linux-kernel - Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV

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Message-ID: <20171110075914.mcjp2niadqszo3yb@hirez.programming.kicks-ass.net>
Date:   Fri, 10 Nov 2017 08:59:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Radim Krcmar <rkrcmar@...hat.com>,
        Pankaj Gupta <pagupta@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Matt Wilson <msw@...zon.com>, Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Waiman Long <longman@...hat.com>, kvm <kvm@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Jan H . Schoenherr" <jschoenh@...zon.de>,
        Anthony Liguori <aliguori@...zon.com>
Subject: Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set

On Fri, Nov 10, 2017 at 10:07:56AM +0800, Wanpeng Li wrote:

> >> Also, you should not put cpumask_t on stack, that's 'broken'.
> 
> Thanks pointing out this. I found a useful comments in arch/x86/kernel/irq.c:
> 
> /* These two declarations are only used in check_irq_vectors_for_cpu_disable()
>  * below, which is protected by stop_machine().  Putting them on the stack
>  * results in a stack frame overflow.  Dynamically allocating could result in a
>  * failure so declare these two cpumasks as global.
>  */
> static struct cpumask affinity_new, online_new;

That code no longer exists.. Also not entirely sure how it would be
helpful.

What you probably want to do is have a per-cpu cpumask, since
flush_tlb_others() is called with preemption disabled. But you probably
don't want an unconditionally allocated one, since most kernels will not
in fact be PV.

So you'll want something like:

	static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);

And then you need something like:

	for_each_possible_cpu(cpu) {
		zalloc_cpumask_var_node(per_cpu_ptr(&__pb_tlb_mask, cpu),
					GFP_KERNEL, cpu_to_node(cpu));
	}

before you set the pv-op or so.