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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Fri, 10 Nov 2017 16:04:48 +0800
Message-ID: <CANRm+CwVaE70_Aod8s6hef_rNsRXovkfisbCj6EABCzOUR6bOg@mail.gmail.com>
Subject: Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Radim Krcmar <rkrcmar@redhat.com>,
        Pankaj Gupta <pagupta@redhat.com>,
        Eduardo Valentin <eduval@amazon.com>,
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2017-11-10 15:59 GMT+08:00 Peter Zijlstra <peterz@infradead.org>:
> 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.

Thanks Peterz, :) I will have a try.

Regards,
Wanpeng Li

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