linux-kernel - Re: [RFC patch] cmpxchg_double: remove local variables to get better performance

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Date:	Fri, 02 Mar 2012 23:12:54 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	jeremy@...p.org,
	"asit.k.mallick@...el.com" <asit.k.mallick@...el.com>,
	"x86@...nel.org" <x86@...nel.org>, tglx@...utronix.de,
	Andi Kleen <ak@...ux.intel.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [RFC patch] cmpxchg_double: remove local variables to get better
 performance

On 03/02/2012 05:11 PM, Jan Beulich wrote:

>>>> On 02.03.12 at 10:00, Alex Shi <alex.shi@...el.com> wrote:
>> Yes, we can use cast for intermediate data. And actually, current kernel
>> has live mis-used case on cmpxchg(), that I plan to point out too. 
>>
>> -- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -203,12 +203,12 @@ static bool make_all_cpus_request(struct kvm *kvm, 
>> unsigned int req)
>>
>>  void kvm_flush_remote_tlbs(struct kvm *kvm)
>>  {
>> -       int dirty_count = kvm->tlbs_dirty;
>> +       long dirty_count = kvm->tlbs_dirty;
>>
>>         smp_mb();
>>         if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
>>                 ++kvm->stat.remote_tlb_flush;
>> -       cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
>> +       cmpxchg(&kvm->tlbs_dirty, dirty_count, 0L);
> 
> Indeed - the cmpxchg would fail if the value doesn't fit. But this is not
> to say that in certain cases it isn't valid to pass an int for the second
> and/or third argument. (And quite likely the issue here is theoretical
> only anyway.)


It may cause potential issue, if it is tlbs_dirty mis-used here, not
dirty_count. If so, it may cause data damage.

> 
> In particular, requiring an L suffix here on literals should be avoided.


Even the each macro may save 0x40 bytes text, and bring more 10%
execution speed?

> 
> Jan
> 
>>  }
> 
> 


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