On Fri, Sep 01, 2017 at 10:58:43AM +0300, Amir Goldstein wrote: > On Thu, Aug 31, 2017 at 11:10 PM, Brian Foster <bfoster@xxxxxxxxxx> wrote: > > On Thu, Aug 31, 2017 at 10:20:19PM +0300, Amir Goldstein wrote: > >> On Thu, Aug 31, 2017 at 7:39 PM, Brian Foster <bfoster@xxxxxxxxxx> wrote: > ... > >> > If we do something > >> > like the above, I wonder if that means we could wait for the submit == > >> > complete if we observe submit was bumped since it was initially sampled > >> > above (rather than issue another flush, which would be necessary if a > >> > submit hadn't occurred))..? > >> > > >> > If we do end up with something like this, I think it's a bit cleaner to > >> > stuff the counter(s) in the xfs_buftarg structure and update them from > >> > the generic buffer submit/completion code based on XBF_FLUSH. FWIW, I > >> > suspect we could also update said counter(s) from > >> > xfs_blkdev_issue_flush(). > >> > > >> > >> I think what you are suggesting is to optimize more cases which are > >> not optimized now. That is probably possible, but also more complicated > >> to get right and not sure if the workloads that gain from this are important > >> enough. > >> > > > > Not necessarily. I'm just suggesting that the code could be factored > > more generically/elegantly such that the logic is easier to follow. That > > may facilitate optimizing more cases, but that's a secondary benefit. In > > practice, the log buffer code is the only place we actually set > > XBF_FLUSH, for example. > > > > I guess that makes sense. > Although it is going to end up with more code, so if we are not going for > optimization for more cases (i.e. subsequent fdatasync) we should consider > if the extra code is worth it. > Incrementally more than Christoph's patch. I don't think that's an issue. > > > >> If I am not mistaken the way to fix the current optimization is to record > >> the last SYNC_DONE lsn (which is sort of what Christoph suggested) > >> and the last WANY_SYNC|ACTIVE lsn. > >> After file_write_and_wait() need to save pre_sync_lsn and before > >> return need to make sure that post_sync_lsn >= pre_sync_lsn or > >> issue a flush. > >> > > > > Perhaps, but I'm not quite following what you mean by pre/post LSNs. > > Note that I believe log buffers can complete out of order, if that is > > relevant here. Either way, this still seems like underhanded logic > > IMO... > > > > If the requirement is a simple "issue a flush if we can't detect that > > one has submitted+completed on this device since our writeback > > completed" rule, why intentionally obfuscate that with internal log > > buffer state such as log buffer header LSN and log state machine values? > > Just track flush submission/completions as you suggested earlier and the > > fsync logic is much easier to follow. Then we don't need to work > > backwards from the XFS logging infrastructure just to try and verify > > whether a flush has occurred in all cases. :) > > > > Your argument makes a lot of sense. I'm just trying to be extra cautious > and looking for a small step solution. As Darrick wrote.. "safety first" :) > Sure. FWIW, I think your patch suits that purpose as it basically disables the shady bits of the optimization. It looks like it's already in Darrick's for-next branch and Cc'd to stable as well. A cleaner rework of this mechanism can come after, hopefully restore the full optimization safely and perhaps clean out the whole log_flushed thing. Brian > Amir. > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html
