Messages in this thread | | | Subject | Re: [PATCH 3/3] KVM: x86/vPMU: Add lazy mechanism to release perf_event per vPMC | From | Like Xu <> | Date | Wed, 9 Oct 2019 11:14:56 +0800 |
| |
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. >
| |