linux-kernel - Re: [PATCH bpf-next 1/5] perf/core: Add PERF_FORMAT_LOST read

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Date:   Fri, 27 Sep 2019 14:28:43 -0700
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Jiri Olsa" <jolsa@...hat.com>
Cc:     <bpf@...r.kernel.org>, <songliubraving@...com>, <yhs@...com>,
        <andriin@...com>, <peterz@...radead.org>, <mingo@...hat.com>,
        <acme@...nel.org>, <ast@...com>,
        <alexander.shishkin@...ux.intel.com>, <namhyung@...nel.org>,
        <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/5] perf/core: Add PERF_FORMAT_LOST
 read_format

Hi Jiri,

On Tue Sep 24, 2019 at 10:33 AM Jiri Olsa wrote:
> On Tue, Sep 17, 2019 at 06:30:52AM -0700, Daniel Xu wrote:
> 
> SNIP
> 
> > +	PERF_FORMAT_MAX = 1U << 5,		/* non-ABI */
> >  };
> >  
> >  #define PERF_ATTR_SIZE_VER0	64	/* sizeof first published struct */
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 0463c1151bae..ee08d3ed6299 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -1715,6 +1715,9 @@ static void __perf_event_read_size(struct perf_event *event, int nr_siblings)
> >  	if (event->attr.read_format & PERF_FORMAT_ID)
> >  		entry += sizeof(u64);
> >  
> > +	if (event->attr.read_format & PERF_FORMAT_LOST)
> > +		entry += sizeof(u64);
> > +
> >  	if (event->attr.read_format & PERF_FORMAT_GROUP) {
> >  		nr += nr_siblings;
> >  		size += sizeof(u64);
> > @@ -4734,6 +4737,24 @@ u64 perf_event_read_value(struct perf_event *event, u64 *enabled, u64 *running)
> >  }
> >  EXPORT_SYMBOL_GPL(perf_event_read_value);
> >  
> > +static struct pmu perf_kprobe;
> > +static u64 perf_event_lost(struct perf_event *event)
> > +{
> > +	struct ring_buffer *rb;
> > +	u64 lost = 0;
> > +
> > +	rcu_read_lock();
> > +	rb = rcu_dereference(event->rb);
> > +	if (likely(!!rb))
> > +		lost += local_read(&rb->lost);
> > +	rcu_read_unlock();
> > +
> > +	if (event->attr.type == perf_kprobe.type)
> > +		lost += perf_kprobe_missed(event);
> 
> not sure what was the peterz's suggestion, but here you are mixing
> ring buffer's lost count with kprobes missed count, seems wrong

To be honest, I'm not 100% sure what the correct semantics here should
be. I thought it might be less misleading if we included ring buffer
related misses as well.

Regardless, I am ok with either.

> maybe we could add PERF_FORMAT_KPROBE_MISSED

I think the feedback from the last patchset was that we want to keep
the misses unified.

Peter, do you have any thoughts?

Thanks,
Daniel