Re: [PATCH] x86/console: process softirqs between warning prints

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.02.2022 13:07, Roger Pau Monné wrote:
> On Thu, Feb 17, 2022 at 12:54:57PM +0100, Jan Beulich wrote:
>> On 17.02.2022 09:28, Roger Pau Monne wrote:
>>> Process softirqs while printing end of boot warnings. Each warning can
>>> be several lines long, and on slow consoles printing multiple ones
>>> without processing softirqs can result in the watchdog triggering:
>>>
>>> (XEN) [   22.277806] ***************************************************
>>> (XEN) [   22.417802] WARNING: CONSOLE OUTPUT IS SYNCHRONOUS
>>> (XEN) [   22.556029] This option is intended to aid debugging of Xen by 
>>> ensuring
>>> (XEN) [   22.696802] that all output is synchronously delivered on the 
>>> serial line.
>>> (XEN) [   22.838024] However it can introduce SIGNIFICANT latencies and 
>>> affect
>>> (XEN) [   22.978710] timekeeping. It is NOT recommended for production use!
>>> (XEN) [   23.119066] ***************************************************
>>> (XEN) [   23.258865] Booted on L1TF-vulnerable hardware with 
>>> SMT/Hyperthreading
>>> (XEN) [   23.399560] enabled.  Please assess your configuration and choose 
>>> an
>>> (XEN) [   23.539925] explicit 'smt=<bool>' setting.  See XSA-273.
>>> (XEN) [   23.678860] ***************************************************
>>> (XEN) [   23.818492] Booted on MLPDS/MFBDS-vulnerable hardware with 
>>> SMT/Hyperthreading
>>> (XEN) [   23.959811] enabled.  Mitigations will not be fully effective.  
>>> Please
>>> (XEN) [   24.100396] choose an explicit smt=<bool> setting.  See XSA-297.
>>> (XEN) [   24.240254] *************************************************(XEN) 
>>> [   24.247302] Watchdog timer detects that CPU0 is stuck!
>>> (XEN) [   24.386785] ----[ Xen-4.17-unstable  x86_64  debug=y  Tainted:   C 
>>>    ]----
>>> (XEN) [   24.527874] CPU:    0
>>> (XEN) [   24.662422] RIP:    e008:[<ffff82d04025b84a>] 
>>> drivers/char/ns16550.c#ns16550_tx_ready+0x3a/0x90
>>>
>>> Fixes: ee3fd57acd ('xen: add warning infrastructure')
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> ---
>>>  xen/common/warning.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/xen/common/warning.c b/xen/common/warning.c
>>> index 0269c6715c..e6e1404baf 100644
>>> --- a/xen/common/warning.c
>>> +++ b/xen/common/warning.c
>>> @@ -30,6 +30,7 @@ void __init warning_print(void)
>>>      {
>>>          printk("%s", warnings[i]);
>>>          printk("***************************************************\n");
>>> +        process_pending_softirqs();
>>>      }
>>
>> To be honest, I'm not convinced. This gets us pretty close to needing
>> to process softirqs after _every_ line of output. If a console is this
>> slow, the problem imo needs dealing with there (and according to irc
>> we appear on a helpful track there already), not by sprinkling more
>> process_pending_softirqs() all over the code.
> 
> There could be up to 20 warning messages of unknown length, so I do
> think we need some processing of softirqs in the loop.

Hmm, yes, you have a point there.

> If you prefer I could do softirq processing only every 4? messages,
> but I'm not sure it's worth it. Also there's no indication of the
> length of messages, so IMO it's safer to just process softirqs after
> each.

No, that's indeed not worth it.

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan