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. CPUs that are idle for long periods of time can fall back much farther. I have seen systems with CPUs having rdp->gp_seq thousands of grace periods behind. So yes, this can happen when there are RCU CPU stall warnings, but it can also happen other ways. > 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. That is quite true. > 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 > 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. If you have not yet read through Documentation/RCU/stallwarn.rst, please do so. There are many potential underlying causes of RCU CPU stall warnings, most of which are simply bugs that need to be fixed. One common example bug is a very long in-kernel loop that is missing a cond_resched() -- it is after all hard to provide 500-millisecond response time when your kernel has a 21-second tight loop. Now, if you are seeing RCU CPU stall warnings for no apparent reason, let's take a look and find root cause. Thanx, Paul
