Re: [Xen-devel] [PATCH 1/8] cpupools: fix state when downing a CPU failed

[Juergen Gross <jgross@xxxxxxxx>]

On 16/07/18 11:17, Jan Beulich wrote:
>>>> On 13.07.18 at 11:02, <jgross@xxxxxxxx> wrote:
>> On 11/07/18 14:04, Jan Beulich wrote:
>>> While I've run into the issue with further patches in place which no
>>> longer guarantee the per-CPU area to start out as all zeros, the
>>> CPU_DOWN_FAILED processing looks to have the same issue: By not zapping
>>> the per-CPU cpupool pointer, cpupool_cpu_add()'s (indirect) invocation
>>> of schedule_cpu_switch() will trigger the "c != old_pool" assertion
>>> there.
>>>
>>> Clearing the field during CPU_DOWN_PREPARE is too early (afaict this
>>> should not happen before cpu_disable_scheduler()). Clearing it in
>>> CPU_DEAD and CPU_DOWN_FAILED would be an option, but would take the same
>>> piece of code twice. Since the field's value shouldn't matter while the
>>> CPU is offline, simply clear it in CPU_ONLINE and CPU_DOWN_FAILED, but
>>> only for other than the suspend/resume case (which gets specially
>>> handled in cpupool_cpu_remove()).
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>> ---
>>> TBD: I think this would better call schedule_cpu_switch(cpu, NULL) from
>>>      cpupool_cpu_remove(), but besides that - as per above - likely
>>>      being too early, that function has further prereqs to be met. It
>>>      also doesn't look as if cpupool_unassign_cpu_helper() could be used
>>>      there.
>>>
>>> --- a/xen/common/cpupool.c
>>> +++ b/xen/common/cpupool.c
>>> @@ -778,6 +778,8 @@ static int cpu_callback(
>>>      {
>>>      case CPU_DOWN_FAILED:
>>>      case CPU_ONLINE:
>>> +        if ( system_state <= SYS_STATE_active )
>>> +            per_cpu(cpupool, cpu) = NULL;
>>>          rc = cpupool_cpu_add(cpu);
>>
>> Wouldn't it make more sense to clear the field in cpupool_cpu_add()
>> which already is testing system_state?
> 
> Hmm, this may be a matter of taste: I consider the change done here
> a prereq to calling the function in the first place. As said in the
> description, I actually think this should come earlier, and it's just that
> I can't see how to cleanly do so.
> 
>> Modifying the condition in cpupool_cpu_add() to
>>
>>   if ( system_state <= SYS_STATE_active )
>>
>> at the same time would have the benefit to catch problems in case
>> suspending cpus is failing during SYS_STATE_suspend (I'd expect
>> triggering the first ASSERT in schedule_cpu_switch() in this case).
> 
> You mean the if() there, not the else? If so - how would the "else"
> body then ever be reached? IOW if anything I could only see the
> "else" to become "else if ( system_state <= SYS_STATE_active )".

Bad wording on my side.

I should have written "the condition in cpupool_cpu_add() should match
if ( system_state <= SYS_STATE_active )."

So: "if ( system_state > SYS_STATE_active )", as the test is for the
other case.


Juergen

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