On Thu, Oct 11, 2012 at 11:34:14AM -0400, Paul Gortmaker wrote: > [Re: Crap, ksoftirqd/0 looping forever in softirq] On 11/10/2012 (Thu 10:21) Paul Gortmaker wrote: > > > [Crap, ksoftirqd/0 looping forever in softirq] On 10/10/2012 (Wed 20:16) Paul Gortmaker wrote: > > > > > Seeing the above message, I added a trivial change to print out the > > > Mask and Pending, and I consistently saw it was (both p/m) in every > > > case `was __IRQF_TIMER (0x200). > > > > Of cource it helps if I look up the value in the right list. > > I guess the ffs had me thinking "bits", instead of looking up > > the value in the softirq enum list. > > > > When I add a change to record the last vecs used and dump them > > out when the Crap message is triggered (using softirq_to_name > > instead of looking things up in random tables!) I get this: > > > > [ 20.887137] Crap, ksoftirqd/0 looping forever in softirq > > [ 20.887150] Last: RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU > > [ 20.920168] Crap, ksoftirqd/0 looping forever in softirq > > [ 20.920179] Last: RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU > > [ 20.954200] Crap, ksoftirqd/0 looping forever in softirq > > [ 20.954213] Last: RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU RCU > > Here is a trace, which I think shows us that we call the > RCU softirq, which eventually calls invoke_rcu_core, which > is just "raise_softirq(RCU_SOFTIRQ);" so the loop which is > testing on current->softirqs_raised will never see it zero. > > We clear it on entry with: > current->softirqs_raised &= ~mask; > but after we call: > do_single_softirq(i, need_rcu_bh_qs); > it will be re-armed by invoke_rcu_core. > > 0xffffffff81044d9d in __raise_softirq_irqoff (nr=9) at kernel/softirq.c:608 > 0xffffffff81044e29 in raise_softirq_irqoff (nr=<optimized out>) at kernel/softirq.c:618 > 0xffffffff810452e1 in raise_softirq (nr=<optimized out>) at kernel/softirq.c:744 > 0xffffffff810bd4e1 in invoke_rcu_core () at kernel/rcutree.c:1878 > rcu_do_batch (rdp=0xffff880017c0bf80, rsp=<optimized out>) at kernel/rcutree.c:1635 > invoke_rcu_callbacks (rdp=0xffff880017c0bf80, rsp=0xffffffff81c2b140) at kernel/rcutree.c:1870 > __rcu_process_callbacks (rsp=0xffffffff81c2b140) at kernel/rcutree.c:1842 > rcu_process_callbacks (unused=<optimized out>) at kernel/rcutree.c:1854 > 0xffffffff8104466b in handle_softirq (need_rcu_bh_qs=1, cpu=0, vec_nr=9) at kernel/softirq.c:149 > do_single_softirq (need_rcu_bh_qs=1, which=9) at kernel/softirq.c:457 > do_current_softirqs (need_rcu_bh_qs=1) at kernel/softirq.c:500 > > Does that sound right? (Following up on IRC discussion) Yep, if a given CPU has a large number of callbacks, it will execute them in batches of 10 (controlled by rcutree.blimit module parameter). It will re-raise RCU_SOFTIRQ at the end of each batch. The point is to allow other softirq handlers to run. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
