Re: linux-next: Tree for Feb 4

From: Sedat Dilek
Date: Wed Feb 04 2015 - 20:53:11 EST

On Thu, Feb 5, 2015 at 2:51 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx>
>> >> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>> >> >
>> >> > 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?

- Sedat -

>
> Thanx, Paul
>
>> - Sedat -
>>
>> [1] http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=d17d8f9dedb9dd76fd540a5c497101529d9eb25a
>>
>
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