Re: RCU: rcu stall issues and an approach to the fix

["Paul E. McKenney" <paulmck@kernel.org>]

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