time < elapsed)
> > > > > + goto snapshot;
> > > >
> > > > Why this goto snapshot and not simply return? This is the second call
> > > > (bdi_update_bandwidth equivalent).
> > >
> > > Good question. The loop inside balance_dirty_pages() normally run only
> > > once, however wb_writeback() may loop over and over again. If we just
> > > return here, the condition
> > >
> > > (jiffies - bdi->write_bandwidth_update_time < elapsed)
> > >
> > > cannot be reset, then future bdi_update_bandwidth() calls in the same
> > > wb_writeback() loop will never find it OK to update the bandwidth.
> >
> > But the thing is, you don't want to reset that, it might loop so fast
> > you'll throttle all of them, if you keep the pre-throttle value you'll
> > eventually pass, no?
>
> It (let's name it A) only resets the _local_ vars bw_* when it's sure
> by the condition
>
> (jiffies - bdi->write_bandwidth_update_time < elapsed)
this will be true if someone else has _done_ overlapped estimation,
otherwise it will equal:
jiffies - bdi->write_bandwidth_update_time == elapsed
Sorry the comment needs updating.
Thanks,
Fengguang
> that someone else (name B) has updated the _global_ bandwidth in the
> time range we planned. So there may be some time in A's range that is
> not covered by B, but sure the range is not totally bypassed without
> updating the bandwidth.
>
> > > It does assume no races between CPUs.. We may need some per-cpu based
> > > estimation.
> >
> > But that multi-writer race is valid even for the balance_dirty_pages()
> > call, two or more could interleave on the bw_time and bw_written
> > variables.
>
> The race will only exist in each task's local vars (their bw_* will
> overlap). But the update bdi->write_bandwidth* will be safeguarded
> by the above check. When the task is scheduled back, it may find
> updated write_bandwidth_update_time and hence give up his estimation.
> This is rather tricky..
>
> Thanks,
> Fengguang
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