linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <CA+55aFxms_9g9bVuVmbMa6Bx78yQGb34RSoE5DEtNt8Le-bRPA@mail.gmail.com>
Date:	Tue, 2 Dec 2014 16:09:31 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Dave Jones <davej@...hat.com>, Chris Mason <clm@...com>,
	Dâniel Fraga <fragabr@...il.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 Tue, Dec 2, 2014 at 3:32 PM, Sasha Levin <sasha.levin@...cle.com> wrote:
>
> I've disabled lock debugging to see if anything new will show up, and hit
> something that may be related:

Very interesting. But your source code doesn't match mine - can you
say what that

    kernel/sched/fair.c:4541:17

line is?

There are at least five multiplications there (all inlined):

 - "imbalance*min_load" from find_idlest_group()

 - "factor * p->wakee_flips" in wake_wide()

 - at least three in wake_affine:

    "prev_eff_load *= capacity_of(this_cpu)"
    "this_eff_load *= this_load + effective_load(tg, this_cpu, weight, weight)"
    "prev_eff_load *= load + effective_load(tg, prev_cpu, 0, weight)"

(There are other multiplications too, but they are by constants afaik
and don't match yours).

None of those seem to have anything to do with the 3.16..3.17 changes,
but I might be missing something, and obviously this also might have
nothing to do with the problems anyway.

Adding Ingo/PeterZ to the participants again.

                 Linus


---
> [  787.894288] ================================================================================
> [  787.897074] UBSan: Undefined behaviour in kernel/sched/fair.c:4541:17
> [  787.898981] signed integer overflow:
> [  787.900066] 361516561629678 * 101500 cannot be represented in type 'long long int'
> [  787.900066] ubsan_epilogue (lib/ubsan.c:159)
> [  787.900066] handle_overflow (lib/ubsan.c:191)
> [  787.900066] ? __do_page_fault (arch/x86/mm/fault.c:1220)
> [  787.900066] ? local_clock (kernel/sched/clock.c:392)
> [  787.900066] __ubsan_handle_mul_overflow (lib/ubsan.c:218)
> [  787.900066] select_task_rq_fair (kernel/sched/fair.c:4541 kernel/sched/fair.c:4755)
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