* Robert Richter <r...@kernel.org> wrote:
> On 26.06.13 10:24:08, Borislav Petkov wrote:
> > On Wed, Jun 26, 2013 at 10:12:23AM +0200, Robert Richter wrote:
> > > We get a new fd by opening the persistent event with the syscall.
> > > There would be 2 new ioctls:
> > >
> > > ioctl(fd, PERF_EVENT_IOC_DETACH, 0);
> > > ioctl(fd, PERF_EVENT_IOC_ATTACH, 0);
> > >
> > > This would be fine and reuses existing infrastructure.
> >
> > Well, how are you going to say that you want to open an already existing
> > persistent event or your want to create exactly the same persistent
> > event? Are we even going to allow identical persistent events to
> > coexist?
>
> Here is the scenario:
Looks mostly good - with a few suggestions:
>
> Creating a persistent event from userspace:
>
> * A process opens a system-wide event with the syscall and gets a fd.
Should this really be limited to system-wide events?
> * The process mmaps the buffer.
> * The process does an ioctl to detach the process which increases the
> events and buffers refcount. The event is listed as 'persistent' in
> sysfs with a unique id.
> * The process closes the fd. Event and buffer remain in the system
> since the refcounts are not zero.
>
> Opening a persistent event:
>
> * A process scans sysfs for persistent events.
> * To open the event it sets up the event attr according to sysfs.
Basically it would just put some ID (found in sysfs) into the attr and set
attr.persistent=1 - not any other information, right?
If it knows the ID straight away (the user told it, or it remembers it
from some other file such as a temporary file, etc.) then it does not even
have to scan sysfs.
[ How about to additional logic: attr.persistent=1 && attr.config==0 means
a new persistent event is created straight away - no ioctl is needed to
detach it explicitly. ]
> * The persistent event is opened with the syscall, the process gets a
> new fd of the event.
> * The process attaches to the event buffer with mmap.
Yes. And gets the pre-existing event and mmap buffer.
> Releasing a persistent event:
>
> * A process opens a persistent event and gets a fd.
> * The process does an ioctl to attach the process which decreases the
> refcounts. The sysfs entry is removed.
> * The process closes the fd.
> * After all processes that are tied to the event closed their event's
> fds, the persistent event and its buffer is released.
>
> Sounds like a plan?
It does :-)
I'm sure there will be some details going down that path, but it looks
workable at first glance.
Note, for tracing the PERF_FLAG_FD_OUTPUT method of multiplexing multiple
events onto a single mmap buffers is probably useful (also usable via the
PERF_EVENT_IOC_SET_OUTPUT ioctl()), so please make sure the scheme works
naturally with that model as well, not just with 1:1 event+buffer
mappings.
See the uses of PERF_EVENT_IOC_SET_OUTPUT in tools/perf/.
Thanks,
Ingo
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