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
