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    SubjectRe: [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@infradead.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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