On Fri, Jul 23, 2021 at 08:29:53AM +0800, Boqun Feng wrote: > On Thu, Jul 22, 2021 at 04:08:06PM -0400, donghai qiao wrote: > > RCU experts, > > > > When you reply, please also keep me CC'ed. > > > > The problem of RCU stall might be an old problem and it can happen quite often. > > As I have observed, when the problem occurs, at least one CPU in the system > > on which its rdp->gp_seq falls behind others by 4 (qs). > > > > e.g. On CPU 0, rdp->gp_seq = 0x13889d, but on other CPUs, their > > rdp->gp_seq = 0x1388a1. > > > > Because RCU stall issues can last a long period of time, the number of callbacks > > in the list rdp->cblist of all CPUs can accumulate to thousands. In > > the worst case, > > it triggers panic. > > > > When looking into the problem further, I'd think the problem is related to the > > Linux scheduler. When the RCU core detects the stall on a CPU, rcu_gp_kthread > > would send a rescheduling request via send_IPI to that CPU to try to force a > > context switch to make some progress. However, at least one situation can fail > > this effort, which is when the CPU is running a user thread and it is the only > > user thread in the rq, then this attempted context switching will not happen > > immediately. In particular if the system is also configured with NOHZ_FULL for > > Correct me if I'm wrong, if a CPU is solely running a user thread, how > can that CPU stall RCU? Because you need to be in a RCU read-side > critical section to stall RCU. Or the problem you're talking here is > about *recovering* from RCU stall? Excellent point, Boqun! Donghai, have you tried reproducing this using a kernel built with CONFIG_RCU_EQS_DEBUG=y? Thanx, Paul > Regards, > Boqun > > > the CPU and as long as the user thread is running, the forced context > > switch will > > never happen unless the user thread volunteers to yield the CPU. I think this > > should be one of the major root causes of these RCU stall issues. Even if > > NOHZ_FULL is not configured, there will be at least 1 tick delay which can > > affect the realtime kernel, by the way. > > > > But it seems not a good idea to craft a fix from the scheduler side because > > this has to invalidate some existing scheduling optimizations. The current > > scheduler is deliberately optimized to avoid such context switching. So my > > question is why the RCU core cannot effectively update qs for the stalled CPU > > when it detects that the stalled CPU is running a user thread? The reason > > is pretty obvious because when a CPU is running a user thread, it must not > > be in any kernel read-side critical sections. So it should be safe to close > > its current RCU grace period on this CPU. Also, with this approach we can make > > RCU work more efficiently than the approach of context switch which needs to > > go through an IPI interrupt and the destination CPU needs to wake up its > > ksoftirqd or wait for the next scheduling cycle. > > > > If my suggested approach makes sense, I can go ahead to fix it that way. > > > > Thanks > > Donghai
