linux-kernel - Re: [PATCH 3/3] KVM: x86/vPMU: Add lazy mechanism to release perf

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Date:   Wed, 9 Oct 2019 11:14:56 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        rkrcmar@...hat.com, sean.j.christopherson@...el.com,
        vkuznets@...hat.com, Jim Mattson <jmattson@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        ak@...ux.intel.com, wei.w.wang@...el.com, kan.liang@...el.com,
        like.xu@...el.com, ehankland@...gle.com, arbel.moshe@...cle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: x86/vPMU: Add lazy mechanism to release
 perf_event per vPMC

On 2019/10/8 20:11, Peter Zijlstra wrote:
> On Tue, Oct 01, 2019 at 08:33:45PM +0800, Like Xu wrote:
>> Hi Peter,
>>
>> On 2019/10/1 16:23, Peter Zijlstra wrote:
>>> On Mon, Sep 30, 2019 at 03:22:57PM +0800, Like Xu wrote:
>>>> +	union {
>>>> +		u8 event_count :7; /* the total number of created perf_events */
>>>> +		bool enable_cleanup :1;
>>>
>>> That's atrocious, don't ever create a bitfield with base _Bool.
>>
>> I saw this kind of usages in the tree such as "struct
>> arm_smmu_master/tipc_mon_state/regmap_irq_chip".
> 
> Because other people do tasteless things doesn't make it right.
> 
>> I'm not sure is this your personal preference or is there a technical
>> reason such as this usage is not incompatible with union syntax?
> 
> Apparently it 'works', so there is no hard technical reason, but
> consider that _Bool is specified as an integer type large enough to
> store the values 0 and 1, then consider it as a base type for a
> bitfield. That's just disguisting.

It's reasonable. Thanks.

> 
> Now, I suppose it 'works', but there is no actual benefit over just
> using a single bit of any other base type.
> 
>> My design point is to save a little bit space without introducing
>> two variables such as "int event_count & bool enable_cleanup".
> 
> Your design is questionable, the structure is _huge_, and your union has
> event_count:0 and enable_cleanup:0 as the same bit, which I don't think
> was intentional.
> 
> Did you perhaps want to write:
> 
> 	struct {
> 		u8 event_count : 7;
> 		u8 event_cleanup : 1;
> 	};
> 
> which has a total size of 1 byte and uses the low 7 bits as count and the
> msb as cleanup.

Yes, my union here is wrong and let me fix it in the next version.

> 
> Also, the structure has plenty holes to stick proper variables in
> without further growing it.

Yes, we could benefit from it.

> 
>> By the way, is the lazy release mechanism looks reasonable to you?
> 
> I've no idea how it works.. I don't know much about virt.
>