On Tue, Jun 22, 2010 at 09:27:35PM +0800, Jan Kara wrote: > On Tue 22-06-10 12:07:27, Wu Fengguang wrote: > > On Mon, Jun 21, 2010 at 03:42:39PM +0200, Jan Kara wrote: > > > On Fri 18-06-10 12:21:37, Peter Zijlstra wrote: > > > > On Thu, 2010-06-17 at 20:04 +0200, Jan Kara wrote: > > > > > + if (bdi_stat(bdi, BDI_WRITTEN) >= bdi->wb_written_head) > > > > > + bdi_wakeup_writers(bdi); > > > > > > > > For the paranoid amongst us you could make wb_written_head s64 and write > > > > the above as: > > > > > > > > if (bdi_stat(bdi, BDI_WRITTEN) - bdi->wb_written_head > 0) > > > > > > > > Which, if you assume both are monotonic and wb_written_head is always > > > > within 2^63 of the actual bdi_stat() value, should give the same end > > > > result and deal with wrap-around. > > > > > > > > For when we manage to create a device that can write 2^64 pages in our > > > > uptime :-) > > > OK, the fix is simple enough so I've changed it, although I'm not > > > paranoic enough ;) (I actually did the math before writing that test). > > > > a bit more change :) > > > > type: > > > > - u64 wb_written_head > > + s64 wb_written_head > > > > resetting: > > > > - bdi->wb_written_head = ~(u64)0; > > + bdi->wb_written_head = 0; > > > > setting: > > > > bdi->wb_written_head = bdi_stat(bdi, BDI_WRITTEN) + wc->written; > > + bdi->wb_written_head |= 1; > > > > testing: > > > > if (bdi->wb_written_head && > > bdi_stat(bdi, BDI_WRITTEN) - bdi->wb_written_head > 0) > > > > This avoids calling into bdi_wakeup_writers() pointlessly when no one > > is being throttled (which is the normal case). > Actually, I've already changed wb_written_head to s64. I kept setting > wb_written_head to s64 maximum. That also avoids calling into > bdi_wakeup_writers() unnecessarily... Ah OK, I forgot bdi_stat() calls percpu_counter_read_positive() which is always in range [0, s64 max]. Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>