Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set

[Wanpeng Li]

2017-11-10 1:15 GMT+08:00 Peter Zijlstra <peterz@xxxxxxxxxxxxx>:
> 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