linux-kernel - Re: [PREEMPT_RT]: Using printk-rt-aware.patch in non-rt kernel

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Message-ID: <20180726153857.abxfebvxjcp4jqgx@linutronix.de>
Date:   Thu, 26 Jul 2018 17:38:58 +0200
From:   Sebastian Andrzej Siewior <sebastian.siewior@...utronix.de>
To:     Pintu Kumar <pintu.ping@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-rt-users@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PREEMPT_RT]: Using printk-rt-aware.patch in non-rt kernel

On 2018-07-18 20:19:15 [+0530], Pintu Kumar wrote:
> Hi All,
Hi,

> I have a question about PREEMPT_RT patch for 3.10 kernel.
> I am trying to port this rt patch: 0224-printk-rt-aware.patch.patch
> (see the patch below), in non-rt kernel.
> I could able to successfully apply this patch after replacing:
> migrate_enable/disable with preempt_enable/disable.
> Things were working fine so far.

Why? 

> Then, I noticed that "printk" from interrupt context is not appearing
> on console (minicom), if console shell is active.
> However, it appears on the dmesg output.
> 
> So, I am wondering, what could be the cause of this patch for this
> behavior in non-rt kernel?
> Is this the expected behavior, even on PREEMPT_RT kernel?

I *think* it is but there are hardly any prints from atomic context for
anyone to notice. And the next print from non-atomic context should
flush it out.

> Is there a way we can fix this issue on non-rt kernel?
I would guess that a wakeup to klogd would fix it.

> Please help us with your suggestions.
> 
> Thanks,
> Pintu

Sebastian