On Tue, 2018-01-23 at 09:42 -0800, Linus Torvalds wrote: > On Tue, Jan 23, 2018 at 8:57 AM, Paolo Abeni <pab...@redhat.com> wrote: > > > > > Or is it that the workqueue execution is simply not yielding for some > > > reason? > > > > It's like that. > > > > I spent little time on it, so I haven't many data point. I'll try to > > investigate the scenario later this week. > > Hmm. workqueues seem to use cond_resched_rcu_qs(), which does a > cond_resched() (and a RCU quiescent note). > > But I wonder if the test triggers the "lets run lots of workqueue > threads", and then the single-threaded user space just gets blown out > of the water by many kernel threads. Each thread gets its own "fair" > amount of CPU, but..
Niklas suggested a possible relation with CONFIG_IRQ_TIME_ACCOUNTING=y and indeed he was right. The patched kernel under test had CONFIG_IRQ_TIME_ACCOUNTING set, and very little CPU time was accounted to the kworker: [2125 is the relevant kworker's pid] grep sum_exec_runtime /proc/2125/sched; sleep 10; grep sum_exec_runtime /proc/2125/sched se.sum_exec_runtime : 13408.239286 se.sum_exec_runtime : 13456.907197 despite such process was processing a lot of packets and basically burning a CPU. Switching CONFIG_IRQ_TIME_ACCOUNTING off I see the expected behaviour: top reports that the user space process and kworker share the CPU almost fairly and the user space process is able to receive a reasonable amount of packets. Paolo