Re: [PATCH tip/core/rcu 04/20] metag: Use common outgoing-CPU-notification code

Paul E. McKenney
2015-03-10T16:59:50Z

On Tue, Mar 10, 2015 at 03:30:42PM +0000, James Hogan wrote:
> Hi Paul,
>
> On 03/03/15 17:42, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> >
> > This commit removes the open-coded CPU-offline notification with new
> > common code. This change avoids calling scheduler code using RCU from
> > an offline CPU that RCU is ignoring. This commit is compatible with
> > the existing code in not checking for timeout during a prior offline
> > for a given CPU.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: James Hogan <james.hogan@imgtec.com>
> > Cc: <linux-metag@vger.kernel.org>
>
> I gave this a try via linux-next, but unfortunately it causes the
> following warning every time a CPU goes down:
> META213-Thread0 DSP [LogF] CPU1: unable to kill

That is certainly not what I had in mind, thank you for finding this!

> If I add printks, I see that the state on entry to both cpu_wait_death
> and cpu_report_death is already CPU_POST_DEAD, suggesting that it hasn't
> changed from its initial value.
>
> Should arches other than x86 now be calling cpu_set_state_online()? The
> patchlet below seems to resolve it for Meta (not sure if that is the
> best place in the startup sequence to do it, perhaps it doesn't matter).
>
> diff --git a/arch/metag/kernel/smp.c b/arch/metag/kernel/smp.c
> index ac3a199e33e7..430e379ec71f 100644
> --- a/arch/metag/kernel/smp.c
> +++ b/arch/metag/kernel/smp.c
> @@ -383,6 +383,7 @@ asmlinkage void secondary_start_kernel(void)
> * OK, now it's safe to let the boot CPU continue
> */
> set_cpu_online(cpu, true);
> + cpu_set_state_online(cpu);
> complete(&cpu_running);
>
> /*
>
> Looking at the comment before cpu_set_state_online:
> > /*
> > * Mark the specified CPU online.
> > *
> > * Note that it is permissible to omit this call entirely, as is
> > * done in architectures that do no CPU-hotplug error checking.
> > */
>
> Which suggests it wasn't wrong to omit it before your patches came
> along.

And that suggestion is quite correct. The idea was indeed to accommodate
architectures that do not do error checking.

Does the following patch (on top of current -next) remove the need for
your addition of cpu_set_state_online() above?

Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 18688e0b0422..80400e019c86 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -460,7 +460,7 @@ bool cpu_report_death(void)

do {
oldstate = atomic_read(&per_cpu(cpu_hotplug_state, cpu));
- if (oldstate == CPU_ONLINE)
+ if (oldstate == CPU_ONLINE || CPU_POST_DEAD)
newstate = CPU_DEAD;
else
newstate = CPU_DEAD_FROZEN;