Messages in this thread | | | Date | Fri, 22 Oct 2010 09:10:32 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 2/2 v2] rcu,cleanup: simplify the code when cpu is dying |
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On Fri, Oct 22, 2010 at 03:35:13PM +0800, Lai Jiangshan wrote: > On 10/21/2010 03:25 AM, Paul E. McKenney wrote: > > On Wed, Oct 20, 2010 at 02:13:06PM +0800, Lai Jiangshan wrote: > >> When we handle cpu notify DYING, the whole system is stopped except > >> current CPU, so we can touch any data, and we remove the orphan_cbs_tail > >> and send the callbacks to the dest CPU directly. > > > > Queued along with the documentation/comment patch below, thank you!!! > > (Of course, please let me know if you see problems with my patch.) > > Your patch is good for me, please queue it, thanks.
Very good, done!
> > One remaining question... You use cpumask_any() to select the destination > > CPU, which sounds good until you look at its definition. The problem > > is that cpumask_any() always chooses the lowest-numbered online CPU. > > So imagine a (say) 64-CPU system and suppose that CPU 0 remains online. > > Suppose further that the other 63 CPUs each execute some workload that > > generates lots of RCU callbacks (perhaps creating then removing a large > > source tree), and periodically go offline and come back online. > > > > All of the RCU callbacks from CPUs 1-63 could easily end up getting > > dumped onto CPU 0's callback lists. It is easy to imagine that CPU 0 > > might not be able to invoke these callbacks as fast as the other CPUs > > could generate them. > > > > Or am I missing something? > > It happens in the worst case. It may also happen before this patch. > > Before this patch, the callback move to the receive-CPU who handles the CPU_DEAD > event, and this CPU may be always cpu#0 in the worst case, the problem happens. > > And it's not help if I introduce a choose_receive_cpu_very_smart(), > Suppose further that the other 63 CPUs each execute some workload that > generates lots of RCU callbacks (perhaps creating then removing a large > source tree), and periodically go offline and come back online. In worse > case, in some period, there is only cpu#0 online, So all of the RCU callbacks > from CPUs 1-63 could easily end up getting dumped onto CPU 0's callback lists. > It is easy to imagine that CPU 0 might not be able to invoke these callbacks > as fast as the other CPUs could generate them. > > Another bad case(it may happens without this patch/with this patch > /with choose_receive_cpu_very_smart()): > Live-Lock, suppose cpu#A and cpu#B periodically go offline and come > back online, the callback may be moved from A to B and from B to A > periodically, no callback is handled.
Agreed, it -could- happen before in the worst case, but it required very bad luck for the task adopting the callbacks to always be the same. In contrast, cpumask_any() will always pick on the same CPU.
That said, your approach called out below is intriguing...
> To fix these problems(it does really very hardly happen), we must force > all adopted callbacks are called before next cpu-offline. so we can use > work_on_cpu() or rcu_barrier() to do this. To make the code simpler, I will > use rcu_barrier().
This approach is nice, but requires extensive testing -- a start would be a script that randomly onlines and offlines CPUs while rcutorture is running in the background. If you have not already done so, could you please give this an over-the-weekend test on the largest system you have access to?
Thanx, Paul
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