linux-kernel - Re: [PATCH] percpu-refcount: Use normal instead of RCU-sched"

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Date:   Thu, 7 Nov 2019 18:24:34 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Dennis Zhou <dennis@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] percpu-refcount: Use normal instead of RCU-sched"

On 2019-11-07 11:55:19 [-0500], Dennis Zhou wrote:
> On Thu, Nov 07, 2019 at 05:28:42PM +0100, Sebastian Andrzej Siewior wrote:
> > > I just want to clarify a little bit. Is this patch aimed at fixing an
> > > issue with RT kernels specifically? 
> > 
> > Due to the implications of preempt_disable() on RT kernels it fixes
> > problems with RT kernels.
> > 
> 
> Great, do you mind adding this explanation with what the implications
> are in the commit message?

some RCU section here invoke callbacks which acquire spinlock_t locks.
This does not work on RT with disabled preemption.

> > > It'd also be nice to have the
> > > numbers as well as if the kernel was RT or non-RT.
> > 
> > The benchmark was done on a CONFIG_PREEMPT kernel. As said in the commit
> > log, the numbers were mostly the same, I can re-run the test and post
> > numbers if you want them.
> > This patch makes no difference on PREEMPT_NONE or PREEMPT_VOLUNTARY
> > kernels.
> > 
> 
> I think a more explicit explanation in the commit message would suffice.

What do you mean by "more explicit explanation"? The part with the
numbers or that it makes no difference for PREEMPT_NONE and
PREEMPT_VOLUNTARY?

> Thanks,
> Dennis

Sebastian