linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Thu, 4 Dec 2014 10:30:38 -0500
From:	Chris Mason <clm@...com>
To:	Dave Hansen <dave.hansen@...el.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Dave Jones <davej@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Thu, Dec 4, 2014 at 10:22 AM, Dave Hansen <dave.hansen@...el.com> 
wrote:
> On 12/03/2014 09:49 PM, Linus Torvalds wrote:
>>  On Wed, Dec 3, 2014 at 7:15 PM, Chris Mason <clm@...com> wrote:
>>> 
>>>  One guess is that trinity is generating a huge number of tlb
>>>  invalidations over sparse and horrible ranges.  Perhaps the old 
>>> code was
>>>  falling back to full tlb flushes before Dave Hansen's string of 
>>> fixes?
>> 
>>  Hmm. I agree that we've had some of the backtraces look like TLB
>>  flushing might be involved. Not all, though. And I'm not seeing 
>> where
>>  a loop over up to 33 pages should matter over doing a full TLB 
>> flush.
>> 
>>  What *might* matter is if we somehow get that number wrong, and the 
>> loops like
>> 
>>                          addr = f->flush_start;
>>                          while (addr < f->flush_end) {
>>                                  __flush_tlb_single(addr);
>>                                  addr += PAGE_SIZE;
>>                          }
>> 
>>  ends up looping a *lot* due to some bug, and then the IPI itself 
>> would
>>  take so long that the watchdog could trigger.
>> 
>>  But I do not see how that could actually happen. As far as I can 
>> tell,
>>  either the number of pages is limited to less than 33, or we have 
>> that
>>   TLB_FLUSH_ALL case.
>> 
>>  Do  you see something I don't?
> 
> The one thing I _do_ see now is a missed TLB flush is we're flushing 
> one
> page at the end of the address space.  We'd overflow flush_end back so
> flush_end=0:
> 
>         if (!f->flush_end)
>                 f->flush_end = f->flush_start + PAGE_SIZE; <-- 
> overflow
> 
> and we'll never enter the while loop where we actually do the flush:
> 
>                         while (addr < f->flush_end) {
>                                 __flush_tlb_single(addr);
>                                 addr += PAGE_SIZE;
>                         }
> 
> But we have a hole up there on x86_64, so this will never happen in
> practice there.  It might theoretically apply to 32-bit, but this 
> still
> doesn't help with the bug.
> 
> Oh, and the tracepoint is spitting out bogus numbers because we need
> some parenthesis around the 'nr_pages' calculation.

Yeah, I didn't see any problems with your changes, but I was hoping 
that even a small change like doing 33 flushes at a time was pushing 
Dave's box just over the line.

-chris



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