Re: linux-next: Tree for Feb 4

[Sedat Dilek <sedat.dilek@gmail.com>]

On Thu, Feb 5, 2015 at 1:10 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Wed, Feb 04, 2015 at 03:51:15PM -0800, Paul E. McKenney wrote:
>> On Wed, Feb 04, 2015 at 11:59:31PM +0100, Rafael J. Wysocki wrote:
>> > On Wednesday, February 04, 2015 01:53:58 PM Paul E. McKenney wrote:
>> > > On Wed, Feb 04, 2015 at 10:54:07PM +0100, Rafael J. Wysocki wrote:
>> > > > On Wednesday, February 04, 2015 09:18:03 PM Sedat Dilek wrote:
>
> [ . . . ]
>
>> > > > > [ 1144.482666] Disabling non-boot CPUs ...
>> > > > > [ 1144.483000] intel_pstate CPU 1 exiting
>> > > > > [ 1144.486064]
>> > > > > [ 1144.486065] ===============================
>> > > > > [ 1144.486067] smpboot: CPU 1 didn't die...
>> > > > > [ 1144.486067] [ INFO: suspicious RCU usage. ]
>> > > > > [ 1144.486069] 3.19.0-rc7-next-20150204.1-iniza-small #1 Not tainted
>> > > > > [ 1144.486070] -------------------------------
>> > > > > [ 1144.486072] include/trace/events/tlb.h:35 suspicious
>> > > > > rcu_dereference_check() usage!
>> > > > > [ 1144.486073]
>> > > > > [ 1144.486073] other info that might help us debug this:
>> > > > > [ 1144.486073]
>> > > > > [ 1144.486074]
>> > > > > [ 1144.486074] RCU used illegally from offline CPU!
>> > > > > [ 1144.486074] rcu_scheduler_active = 1, debug_locks = 0
>> > > > > [ 1144.486076] no locks held by swapper/1/0.
>> > > > > [ 1144.486076]
>> > > > > [ 1144.486076] stack backtrace:
>> > > > > [ 1144.486079] CPU: 1 PID: 0 Comm: swapper/1 Not tainted
>> > > > > 3.19.0-rc7-next-20150204.1-iniza-small #1
>> > > > > [ 1144.486080] Hardware name: SAMSUNG ELECTRONICS CO., LTD.
>> > > > > 530U3BI/530U4BI/530U4BH/530U3BI/530U4BI/530U4BH, BIOS 13XK 03/28/2013
>> > > > > [ 1144.486085]  0000000000000001 ffff88011a44fe18 ffffffff817e370d
>> > > > > 0000000000000011
>> > > > > [ 1144.486088]  ffff88011a448290 ffff88011a44fe48 ffffffff810d6847
>> > > > > ffff8800c66b9600
>> > > > > [ 1144.486091]  0000000000000001 ffff88011a44c000 ffffffff81cb3900
>> > > > > ffff88011a44fe78
>> > > > > [ 1144.486092] Call Trace:
>> > > > > [ 1144.486099]  [<ffffffff817e370d>] dump_stack+0x4c/0x65
>> > > > > [ 1144.486104]  [<ffffffff810d6847>] lockdep_rcu_suspicious+0xe7/0x120
>> > >
>> > > As near as I can tell, idle_task_exit() is running on an offline CPU,
>> > > then calling switch_mm() which contains trace_tlb_flush(), which uses RCU.
>> > > And RCU is objecting to being used from a CPU that it is ignoring.
>> > >
>> > > One approach would be to push RCU's idea of when the CPU goes offline
>> > > down into arch code in this case, using some Kconfig symbol and
>> > > the usual conditional compilation.  Another approach would be to
>> > > invoke the trace calls under cpu_online(), for example, for the
>> > > first such call in switch_mm():
>> > >
>> > >   if (cpu_online(smp_processor_id()))
>> > >           trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
>> > >
>> > > The compiler would discard this if tracing was disabled.
>> >
>> > That looks like less intrusive to me.
>>
>> One possible concern is increased context-switch path length, but that
>> would only be the case where tracing is enabled by default.
>
> Nevertheless, here is an untested patch.  Does it help?

No bedtime :-)

I tried with a revert of...

commit 5f1dedac9adb6259bb7b62a923bd7c247a2f2d5b
rcu: Handle outgoing CPUs on exit from idle loop

...and offlining cpu1 seems not to produce the trace...

[  115.280244] PPP BSD Compression module registered
[  115.288761] PPP Deflate Compression module registered
[  162.935524] intel_pstate CPU 1 exiting
[  162.949729] smpboot: CPU 1 is now offline

Will try the patch.

- Sedat -

>
>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> x86: Omit switch_mm() tracing for offline CPUs
>
> The architecture-specific switch_mm() function can be called by offline
> CPUs, but includes event tracing, which cannot be legally carried out
> on offline CPUs.  This results in a lockdep-RCU splat.  This commit fixes
> this splat by omitting the tracing when the CPU is offline.
>
> Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index 40269a2bf6f9..7e7f2445fbc9 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -47,7 +47,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>
>                 /* Re-load page tables */
>                 load_cr3(next->pgd);
> -               trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
> +               if (cpu_online(smp_processor_id()))
> +                       trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
>
>                 /* Stop flush ipis for the previous mm */
>                 cpumask_clear_cpu(cpu, mm_cpumask(prev));
> @@ -84,7 +85,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>                          * to make sure to use no freed page tables.
>                          */
>                         load_cr3(next->pgd);
> -                       trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
> +                       if (cpu_online(smp_processor_id()))
> +                               trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
>                         load_LDT_nolock(&next->context);
>                 }
>         }
>