Subject: Re: [RFC PATCH powerpc] Fix a lazy irq related WARING in arch_local_irq_restore()

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 18 Oct 2012 10:54:16 +1100

On Wed, 2012-09-26 at 18:10 +0800, Li Zhong wrote:

 ../...

Sorry got distracted, got back on this patch today:

> > We might need to "sanitize" the enable state in the PACA before we
> > actually enter NAP or in the return from NAP code, like we do for normal
> > idle code...
> 
> Hi Ben, 
> 
> After some further reading of the code, I updated the code as following,
> but I'm not very sure, and guess it most probably has some issues ...
> Could you please help to review and give your comments? 

I think it's still not right, see below

> In extended_cede_processor(), if there is still lazy irq pending, I used
> local_irq_enable() to make sure the irq replayed and flags cleared, but
> I'm not sure whether it is a proper way. 

Right but that will break things for idle. In idle, if you have
re-enabled interrupts, you need to return and essentially abort the
attempt at going to nap mode, because the interrupt might have set need
resched. That's why we normally just check if something's pending and
return, letting the upper levels re-enable interrupts and do all the
dirty work for us.

Now, hotplug might differ here in what it needs, but in any case,
extended_cede_processor doesn't seem to be the right place to handle it,
at best that function should return if it thinks there's something that
needs to be done and let the upper layers deal with it appropriately.

> In pseries_mach_cpu_die(), I added local_irq_disable() after cede, and
> prepare for the start_secondary_resume(), but I'm not sure whether we
> also need a hard_irq_disable() here. 

You probably do if it's going to go back to the start secondary path. It
shouldn't hurt in any case as long as start_secondary_resume()
eventually does a local_irq_enable().

> I'm still a little confused by the meaning of PACA_IRQ_HARD_DIS in
> irq_happened. From the checking at the warning point, it seems only
> irq_happened equaling 0x1(PACA_IRQ_HARD_DIS) means hard irqs are
> disabled.

No. They are disabled if any bit in there corresponding to a "level"
interrupt is set as well. Only the "edge" interrupts (and decrementer
which we treat as edge and reset to a high value when it kicks) are
ignored for the sake of HW irq state.

The reason we have this IRQ_HARD_DIS bit is to indicate that a 'manual'
hard disabling occurred (by opposition to one happening as a result of
an external interrupt).

We need that so we can avoid doing an mfspr() in local_irq_enable() and
entirely rely on the content of irq_happened to know whether interrupts
are hard enabled or hard disabled.

We do that because mfspr is a fairly expensive instruction. But that
means that we need to make sure we always have a consistent content in
irq_happened. That's also why I've added all those sanity checks if you
enable IRQ tracing.

> Is it possible to set this bit at anyplace the hard irqs are disabled,
> so then we could check whether this bit is set to know whether hard irqs
> are disabled? Then it seems that in MASKED_INTERRUPT, we need set this
> bit where MSR_EE is cleared for something other than decrementer. Maybe
> I missed too much things?

Either that bit or PACA_IRQ_EE. Both indicate that interrupts are hard
disabled.

There are some rare cases where do do change MSR:EE without touching
those bits, only when we're going to restore it shortly afterward in the
kernel asm exception entry/exit path for example.
 
Cheers,
Ben.

> Thanks, Zhong 
> 
> ===================
> diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> index 64c97d8..b5f7597 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> @@ -130,6 +130,8 @@ static void pseries_mach_cpu_die(void)
>  			extended_cede_processor(cede_latency_hint);
>  		}
>  
> +		local_irq_disable();
> +
>  		if (!get_lppaca()->shared_proc)
>  			get_lppaca()->donate_dedicated_cpu = 0;
>  		get_lppaca()->idle = 0;
> diff --git a/arch/powerpc/platforms/pseries/plpar_wrappers.h b/arch/powerpc/platforms/pseries/plpar_wrappers.h
> index 13e8cc4..07560d8 100644
> --- a/arch/powerpc/platforms/pseries/plpar_wrappers.h
> +++ b/arch/powerpc/platforms/pseries/plpar_wrappers.h
> @@ -2,6 +2,7 @@
>  #define _PSERIES_PLPAR_WRAPPERS_H
>  
>  #include <linux/string.h>
> +#include <linux/irqflags.h>
>  
>  #include <asm/hvcall.h>
>  #include <asm/paca.h>
> @@ -41,7 +42,19 @@ static inline long extended_cede_processor(unsigned long latency_hint)
>  	u8 old_latency_hint = get_cede_latency_hint();
>  
>  	set_cede_latency_hint(latency_hint);
> +
> +	while (!prep_irq_for_idle()) {
> +		local_irq_enable();
> +		local_irq_disable();
> +	}
> +
>  	rc = cede_processor();
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +		/* Ensure that H_CEDE returns with IRQs on */
> +		if (WARN_ON(!(mfmsr() & MSR_EE)))
> +			__hard_irq_enable();
> +#endif
> +
>  	set_cede_latency_hint(old_latency_hint);
>  
>  	return rc;
> ===================
> 
> 
> > 
> > Cheers,
> > Ben.
> > 
> > > [   56.618846] WARNING: at arch/powerpc/kernel/irq.c:240
> > > [   56.618851] Modules linked in: rcutorture ipv6 dm_mod ext3 jbd mbcache sg sd_mod crc_t10dif ibmvscsic scsi_transport_srp scsi_tgt ibmveth
> > > [   56.618883] NIP: c00000000000ff94 LR: c00000000067a5e0 CTR: 0000000000000001
> > > [   56.618889] REGS: c0000001fef6bbe0 TRAP: 0700   Not tainted  (3.6.0-rc1-autokern1)
> > > [   56.618894] MSR: 8000000000029032 <SF,EE,ME,IR,DR,RI>  CR: 42000082  XER: 20000000
> > > [   56.618913] SOFTE: 1
> > > [   56.618916] CFAR: c00000000067a5dc
> > > [   56.618920] TASK = c0000001feed79a0[0] 'swapper/5' THREAD: c0000001fef68000 CPU: 5
> > > GPR00: 0000000000000001 c0000001fef6be60 c000000000f9ca08 0000000000000001 
> > > GPR04: 0000000000000001 0000000000000008 0000000000000001 0000000000000000 
> > > GPR08: 0000000000000000 c0000001feed79a0 0008a80000000000 0000000000000000 
> > > GPR12: 0000000022000082 c00000000f330f00 c0000001fef6bf90 000000000f394b4c 
> > > GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
> > > GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
> > > GPR24: c000000000fe8f80 0000000000000008 0000000000000028 0000000000000000 
> > > GPR28: 0000000000000000 0000000000000020 c000000000f1ab40 0000000000000001 
> > > [   56.619014] NIP [c00000000000ff94] .arch_local_irq_restore+0x34/0xa0
> > > [   56.619020] LR [c00000000067a5e0] .start_secondary+0x368/0x37c
> > > [   56.619025] Call Trace:
> > > [   56.619030] [c0000001fef6be60] [c000000001ba0500] 0xc000000001ba0500 (unreliable)
> > > [   56.619038] [c0000001fef6bed0] [c00000000067a5e0] .start_secondary+0x368/0x37c
> > > [   56.619046] [c0000001fef6bf90] [c000000000009380] .start_secondary_resume+0x10/0x14
> > > [   56.619052] Instruction dump:
> > > [   56.619056] f8010010 f821ff91 986d022a 2fa30000 419e0054 880d022b 78000621 41820048 
> > > [   56.619071] 2f800001 40de0064 7c0000a6 78008fe2 <0b000000> 2fa00000 40de0050 38000000 
> > > [   56.619088] ---[ end trace 0199c0d783d7f9ba ]---
> > > 
> > > Reported-by: Paul E. McKenney <[email-masked]>
> > > Signed-off-by: Li Zhong <[email-masked]>
> > > ---
> > >  arch/powerpc/platforms/pseries/hotplug-cpu.c |    2 ++
> > >  1 files changed, 2 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> > > index 64c97d8..8de539a 100644
> > > --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> > > +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> > > @@ -137,6 +137,8 @@ static void pseries_mach_cpu_die(void)
> > >  		if (get_preferred_offline_state(cpu) == CPU_STATE_ONLINE) {
> > >  			unregister_slb_shadow(hwcpu);
> > >  
> > > +			__hard_irq_disable();
> > > +
> > >  			/*
> > >  			 * Call to start_secondary_resume() will not return.
> > >  			 * Kernel stack will be reset and start_secondary()
> > 
> > 
>