linux-kernel - Re: [PATCH RT v2] arm64: fpsimd: use a local_lock() in addition to local_bh

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Date:   Wed, 18 Jul 2018 12:36:57 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Mike Galbraith <efault@....de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH RT v2] arm64: fpsimd: use a local_lock() in addition to
 local_bh_disable()

On 2018-07-18 12:28:48 [+0200], Mike Galbraith wrote:
> > Okay, so you did not test this because you can't compile.
> 
> Nope, the running kernel, the one that is doing the segfaulting etc,
> has the patches applied.
> 
> It is exhibiting that symptom because those patches do not cure this
> symptom, one which I verified to be present in virgin 4.14-rt as well. 
> The pseudo-patch I sent, disabling preemption where it is assumed to be
> disabled instead, does cure it.  With preemption so disabled, I can
> beat on affected kernels (>=4.14-rt) as long as I like.

ah. so gcc shows the problem instead gcc explodes with the patch
applied. Okay. Let me stare at this a little more…

> This particular 48 core Cavium is very slow, maybe that makes it easier
> to reproduce, dunno.  According to pipe-test, the thing is essentially
> a dozen RPi super-glued together.  pipe-test pinned to a single core
> can only context switch at ~40KHz with PREEMPT_RT, or ~90 with
> NOPREEMPT, comparable to measurement done in real deal RPi.
> 
> 	-Mike

Sebastian