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Date:   Fri, 10 Nov 2017 10:07:56 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
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

2017-11-10 1:15 GMT+08:00 Peter Zijlstra <peterz@...radead.org>:
> On Thu, Nov 09, 2017 at 06:12:41PM +0100, Peter Zijlstra wrote:
>> On Thu, Nov 09, 2017 at 05:45:23PM +0100, Radim Krcmar wrote:
>> > 2017-11-09 17:17+0100, Peter Zijlstra:
>> > > On Thu, Nov 09, 2017 at 05:05:36PM +0100, Radim Krcmar wrote:
>> > > > 2017-11-09 10:53-0500, Pankaj Gupta:
>> > > > > 2] PV TLB should also behave as per option PV_DEDICATED for better performance.
>> > > >
>> > > > Right,
>> > >
>> > > Shouldn't KVM do flush_tlb_other() in any case? Not sure how
>> > > PV_DEDICATED can help with that.
>> >
>> > It will, the suggestion was based on recent extension of the
>> > flush_tlb_others implementaion, https://lkml.org/lkml/2017/11/8/1146.
>> >
>> > PV_TLB_FLUSH allows a guest to set a flush bit instead of sending flush
>> > IPI if the target VCPU is not running.  This would be a waste of time
>> > with PV_DEDICATED as all VCPUs are expected to always running.
>> >
>> > With PV_DEDICATED, the guest should keep using native_flush_tlb_others.
>>
>> Is saving that for_each_cpu() really worth the effort compared to the
>> cost of actually doing the IPIs and CR3 write?
>>
>> 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;

>
> Also, you'll want to use __cpumask_clear_cpu() there.

Will do.

Regards,
Wanpeng Li

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