linux-kernel - Re: [PATCH] random: use raw spinlocks for use on RT

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Message-ID: <Yw5dqweo0bQDvPkP@linutronix.de>
Date:   Tue, 30 Aug 2022 20:57:47 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH] random: use raw spinlocks for use on RT

On 2022-08-30 11:24:33 [-0400], Jason A. Donenfeld wrote:
> Hi Sebastian,
Hi Jason,

> On Tue, Aug 30, 2022 at 12:13:44PM +0200, Sebastian Andrzej Siewior wrote:
> > The first patch did so yes. The second simply retried in two secs and
> > this shouldn't be problematic.
> 
> This seemed pretty bad too, because now you potentially miss up to 2
> seconds of messages AND it adds more complexity.

It is early at boot and it could be reduced to one if it helps. I
remember you had a suggestion where we would lose always the first print
out on RT you said it is okay since you can't rely on that…

> I'm fine with changing things up to accommodate RT, but not when the
> result is so obviously worse than before.

I don't think it is worse. This is your opinion and I did not hear any
other feedback so far.

> In my tests I can't see any latency difference with using raw spinlocks
> in random.c. Maybe I'm doing things wrong? But I'm not seeing anything
> change...

You need to look at the maximum latency that may happen. Also the other
thing is that there is no need to add raw_spinlock_t locking if it can
be avoided.

> Jason

Sebastian