On Mon, Apr 23, 2018 at 09:47:42AM -0400, Steven Rostedt wrote: > On Mon, 23 Apr 2018 05:40:00 -0700 > "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote: > > > > I'm confused.. why is having this conditional on TRACEPOINT_BENCHMARK a > > > sane idea? > > > > Because the TRACEPOINT_BENCHMARK tests are insane, so a similar > > level of insanity is required to make things work. Plus having this > > be unconditional would not be good for performance, as 0day has been > > telling me frequently over the past couple of years. > > Just for some context. The tracepoint benchmark (which should never be > enabled in any production machine), will start a thread when the > benchmark trace event is enabled. This thread will never exit (until > the trace event is disabled), and does a benchmark loop and constantly > calls "cond_resched()" to allow other tasks to run. The point is, this > thread will never have a quiescent state for task_rcu, unless we tell > rcu that cond_resched() is a quiescent state. But this is only required > because the tracepoint benchmark has this nasty thread, that is only > used for debugging and benchmarking the tracepoint (during development). > > I also suggested having a direct call into RCU from the thread to tell > RCU that it entered a quiescent state, but Paul didn't like that idea > as it caused the tracepoint benchmark to call too deep into RCU > internals. > > http://lkml.kernel.org/r/20180227153646.gd3...@linux.vnet.ibm.com
That thread using cond_resched_task_rcu_qs() seems like a _lot_ better than having cond_resched() semantics change depending on random !scheduler config parameters.
