linux-kernel - Re: [RFC][PATCH 7/7] sched: Replace rq::wake

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Date:   Fri, 5 Jun 2020 16:09:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     tglx@...utronix.de, frederic@...nel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org, cai@....pw,
        mgorman@...hsingularity.net
Subject: Re: [RFC][PATCH 7/7] sched: Replace rq::wake_list

On Fri, Jun 05, 2020 at 06:33:38AM -0700, Guenter Roeck wrote:

> I have not made the C standard. You point out yourself a possible explicit
> culprit: struct randomization. 

The randomization crud is very much outside the C spec.

> That by itself shows that you can not rely
> on two elements of different structures having the same alignment,

Randomization does not change the alignment, if it were to do that it
would break all sorts. All it does is change the order of elements.

> which is pretty much exactly what I said (and may explain why observing
> the problem seemed to at least somewhat depend on the weather).

A normal C compiler will have deterministic layout, otherwise unions and
union based type punning would not be a thing.