Re: [PATCH v13 2/8] perf evlist: Introduce aux evlist

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Jul 06, 2016 at 08:16:52PM +0800, Wangnan (F) wrote:
> 
> 
> On 2016/7/6 19:36, Jiri Olsa wrote:
> > On Mon, Jul 04, 2016 at 06:20:03AM +0000, Wang Nan wrote:
> > 
> > SNIP
> > 
> > > +struct perf_evlist *perf_evlist__new_aux(struct perf_evlist *parent)
> > > +{
> > > +	struct perf_evlist *evlist;
> > > +
> > > +	if (perf_evlist__is_aux(parent)) {
> > > +		pr_err("Internal error: create aux evlist from another aux evlist\n");
> > > +		return NULL;
> > > +	}
> > > +
> > > +	evlist = zalloc(sizeof(*evlist));
> > > +	if (!evlist)
> > > +		return NULL;
> > > +
> > > +	perf_evlist__init(evlist, parent->cpus, parent->threads);
> > > +	evlist->parent = parent;
> > > +	INIT_LIST_HEAD(&evlist->list);
> > > +	list_add(&evlist->list, &parent->children);
> > I understand there's some reason for separating maps with and
> > without overwrite set, but I'm missing it.. why is that?
> 
> You are asking overwrite, not write_backward?
> 
> Overwrite mapping needs to be mapped without PROT_WRITE, so its
> control page is also read only, so perf_evlist__mmap_consume() is
> not able to use, and there's no way to tell kernel to where we have
> read. Kernel overwrite old records when its full. Compare with normal
> mapping: perf uses perf_evlist__mmap_consume() to tell kernel the
> last byte it has read, so kernel stop writing data to it when it full,
> and issues LOST event. This is the reason we need to separate maps
> with and without overwrite set.
> 
> For write backward: kernel write data in different direction, so
> requires map separation.

I dont like the idea of duplicating whole perf_evlist
in order just to map some events with overwrite/backward

perf_evlist carries all the other info about events,
not just memory maping..

I think it'd be better to do it some other way, like:

  - we have mmaps for events/evsels, so you're able to map
    it differently with or without PROT_WRITE even in current
    design.. there's struct perf_mmap that can carry that info
    then it's the matter of reading/processing those maps
    that needs to change.. new perf_evlist interface

  - we could keep separate struct perf_mmap arrays for forward
    and backward/overwrite maps

  - ...

I understand both mapping need different treatment,
but I think that should be encapsulated within the
struct perf_evlist interface 

thanks,
jirka