linux-kernel - Re: v4.16+ seeing many unaligned access in dequeue_task

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Date:   Wed, 4 Apr 2018 18:53:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Norbert Manthey <nmanthey@...zon.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: v4.16+ seeing many unaligned access in dequeue_task_fair() on
 IA64

On Wed, Apr 04, 2018 at 09:38:56AM -0700, Luck, Tony wrote:
> On Wed, Apr 04, 2018 at 09:25:13AM +0200, Peter Zijlstra wrote:
> > Right, I remember being careful with that. Which again brings me to the
> > RANDSTRUCT thing, which will mess that up.
> 
> No RANDSTRUCT config options set for my build.

Weird though, with or without that patch, my ia64-defconfig gives the
below layout.

$ pahole -EC sched_avg ia64-defconfig/kernel/sched/core.o
die__process_function: tag not supported (INVALID)!
struct sched_avg {
        /* typedef u64 */ long long unsigned int     last_update_time;                   /*     0     8 */
        /* typedef u64 */ long long unsigned int     load_sum;                           /*     8     8 */
        /* typedef u64 */ long long unsigned int     runnable_load_sum;                  /*    16     8 */
        /* typedef u32 */ unsigned int               util_sum;                           /*    24     4 */
        /* typedef u32 */ unsigned int               period_contrib;                     /*    28     4 */
        long unsigned int          load_avg;                                             /*    32     8 */
        long unsigned int          runnable_load_avg;                                    /*    40     8 */
        long unsigned int          util_avg;                                             /*    48     8 */
        struct util_est {
                unsigned int       enqueued;                                             /*    56     4 */
                unsigned int       ewma;                                                 /*    60     4 */
        } util_est; /*    56     8 */
        /* --- cacheline 1 boundary (64 bytes) --- */

        /* size: 64, cachelines: 1, members: 9 */
};

> > Does the below cure things? It makes absolutely no difference for my
> > x86_64-defconfig build, but it puts more explicit alignment constraints
> > on things.
> 
> Yes. That fixes it. No unaligned traps with this patch applied.
> 
> Tested-by: Tony Luck <tony.luck@...el.com>

Awesome, I'll go get it merged, even though I don't understand where it
went wobbly.