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

[Juergen Gross <jgross@xxxxxxxx>]

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?

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).

It should be noted that this scenario is theoretical only, as today
the CPU_DOWN_FAILED case can't happen in the suspend case.


Juergen

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