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

[Mandeep Singh Baines <msb@google.com>]

Srivatsa S. Bhat (srivatsa.bhat@linux.vnet.ibm.com) wrote:
> On 06/21/2012 03:33 AM, mandeep.baines@gmail.com wrote:
> > From: Mandeep Singh Baines <msb@chromium.org>
> > 
> > A cpu in the mm_cpumask could go offline before we send the
> > invalidate IPI causing us to wait forever.
> > 
> > Bug Trace:
> > 
> > <4>[10222.234548] WARNING: at ../../arch/x86/kernel/apic/ipi.c:113 default_send_IPI_mask_logical+0x58/0x73()
> > <5>[10222.234633] Pid: 23605, comm: lmt-udev Tainted: G         C   3.2.7 #1
> > <5>[10222.234639] Call Trace:
> > <5>[10222.234651]  [<8102e666>] warn_slowpath_common+0x68/0x7d
> > <5>[10222.234661]  [<81016c36>] ? default_send_IPI_mask_logical+0x58/0x73
> > <5>[10222.234670]  [<8102e68f>] warn_slowpath_null+0x14/0x18
> > <5>[10222.234678]  [<81016c36>] default_send_IPI_mask_logical+0x58/0x73
> > <5>[10222.234687]  [<8101eec2>] flush_tlb_others_ipi+0x86/0xba
> > <5>[10222.234696]  [<8101f0bb>] flush_tlb_mm+0x5e/0x62
> > <5>[10222.234703]  [<8101e36c>] pud_populate+0x2c/0x31
> > <5>[10222.234711]  [<8101e409>] pgd_alloc+0x98/0xc7
> > <5>[10222.234719]  [<8102c881>] mm_init.isra.38+0xcc/0xf3
> > <5>[10222.234727]  [<8102cbc2>] dup_mm+0x68/0x34e
> > <5>[10222.234736]  [<8139bbae>] ? _cond_resched+0xd/0x21
> > <5>[10222.234745]  [<810a5b7c>] ? kmem_cache_alloc+0x26/0xe2
> > <5>[10222.234753]  [<8102d421>] ? copy_process+0x556/0xda6
> > <5>[10222.234761]  [<8102d641>] copy_process+0x776/0xda6
> > <5>[10222.234770]  [<8102dd5e>] do_fork+0xcb/0x1d4
> > <5>[10222.234778]  [<810a8c96>] ? do_sync_write+0xd3/0xd3
> > <5>[10222.234786]  [<810a94ab>] ? vfs_read+0x95/0xa2
> > <5>[10222.234795]  [<81008850>] sys_clone+0x20/0x25
> > <5>[10222.234804]  [<8139d8c5>] ptregs_clone+0x15/0x30
> > <5>[10222.234812]  [<8139d7f7>] ? sysenter_do_call+0x12/0x26
> > <4>[10222.234818] ---[ end trace 31e095600f50fd48 ]---
> > <3>[10234.880183] BUG: soft lockup - CPU#0 stuck for 11s! [lmt-udev:23605]
> > 
> > Addresses http://crosbug.com/31737
> > 
> > Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: x86@kernel.org
> > Cc: Tejun Heo <tj@kernel.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Stephen Rothwell <sfr@canb.auug.org.au>
> > Cc: Christoph Lameter <cl@gentwo.org>
> > Cc: Olof Johansson <olofj@chromium.org>
> > ---
> >  arch/x86/mm/tlb.c |    7 ++++++-
> >  1 files changed, 6 insertions(+), 1 deletions(-)
> > 
> > diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> > index 5e57e11..010090d 100644
> > --- a/arch/x86/mm/tlb.c
> > +++ b/arch/x86/mm/tlb.c
> > @@ -194,8 +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.
> 
> > -		while (!cpumask_empty(to_cpumask(f->flush_cpumask)))
> > +		while (!cpumask_empty(to_cpumask(f->flush_cpumask))) {
> > +			/* Only wait for online cpus */
> > +			cpumask_and(to_cpumask(f->flush_cpumask),
> > +				    to_cpumask(f->flush_cpumask),
> > +				    cpu_online_mask);
> >  			cpu_relax();
> > +		}
> >  	}
> > 
> >  	f->flush_mm = NULL;
> > 
> 
> That is, how about something like this:
> 

Acked-off-by: Mandeep Singh Baines <msb@chromium.org>

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

Thanks and regards,
Mandeep

> 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
>