Re: [PATCH v5 00/12] fold per-CPU vmstats remotely

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Mar 14, 2023 at 09:59:37AM -0300, Marcelo Tosatti wrote:
> > Why is that a problem? Out-of-sync stats shouldn't cause major problems.
> > Or can they?
> 
> Consider SCHED_FIFO task that is polling the network queue (say
> testpmd).
> 
> 	do {
> 	 	if (net_registers->state & DATA_AVAILABLE) {
> 			process_data)();
> 		}
> 	 } while (!stopped);
> 
> Since this task runs at SCHED_FIFO priority, kworker won't 
> be scheduled to run (therefore per-CPU vmstats won't be
> flushed to global vmstats). 
> 
> Or, if testpmd runs at SCHED_OTHER, then the work item to
> flush per-CPU vmstats causes
> 
> 	testpmd -> kworker
> 	kworker: flush per-CPU vmstats
> 	kworker -> testpmd
> 
> And this might cause undesired latencies to the packets being
> processed by the testpmd task.

This problem is unrelated to the kswapd problem, but both are addressed
by the patchset.