Date:   Wed, 26 Jan 2022 15:51:36 +0100
From:   Juergen Gross <jgross@...e.com>
To:     minyard@....org
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Possible reproduction of CSD locking issue

On 26.01.22 14:56, Corey Minyard wrote:
> On Wed, Jan 26, 2022 at 07:08:22AM +0100, Juergen Gross wrote:
>> On 25.01.22 19:27, Corey Minyard wrote:
>>> We have a customer that had been seeing CSD lock issues on a Centos 7
>>> kernel (unfortunately).  I couldn't find anything or any kernel changes
>>> that might fix it, so I was consdering it was the CSD locking issue you
>>> have been chasing for a while.
>>
>> Is this on bare metal or in a virtualized environment?
> 
> This is bare metal.
> 
> I do think I know what happened.  Here's my analysis...
> 
> csd: Detected non-responsive CSD lock (#1) on CPU#3, waiting 5000000042 ns for CPU#55 flush_tlb_func+0x0/0xb0(0xffff8e0b3e2afbe8).
>   csd: CSD lock (#1) unresponsive.
>   csd: cnt(0000000): 0000->0000 queue
>   csd: cnt(0000001): ffff->0037 idle
> 
> The above means that these events weren't seen, I think.  We can
> ignore them in any case.
> 
>   csd: cnt(63d8dd8): 0003->0037 ipi
>   csd: cnt(63d8dd9): 0003->0037 ping
>   csd: cnt(63d8dda): 0003->ffff pinged
> 
> This is a little confusing.  The first two lines have to be from
> __smp_call_single_queue_debug.  The last line has to be from
> smp_call_function_many.  But you never see the pinged from
> __smp_call_single_queue_debug.

Be careful here. For each event-type/cpu combination there is only one
entry saved. It is still possible to see some events more than once,
as each entry holds information of two events (the location _where_
the entry is stored is determining what just happened, and the _data_
stored at this position tells us what happened one event before that).

So any "missing" entry which type/cpu combination is seen further down
in the trace _might_ be just overwritten by the later entry.

I've used this scheme instead of simple trace buffers as on a large
system any buffer would be overwritten multiple times before the CSD
timeout is over. This way at least the trace entries of the cpus
having problems (the sender and/or the receiver) are still available.

After having found a local reproducer for my problem I could enhance
the patches by using small arrays instead of single entries for each
trace entry location, producing better traces with less "missing"
entries. I didn't post those patches, as they were heavily
intertwined with hypervisor patches for producing traces covering
both, kernel and hypervisor events.

> 
>   csd: cnt(63d8dea): 0035->0037 pinged
> 
> The tail of CPU 53 sending an IPI to CPU 55 in
> __smp_call_single_queue_debug.
> 
>   csd: cnt(63d8deb): ffff->0037 gotipi
>   csd: cnt(63d8dec): ffff->0037 handle
>   csd: cnt(63d8ded): ffff->0037 dequeue (src CPU 0 == empty)
>   csd: cnt(63d8dee): ffff->0037 hdlend (src CPU 0 == early)
> 
> CPU 55 is handling the IPI(s) it was sent earlier.

Right. Everything up to here is completely fine and can be ignored.

> 
>   csd: cnt(63d8e1f): 0003->0037 queue
>   csd: cnt(63d8e20): 0003->0037 ipi
>   csd: cnt(63d8e21): 0003->0037 ping
> 
> In __smp_call_single_queue_debug CPU 3 sends another message to
> CPU 55 and sends an IPI.  But there should be a pinged entry
> after this.
> 
>   csd: cnt(63d8e22): 0003->0037 queue
>   csd: cnt(63d8e23): 0003->0037 noipi

This is interesting. Those are 5 consecutive entries without any
missing in between (see the counter values). Could it be that after
the ping there was an interrupt and the code was re-entered for
sending another IPI? This would clearly result in a hang as seen.


Juergen

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