linux-kernel - Re: [PATCH v7 07/12] perf/x86: no counter allocation support

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Date:   Tue, 9 Jul 2019 11:43:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, ak@...ux.intel.com, kan.liang@...el.com,
        mingo@...hat.com, rkrcmar@...hat.com, like.xu@...el.com,
        jannh@...gle.com, arei.gonglei@...wei.com, jmattson@...gle.com
Subject: Re: [PATCH v7 07/12] perf/x86: no counter allocation support

On Tue, Jul 09, 2019 at 10:58:46AM +0800, Wei Wang wrote:
> On 07/08/2019 10:29 PM, Peter Zijlstra wrote:
> 
> Thanks for the comments.
> 
> > 
> > > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > > index 0ab99c7..19e6593 100644
> > > --- a/include/linux/perf_event.h
> > > +++ b/include/linux/perf_event.h
> > > @@ -528,6 +528,7 @@ typedef void (*perf_overflow_handler_t)(struct perf_event *,
> > >    */
> > >   #define PERF_EV_CAP_SOFTWARE		BIT(0)
> > >   #define PERF_EV_CAP_READ_ACTIVE_PKG	BIT(1)
> > > +#define PERF_EV_CAP_NO_COUNTER		BIT(2)
> > >   #define SWEVENT_HLIST_BITS		8
> > >   #define SWEVENT_HLIST_SIZE		(1 << SWEVENT_HLIST_BITS)
> > > @@ -895,6 +896,13 @@ extern int perf_event_refresh(struct perf_event *event, int refresh);
> > >   extern void perf_event_update_userpage(struct perf_event *event);
> > >   extern int perf_event_release_kernel(struct perf_event *event);
> > >   extern struct perf_event *
> > > +perf_event_create(struct perf_event_attr *attr,
> > > +		  int cpu,
> > > +		  struct task_struct *task,
> > > +		  perf_overflow_handler_t overflow_handler,
> > > +		  void *context,
> > > +		  bool counter_assignment);
> > > +extern struct perf_event *
> > >   perf_event_create_kernel_counter(struct perf_event_attr *attr,
> > >   				int cpu,
> > >   				struct task_struct *task,
> > Why the heck are you creating this wrapper nonsense?
> 
> (please see early discussions: https://lkml.org/lkml/2018/9/20/868)
> I thought we agreed that the perf event created here don't need to consume
> an extra counter.

That's almost a year ago; I really can't remember that and you didn't
put any of that in your Changelog to help me remember.

(also please use: https://lkml.kernel.org/r/$msgid style links)

> In the previous version, we added a "no_counter" bit to perf_event_attr, and
> that will be exposed to user ABI, which seems not good.
> (https://lkml.org/lkml/2019/2/14/791)
> So we wrap a new kernel API above to support this.
> 
> Do you have a different suggestion to do this?
> (exclude host/guest just clears the enable bit when on VM-exit/entry,
> still consumes the counter)

Just add an argument to perf_event_create_kernel_counter() ?