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Date: Wed, 3 Dec 2014 21:49:04 -0800
Message-ID: <CA+55aFz-QhQOQuFPKbxBDAOjJ+fR02m3-unA4iD9kLwhWS38cA@mail.gmail.com>
Subject: Re: frequent lockups in 3.18rc4
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Chris Mason <clm@fb.com>, Thomas Gleixner <tglx@linutronix.de>,
        John Stultz <john.stultz@linaro.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Jones <davej@redhat.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        =?UTF-8?Q?D=C3=A2niel_Fraga?= <fragabr@gmail.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org

On Wed, Dec 3, 2014 at 7:15 PM, Chris Mason <clm@fb.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?

                  Linus
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