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

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Message-ID: <20150311185831.GG5412@linux.vnet.ibm.com>
Date:	Wed, 11 Mar 2015 11:58:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	James Hogan <james.hogan@...tec.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com, linux-metag@...r.kernel.org
Subject: Re: [PATCH tip/core/rcu 04/20] metag: Use common
 outgoing-CPU-notification code

On Wed, Mar 11, 2015 at 11:03:18AM +0000, James Hogan wrote:
> On 10/03/15 16:59, Paul E. McKenney wrote:
> > 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@...ux.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@...ux.vnet.ibm.com>
> >>> Cc: James Hogan <james.hogan@...tec.com>
> >>> Cc: <linux-metag@...r.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?
> 
> Don't forget the "oldstate == ", otherwise it'll work for the wrong
> reason :-/

I clearly wasn't doing well yesterday, was I?  :-/

> Checking for CPU_POST_DEAD does seem to fix the immediate problem,
> however this still leaves open the possibility of a single timeout
> propagating to all further offlines after CPU_DEAD_FROZEN gets set. I've
> confirmed that by adding a delay loop only on the second
> cpu_report_death() call, and sure enough the 2nd and further offlines
> all fail even though the CPU stops immediately after the 2nd one.
> 
> If this check is primarily so that CPU_DEAD_FROZEN is set if
> cpu_wait_death timed out, would it be better to instead check explicitly
> for CPU_BROKEN?
> 
> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> index 18688e0b0422..c697f73d82d6 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_BROKEN)
>  			newstate = CPU_DEAD;
>  		else
>  			newstate = CPU_DEAD_FROZEN;

This does look much better!  I will incorporate this with attribution.

The idea is to support two use cases.  The first use case provides full
checking, and the second provides minimal checking.

Full checking is used by architectures that require that one of the
surviving CPUs so something to help the offlined CPU go offline,
Xen being one example.  In this case, the architecture invokes
cpu_check_up_prepare(), which returns an error code if the CPU did not
go offline properly.  The architecture can choose to return an error or
to provide the offlining help at that point.  The CPU being onlined then
calls cpu_set_state_online().  When the CPU goes offline, it invokes
cpu_report_death(), which can race with the timing out of one of the
surviving CPUs invoking cpu_wait_death().  If cpu_wait_death() times
out first, or if cpu_report_death() is never called, state is set so
that the next call to cpu_check_up_prepare() can react accordingly.

Minimal checking is what metag does.  The cpu_check_up_prepare() and
cpu_set_state_online() functions are never called, just cpu_report_death()
and cpu_wait_death().

And yes, this time I drew state diagrams.  Which I should have done in
the first place.

						Thanx, Paul

> Cheers
> James
> 
> > 
> > 							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;
> > 
> 


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