Re: linux-next: Tree for Feb 4

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

On Thu, Feb 5, 2015 at 2:53 AM, Sedat Dilek <sedat.dilek@gmail.com> wrote:
> On Thu, Feb 5, 2015 at 2:51 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
>> On Thu, Feb 05, 2015 at 02:18:01AM +0100, Sedat Dilek wrote:
>>> On Thu, Feb 5, 2015 at 1:57 AM, Paul E. McKenney
>>> <paulmck@linux.vnet.ibm.com> wrote:
>>> > On Thu, Feb 05, 2015 at 01:30:45AM +0100, Sedat Dilek wrote:
>>> >> 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 :-)
>>> >
>>> > Sorry!  Actually, getting results tomorrow would be plenty OK by me.
>>> >
>>> >> 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...
>>> >
>>> > As expected.  The trace can still appear, but the outgoing CPU needs to
>>> > be delayed by at least one jiffy on its final pass through the idle loop.
>>> > Which can really happen in virtualized environments.
>>> >
>>> >> [  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.
>>> >
>>> > Looking forward to seeing the results!
>>> >
>>> >                                                         Thanx, Paul
>>> >
>>> >> - 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);
>>> >> >                 }
>>> >> >         }
>>> >> >
>>> >>
>>> >
>>>
>>> [ CC involved people of "culprit" commit ]
>>>
>>> OK, this fixes the issue for me.
>>> ( Several s/r and offline/online cpu1. )
>>
>> Very good
>>
>>> I looked through the commits and the problem seems to be introduced with...
>>>
>>> commit d17d8f9dedb9dd76fd540a5c497101529d9eb25a
>>> "x86/mm: Add tracepoints for TLB flushes"
>>>
>>> Can you please add a Fixes-tag?
>>>
>>> Fixes: d17d8f9dedb9 ("x86/mm: Add tracepoints for TLB flushes")
>>
>> Done!
>>
>>> And maybe label your proposal-patch with "x86/mm:" instead of "x86:"?
>>>
>>> Feel free to add my Tested-by.
>>
>> Also done!
>>
>>> Anyway, we should listen to the voices of the involved people.
>>
>> Definitely -- this is but one way to fix this problem.  It is the simplest,
>> so it is the one that I am starting with, but if someone has a better idea,
>> please don't keep it a secret!
>>
>>> Thanks, Paul!
>>
>> And many thanks for your testing efforts, especially your late-night
>> testing efforts!
>
> Will you send a separate patch?
>

Thanks, it's in rcu-next.

commit 33a741a1ea39f1daa821259c3654f5abf91d1690
"x86/mm: Omit switch_mm() tracing for offline CPUs"

- Sedat -

[1] http://git.kernel.org/cgit/linux/kernel/git/paulmck/linux-rcu.git/commit/?h=rcu/next&id=33a741a1ea39f1daa821259c3654f5abf91d1690

- Sedat -