Re: [PATCH v2] x86, mm: only wait for flushes from online cpus

[Mandeep Singh Baines]

Srivatsa S. Bhat (srivatsa.bhat@xxxxxxxxxxxxxxxxxx) wrote:
> On 06/23/2012 03:36 AM, Mandeep Singh Baines wrote:
> > A cpu in the mm_cpumask could go offline before we send the invalidate
> > IPI causing us to wait forever. Avoid this by only waiting for online
> > cpus.
> > 
> > We are seeing a softlockup reporting during shutdown. The stack
> > trace shows us that we are inside default_send_IPI_mask_logical:
> > 
> [...]
> > Changes in V2:
> >   * bitmap_and is not atomic so use a temporary bitmask
> > 
> 
> Looks like I posted my reply to v1. So I'll repeat the same suggestions in
> this thread as well.
> 
> > ---
> >  arch/x86/mm/tlb.c |    9 ++++++++-
> >  1 files changed, 8 insertions(+), 1 deletions(-)
> > 
> > diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> > index d6c0418..231a0b9 100644
> > --- a/arch/x86/mm/tlb.c
> > +++ b/arch/x86/mm/tlb.c
> > @@ -185,6 +185,8 @@ static void flush_tlb_others_ipi(const struct cpumask *cpumask,
> >  	f->flush_mm = mm;
> >  	f->flush_va = va;
> >  	if (cpumask_andnot(to_cpumask(f->flush_cpumask), cpumask, cpumask_of(smp_processor_id()))) {
> > +		DECLARE_BITMAP(tmp_cpumask, NR_CPUS);
> > +
> >  		/*
> >  		 * We have to send the IPI only to
> >  		 * CPUs affected.
> > @@ -192,8 +194,13 @@ static void flush_tlb_others_ipi(const struct cpumask *cpumask,
> >  		apic->send_IPI_mask(to_cpumask(f->flush_cpumask),
> >  			      INVALIDATE_TLB_VECTOR_START + sender);
> > 
> 
> This function is always called with preempt_disabled() right?
> In that case, _while_ this function is running, a CPU cannot go offline
> because of stop_machine(). (I understand that it might go offline in between
> calculating that cpumask and calling preempt_disable() - which is the race
> you are trying to handle).
> 

Ah. Good point. A cpu cannot be remove from the cpu_online_mask while
preemption is disabled because stop_machine() can't run until
preemption is enabled.

./kernel/cpu.c: err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));

> So, why not take the offline cpus out of the way even before sending that IPI?
> That way, we need not modify the while loop below.
> 

Acked-off-by: Mandeep Singh Baines <msb@xxxxxxxxxxxx>

Do you mind re-sending you're patch with a proper sign-off.

Thanks and regards,
Mandeep

> > -		while (!cpumask_empty(to_cpumask(f->flush_cpumask)))
> > +		/* Only wait for online cpus */
> > +		do {
> > +			cpumask_and(to_cpumask(tmp_cpumask),
> > +				    to_cpumask(f->flush_cpumask),
> > +				    cpu_online_mask);
> >  			cpu_relax();
> > +		} while (!cpumask_empty(to_cpumask(tmp_cpumask)));
> >  	}
> > 
> >  	f->flush_mm = NULL;
> > 
> 
> That is, how about something like this:
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 5e57e11..9d387a9 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -186,7 +186,11 @@ static void flush_tlb_others_ipi(const struct cpumask *cpumask,
>  
>         f->flush_mm = mm;
>         f->flush_va = va;
> -       if (cpumask_andnot(to_cpumask(f->flush_cpumask), cpumask, cpumask_of(smp_processor_id()))) {
> +
> +       cpumask_and(to_cpumask(f->flush_cpumask), cpumask, cpu_online_mask);
> +       cpumask_clear_cpu(smp_processor_id(), to_cpumask(f->flush_cpumask));
> +
> +       if (!cpumask_empty(to_cpumask(f->flush_cpumask))) {
>                 /*
>                  * We have to send the IPI only to
>                  * CPUs affected.
> 
> 
> Regards,
> Srivatsa S. Bhat
> IBM Linux Technology Center
> 
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