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

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Date:   Thu, 7 Nov 2019 12:36:53 -0500
From:   Dennis Zhou <dennis@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Dennis Zhou <dennis@...nel.org>, 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 Thu, Nov 07, 2019 at 06:24:34PM +0100, Sebastian Andrzej Siewior wrote:
> 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.
> 

Yeah, so adding a bit in the commit message about why it's an issue for
RT kernels with disabled preemption as I don't believe this is an issue
for non-RT kernels.


> > > > 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?
> 

I just meant the above, the benchmarking is fine.

Thanks,
Dennis