linux-kernel - Re: [PATCH V2 2/4] LoongArch: Fix sleeping in atomic context for PREEMPT

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Message-ID: <20241120080521.ynDZpGQd@linutronix.de>
Date: Wed, 20 Nov 2024 09:05:21 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Huacai Chen <chenhuacai@...ngson.cn>, loongarch@...ts.linux.dev,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-rt-devel@...ts.linux.dev, Xuerui Wang <kernel@...0n.name>,
	Jiaxun Yang <jiaxun.yang@...goat.com>, linux-kernel@...r.kernel.org,
	loongson-kernel@...ts.loongnix.cn
Subject: Re: [PATCH V2 2/4] LoongArch: Fix sleeping in atomic context for
 PREEMPT_RT

On 2024-11-20 15:46:08 [+0800], Huacai Chen wrote:
> > > diff --git a/arch/loongarch/mm/tlb.c b/arch/loongarch/mm/tlb.c
> > > index 5ac9beb5f093..3b427b319db2 100644
> > > --- a/arch/loongarch/mm/tlb.c
> > > +++ b/arch/loongarch/mm/tlb.c
> > > @@ -289,7 +289,7 @@ static void setup_tlb_handler(int cpu)
> > >               /* Avoid lockdep warning */
> > >               rcutree_report_cpu_starting(cpu);
> > >
> > > -#ifdef CONFIG_NUMA
> > > +#if defined(CONFIG_NUMA) && !defined(CONFIG_PREEMPT_RT)
> > >               vec_sz = sizeof(exception_handlers);
> >
> > How does this work with NUMA and RT? You don't allocate memory and
> > everything is fine? Couldn't you pre-allocate the memory on the boot CPU
> > before kicking the CPU to boot? And then just assign the memory here.
> Allocating percpu exception pages on the own node is just an
> optimization, everything can work without this optimization.
> Preallocation is meaningless because all pages come from Node-0.

Don't you have alloc_pages_node() where you can set the node?
And I mean pre-allocation so you don't have to allocate memory here but
already have it ready.

> Huacai

Sebastian