Re: [PATCH] [RFC] s390/kvm: note a quiescing state if we interupt guest mode

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, May 02, 2013 at 10:09:52AM +0200, Christian Borntraeger wrote:
> The SIE instruction is interruptible, so instead of having a guest
> exit on a host interrupt we basically return to guest mode.
> We have some logic in the interrupt handler to check for
> need_resched, machine checks or sigpending to exit SIE the hard
> way, but RCU is currently not handled, leading to several second
> delays on cpu bound guests.
> 
> Lets mark SIE (guest context) as quiescing state in the external
> interrupt handler (hz tick, timers sigp and others) thus making
> RCU working properly again.
> 
> Long term we might want to use proper state tracking (just like
> the dynticks folks) and mark guest state similar to user space
> as an extended grace period, but this is not ready yet.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
> Cc: Dipankar Sarma <dipankar@in.ibm.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Gleb Natapov <gleb@redhat.com>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> ---

Hmmm...  This looks like an interrupt.  Can it interrupt kernel code?
If it can, then we would need to deal with the possibility of it
having interrupted an RCU read-side critical section.  If it somehow is
guaranteed to never interrupt code containing RCU read-side critical
sections (for example, if it is the exception handler for an SIE
instruction in cases where the SIE instruction is illegal), then should
be OK.

							Thanx, Paul

>  arch/s390/kernel/irq.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/s390/kernel/irq.c b/arch/s390/kernel/irq.c
> index 1630f43..d6ccb1d 100644
> --- a/arch/s390/kernel/irq.c
> +++ b/arch/s390/kernel/irq.c
> @@ -244,6 +244,17 @@ void __irq_entry do_extint(struct pt_regs *regs, struct ext_code ext_code,
>  	int index;
> 
>  	old_regs = set_irq_regs(regs);
> +	/*
> +	 * The SIE instruction is interruptible, so instead of having a guest
> +	 * exit on a host interrupt we basically return to guest mode if there
> +	 * is no need_resched, machine check or signal pending. So we can
> +	 * stay in guest mode for several seconds or even minutes. This
> +	 * lets RCU wait for a grace period much too long. In case of PF_VCPU
> +	 * we know that we do not hold any rcu data, so lets claim that a
> +	 * context switch happened, which is a quiescing state.
> +	 */
> +	if (current->flags & PF_VCPU) 
> +		rcu_sched_qs(smp_processor_id());
>  	irq_enter();
>  	if (S390_lowcore.int_clock >= S390_lowcore.clock_comparator) {
>  		/* Serve timer interrupts first. */
> -- 
> 1.8.1.4
>