Messages in this thread | | | Subject | Re: [patch] latency tracer, 2.6.15-rc7 | From | Lee Revell <> | Date | Sun, 01 Jan 2006 00:46:11 -0500 |
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On Sat, 2005-12-31 at 12:14 -0800, Paul E. McKenney wrote: > So it seems to me that Linus's patch is part of the solution, but > needs to also have a global component, perhaps as follows: > > if (unlikely(rdp->count > 100)) { > set_need_resched();
In fact neither of these patches helps, because these RCU callbacks run from a tasklet in softirq context, and softirqs are not preemptible (unless they are running in threads, see below):
_------=> CPU# / _-----=> irqs-off | / _----=> need-resched || / _---=> hardirq/softirq <---------*** ||| / _--=> preempt-depth |||| / ||||| delay cmd pid ||||| time | caller \ / ||||| \ | / <idle>-0 0d.s2 1us : __trace_start_sched_wakeup (try_to_wake_up) <idle>-0 0d.s2 1us : __trace_start_sched_wakeup <<...>-21330> (73 0) <idle>-0 0d.s. 2us : wake_up_process (process_timeout) <idle>-0 0d.s. 3us : tasklet_action (__do_softirq) <idle>-0 0d.s. 4us : rcu_process_callbacks (tasklet_action) <idle>-0 0d.s. 5us : __rcu_process_callbacks (rcu_process_callbacks) <idle>-0 0d.s. 5us : rcu_check_quiescent_state (__rcu_process_callbacks) <idle>-0 0d.s. 6us : __rcu_process_callbacks (rcu_process_callbacks) <idle>-0 0d.s. 7us : rcu_check_quiescent_state (__rcu_process_callbacks) <idle>-0 0d.s. 7us : rcu_do_batch (__rcu_process_callbacks) <idle>-0 0d.s. 8us : dst_rcu_free (rcu_do_batch)
Fortunately softirq preemption is one of the simplest parts of the -rt patch - since the kernel already runs all softirqs in threads under heavy load, it's simply a matter of adding a .config option to always do that.
Linus, would you accept CONFIG_PREEMPT_SOFTIRQS to always run softirqs in threads (default N of course, it certainly has a slight throughput cost) for mainline if Ingo were to submit it?
Lee
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