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

[Joel Fernandes]

On 8/18/2022 10:35 PM, Paul E. McKenney wrote:
> 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.

Thanks a lot for this great debug idea, I will look into it.

Thanks,

 - Joel