linux-kernel - Re: [PATCH next v1 1/2] dump_stack: move cpu lock to printk.c

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Date:   Tue, 1 Jun 2021 15:29:08 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Alexander Potapenko <glider@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH next v1 1/2] dump_stack: move cpu lock to printk.c

On Tue 2021-06-01 09:37:08, John Ogness wrote:
> On 2021-06-01, John Ogness <john.ogness@...utronix.de> wrote:
> >> Is there any particular reason this does
> >>
> >> 	preempt_disable();
> >> 	cpu = smp_processor_id();
> >> 	local_irq_safe();
> >>
> >> instead of
> >>
> >> 	local_irq_safe();
> >> 	cpu = raw_smp_processor_id();
> >>
> >> ?
> >
> > If the lock is owned by another CPU, there is no need to disable
> > interrupts for this CPU. (The local_irq_save() is conditional.)
> 
> The cpu lock implementation from dump_stack() also keeps preemption
> continually enabled while spinning.

I wonder if this might reduce some noise on the CPU cache lines
when disable_preemption()/enable_preemption() actually does something.
But the problem might be only with cmpxchg() in a busy loop.
Peter Zijlstra might know more.

> I used the cpu lock implementation from PREEMPT_RT. But for my v2
> I will adopt the same ordering from dump_stack(), as you are suggesting.

Anyway, please document any changes in the ordering if there are any.
The current commit message sounds like a code move without any
functional changes.

Best Regards,
Petr