Messages in this thread | | | Date | Fri, 22 Oct 2010 15:35:13 +0800 | From | Lai Jiangshan <> | Subject | Re: [PATCH 2/2 v2] rcu,cleanup: simplify the code when cpu is dying |
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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.
> > 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.
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().
Thanks. Lai
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