linux-kernel - Re: [PATCH] x86: UV: raw

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Date:   Tue, 22 May 2018 10:24:22 +0200
From:   Mike Galbraith <gleep@....de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH] x86: UV: raw_spinlock conversion

On Tue, 2018-05-22 at 08:50 +0200, Sebastian Andrzej Siewior wrote:
> 
> Regarding the preempt_disable() in the original patch in uv_read_rtc():
> This looks essential for PREEMPT configs. Is it possible to get this
> tested by someone or else get rid of the UV code? It looks broken for
> "uv_get_min_hub_revision_id() != 1".

I suspect SGI cares not one whit about PREEMPT.

> Why does PREEMPT_RT require migrate_disable() but PREEMPT only is fine
> as-is? This does not look right.

UV is not ok with a PREEMPT config, it's just that for RT it's dirt
simple to shut it up, whereas for PREEMPT, preempt_disable() across
uv_bau_init() doesn't cut it due to allocations, and whatever else I
would have met before ending the whack-a-mole game.

If I were in your shoes, I think I'd just stop caring about UV until a
real user appears.  AFAIK, I'm the only guy who ever ran RT on UV, and
I only did so because SUSE asked me to look into it.. years ago now.

	-Mike