On Fri, May 12, 2017 at 10:11:14AM -0500, Christoph Lameter wrote: > On Fri, 12 May 2017, Marcelo Tosatti wrote: > > > > A bit confused by this one. The vmstat worker is already disabled if there > > > are no updates. Also the patches by Chris Metcalf on data plane mode add a > > > prctl to quiet the vmstat workers. > > > > > > Why do we need more than this? > > > > If there are vmstat statistic updates on a given CPU, and you don't > > want intervention from the vmstat worker, you change the behaviour of > > stat data collection to directly write to the global structures (which > > disables the performance optimization of collecting data in per-cpu > > counters). > > Hmmm.... Ok. That is going to be expensive if you do this for each > individual vmstat update.
In our case, vmstat updates are very rare (CPU is dominated by DPDK). > > This way you can disable vmstat worker (because it causes undesired > > latencies), while allowing vmstatistics to function properly. > > Best then to run the vmstat update mechanism when you leave kernel mode to > get all the updates in one go. Again, vmstat updates are very rare (CPU is dominated by DPDK). > > The prctl from Chris Metcalf patchset allows one to disable vmstat > > worker per CPU? If so, they replace the functionality of the patch > > "[patch 3/3] MM: allow per-cpu vmstat_worker configuration" > > of the -v2 series of my patchset, and we can use it instead. > > > > Is it integrated already? > > The data plane mode patches disables vmstat processing by updating the > vmstats immediately if necessary and switching off the kworker thread. OK this is what my patch set is doing. > So the kworker wont be running until the next time statistics are checked > by the shepherd task from a remote cpu. We don't want kworker thread to ever run. > If the counters have been updated > then the shepherd task will reenable the kworker. This is already merged > and has been working for a long time. Data plan mode has not been merged > yet but the infrastructure in vmstat.c is there because NOHZ needs it too. OK. > > See linux/vmstat.c:quiet_vmstat() > > It would be easy to add a /proc file that allows the quieting of the > vmstat workers for a certain cpu. Just make it call the quiet_vmstat() on > the right cpu. > > This will quiet vmstat down. The shepherd task will check the stats in 2 > second intervals and will then reenable when necessasry. > > Note that we already are updating the global structures directly if the > differential gets too high. Reducing the differential may get you what you > want. Yes, we reduce the differential to 1 (== direct updates to global structures). OK, i'll check if the patches from Chris work for us and then add Tested-by on that. Thanks.
