On Fri, Oct 27, 2023 at 11:17:41PM +0100, Joao Martins wrote: > On 27/10/2023 15:41, Peter Xu wrote: > > On Fri, Oct 27, 2023 at 09:58:03AM +0100, Joao Martins wrote: > >> On 26/10/2023 21:07, Peter Xu wrote: > >>> On Thu, Oct 26, 2023 at 08:33:13PM +0100, Joao Martins wrote: > >>>> Sure. For the fourth patch, feel free to add Suggested-by and/or a Link, > >>>> considering it started on the other patches (if you also agree it is > >>>> right). The > >>>> patches ofc are enterily different, but at least I like to believe the > >>>> ideas > >>>> initially presented and then subsequently improved are what lead to the > >>>> downtime > >>>> observability improvements in this series. > >>> > >>> Sure, I'll add that. > >>> > >>> If you like, I would be definitely happy to have Co-developed-by: with > >>> you, > >>> if you agree. > >> > >> Oh, that's great, thanks! > > > > Great! I apologize on not asking already before a formal patch is post. > > > >> > >>> I just don't know whether that addressed all your need, and > >>> I need some patch like that for our builds. > >> > >> I think it achieves the same as the other series. Or rather it > >> re-implements it > >> but with less compromise on QAPI and made the tracepoints more 'generic' > >> to even > >> other usecases and less specific to the 'checkpoint breakdown'. Which > >> makes the > >> implementation simpler (like we don't need that array storing the > >> checkpoint > >> timestamps) given that it's just tracing and not for QAPI. > > > > Yes. Please also feel free to have a closer look on the exact checkpoints > > in that patch. I just want to make sure that'll be able to service the > > same as the patch you proposed, but with tracepoints, and I don't miss > > anything important. > > > > The dest checkpoints are all new, I hope I nailed them all right as we > > would expect. > > > Yeah, it looks like so; but I am no master of postcopy so I don't have quite > the > cirurgical eye there. > > Perhaps for the loadvm side, 'precopy-bh-enter' suggests some ambiguity (given > that precopy happens on both source and destination?). Perhaps being explicit > in > either incoming-bh-enter? or even prefix checkpoint names by 'source' on > source > side for example to distinguish?
I don't think src has a bottom half; if not considering cleanup_bh as part of migration at all. Destination's BH is a major part of migration. In all cases, let me prefix them with "src"/"dst" then, hopefully even clearer. > > > For src checkpoints, IIRC your patch explicitly tracked return path closing > > while patch 4 only made it just part of final enclosure; the 4th checkpoint > > is after non-iterable sent, until 5th to be the last "downtime-end". It can > > cover more than "return path close": > > > > qemu_savevm_state_complete_precopy_non_iterable <--- 4th checkpoint > > qemu_fflush (after non-iterable sent) > > close_return_path_on_source > > cpu_throttle_stop > > qemu_mutex_lock_iothread > > migration_downtime_end <---- 5th checkpoint > > > > If you see fit or necessary, I can, for example, also add one more > > checkpoint right after close return path. I just didn't know whether it's > > your intention to explicitly check that point. Just let me know if so. > > > It was put there because it was a simple catch-all from the source PoV on how > much the destination is taking. Of course bearing in mind that without > return-path then such metric/tracepoint is not available. On the other hand, > with > migration_downtime_end I think we have the equivalent in that resume > checkpoint. > So we just need to make the diff between that and the non-iterable and I think > we have the same things as I was looking for (even more I would say). > > > Also on whether you prefer to keep a timestamp in the tracepoint itself; > > I only use either "log" or "dtrace"+qemu-trace-stap for tracing: both of > > them contain timestamps already. But I can also add the timestamps > > (offseted by downtime_start) if you prefer. > > > Perhaps it is easy to wrap the checkpoint tracepoint in its own function to > allow extension of something else e.g. add the timestamp (or any other data > into > the checkpoints) or do something in a particular checkpoint of the migration. > The timing function from qemu would give qemu own idea of time (directly > correlable with the downtime metric that applications consume) which would be > more consistent? though I am at too minds on this whether to rely on tracing > stamps or align with what's provided to applications. Yes it should be more consistent. Let me add them into the checkpoints. Thanks, -- Peter Xu
