Re: [PATCH v3 resend 4/6] fs: Move call_rcu() to call_rcu_lazy() in some paths

[Paul E. McKenney]

On Thu, Aug 18, 2022 at 09:21:56PM -0400, Joel Fernandes wrote:
> On Thu, Aug 18, 2022 at 7:05 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, Aug 18, 2022 at 1:23 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > [Sorry, adding back the CC list]
> > >
> > > On Mon, Aug 8, 2022 at 11:45 PM Joel Fernandes (Google)
> > > <joel@xxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > This is required to prevent callbacks triggering RCU machinery too
> > > > quickly and too often, which adds more power to the system.
> > > >
> > > > When testing, we found that these paths were invoked often when the
> > > > system is not doing anything (screen is ON but otherwise idle).
> > >
> > > Unfortunately, I am seeing a slow down in ChromeOS boot performance
> > > after applying this particular patch. It is the first time I could
> > > test ChromeOS boot times with the series since it was hard to find a
> > > ChromeOS device that runs the upstream kernel.
> > >
> > > Anyway, Vlad, Neeraj, do you guys also see slower boot times with this
> > > patch? I wonder if the issue is with wake up interaction with the nocb
> > > GP threads.
> > >
> > > We ought to disable lazy RCU during boot since it would have little
> > > benefit anyway. But I am also concerned about some deeper problem I
> > > did not catch before.
> > >
> > > I'll look into tracing the fs paths to see if I can narrow down what's
> > > causing it. Will also try a newer kernel, I am currently testing on
> > > 5.19-rc4.
> >
> > I got somewhere with this. It looks like queuing CBs as lazy CBs
> > instead of normal CBs, are triggering expedited stalls during the boot
> > process:
> >
> >   39.949198] rcu: INFO: rcu_preempt detected expedited stalls on
> > CPUs/tasks: { } 28 jiffies s: 69 root: 0x0/.
> >
> > No idea how/why lazy RCU CBs would be related to expedited GP issues,
> > but maybe something hangs and causes that side-effect.
> >
> > initcall_debug did not help, as it seems initcalls all work fine, and
> > then 8 seconds after the boot, it starts slowing down a lot, followed
> > by the RCU stall messages. As a next step I'll enable ftrace during
> > the boot to see if I can get more insight. But I believe, its not the
> > FS layer, the FS layer just triggers lazy CBs, but there is something
> > wrong with the core lazy-RCU work itself.
> >
> > This kernel is 5.19-rc4. I'll also try to rebase ChromeOS on more
> > recent kernels and debug.
> 
> More digging, thanks to trace_event= boot option , I find that the
> boot process does have some synchronous waits, and though these are
> "non-lazy", for some reason the lazy CBs that were previously queued
> are making them wait for the *full* lazy duration. Which points to a
> likely bug in the lazy RCU logic. These synchronous CBs should never
> be waiting like the lazy ones:
> 
> [   17.715904]  => trace_dump_stack
> [   17.715904]  => __wait_rcu_gp
> [   17.715904]  => synchronize_rcu
> [   17.715904]  => selinux_netcache_avc_callback
> [   17.715904]  => avc_ss_reset
> [   17.715904]  => sel_write_enforce
> [   17.715904]  => vfs_write
> [   17.715904]  => ksys_write
> [   17.715904]  => do_syscall_64
> [   17.715904]  => entry_SYSCALL_64_after_hwframe
> 
> I'm tired so I'll resume the debug later.

At times like this, I often pull the suspect code into userspace and
run it through its paces.  In this case, a bunch of call_rcu_lazy()
invocations into an empty bypass list, followed by a call_rcu()
invocation, then a check to make sure that the bypass list is no longer
lazy.

							Thanx, Paul