linux-kernel - Re: [PATCH v4 7/8] KVM: Pre-allocate cpumasks for kvm_make_all_cpus_request

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Date:   Fri, 03 Sep 2021 09:20:44 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 7/8] KVM: Pre-allocate cpumasks for
 kvm_make_all_cpus_request_except()

Sean Christopherson <seanjc@...gle.com> writes:

> On Fri, Aug 27, 2021, Vitaly Kuznetsov wrote:
>> Allocating cpumask dynamically in zalloc_cpumask_var() is not ideal.
>> Allocation is somewhat slow and can (in theory and when CPUMASK_OFFSTACK)
>> fail. kvm_make_all_cpus_request_except() already disables preemption so
>> we can use pre-allocated per-cpu cpumasks instead.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  virt/kvm/kvm_main.c | 29 +++++++++++++++++++++++------
>>  1 file changed, 23 insertions(+), 6 deletions(-)
>> 
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 2e9927c4eb32..2f5fe4f54a51 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -155,6 +155,8 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm);
>>  static unsigned long long kvm_createvm_count;
>>  static unsigned long long kvm_active_vms;
>>  
>> +static DEFINE_PER_CPU(cpumask_var_t, cpu_kick_mask);
>> +
>>  __weak void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
>>  						   unsigned long start, unsigned long end)
>>  {
>> @@ -323,14 +325,15 @@ bool kvm_make_all_cpus_request_except(struct kvm *kvm, unsigned int req,
>>  				      struct kvm_vcpu *except)
>>  {
>>  	struct kvm_vcpu *vcpu;
>> -	cpumask_var_t cpus;
>> +	struct cpumask *cpus;
>>  	bool called;
>>  	int i, me;
>>  
>> -	zalloc_cpumask_var(&cpus, GFP_ATOMIC);
>> -
>>  	me = get_cpu();
>>  
>> +	cpus = this_cpu_cpumask_var_ptr(cpu_kick_mask);
>> +	cpumask_clear(cpus);
>> +
>>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>>  		if (vcpu == except)
>>  			continue;
>> @@ -340,7 +343,6 @@ bool kvm_make_all_cpus_request_except(struct kvm *kvm, unsigned int req,
>>  	called = kvm_kick_many_cpus(cpus, !!(req & KVM_REQUEST_WAIT));
>>  	put_cpu();
>>  
>> -	free_cpumask_var(cpus);
>>  	return called;
>>  }
>>  
>> @@ -5581,9 +5583,15 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
>>  		goto out_free_3;
>>  	}
>>  
>> +	for_each_possible_cpu(cpu) {
>> +		if (!alloc_cpumask_var_node(&per_cpu(cpu_kick_mask, cpu),
>> +					    GFP_KERNEL, cpu_to_node(cpu)))
>> +			goto out_free_4;
>
> 'r' needs to be explicitly set to -EFAULT, e.g. in the current code it's
> guaranteed to be 0 here.

Oops, yes. Any particular reason to avoid -ENOMEM? (hope not, will use
this in v5)

>
>> +	}
>> +
>>  	r = kvm_async_pf_init();
>>  	if (r)
>> -		goto out_free;
>> +		goto out_free_5;
>>  
>>  	kvm_chardev_ops.owner = module;
>>  	kvm_vm_fops.owner = module;
>> @@ -5609,7 +5617,11 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
>>  
>>  out_unreg:
>>  	kvm_async_pf_deinit();
>> -out_free:
>> +out_free_5:
>> +	for_each_possible_cpu(cpu) {
>
> Unnecessary braces.
>
>> +		free_cpumask_var(per_cpu(cpu_kick_mask, cpu));
>> +	}
>> +out_free_4:
>>  	kmem_cache_destroy(kvm_vcpu_cache);
>>  out_free_3:
>>  	unregister_reboot_notifier(&kvm_reboot_notifier);
>> @@ -5629,8 +5641,13 @@ EXPORT_SYMBOL_GPL(kvm_init);
>>  
>>  void kvm_exit(void)
>>  {
>> +	int cpu;
>> +
>>  	debugfs_remove_recursive(kvm_debugfs_dir);
>>  	misc_deregister(&kvm_dev);
>> +	for_each_possible_cpu(cpu) {
>
> Same here.
>
>> +		free_cpumask_var(per_cpu(cpu_kick_mask, cpu));
>> +	}
>>  	kmem_cache_destroy(kvm_vcpu_cache);
>>  	kvm_async_pf_deinit();
>>  	unregister_syscore_ops(&kvm_syscore_ops);
>> -- 
>> 2.31.1
>> 
>

-- 
Vitaly