linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <CA+55aFz-QhQOQuFPKbxBDAOjJ+fR02m3-unA4iD9kLwhWS38cA@mail.gmail.com>
Date:	Wed, 3 Dec 2014 21:49:04 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Chris Mason <clm@...com>, Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Linus Torvalds <torvalds@...ux-foundation.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 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?

                  Linus
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