Re: perf_counters issue with self-sampling threads

[Oleg Nesterov <oleg@redhat.com>]

(add Roland)

On 07/29, Peter Zijlstra wrote:
>
> On Mon, 2009-07-27 at 18:51 +0200, stephane eranian wrote:
> >
> > POSIX does not mandate that asynchronous signals be delivered
> > to the thread in which they originated. Any thread in the process
> > may process the signal, assuming it does not have the signal
> > blocked.

Yes. I now nothing about POSIX, but this is what Linux does at least.
I don't think we can/should change this behaviour.

> fcntl(2) for F_SETOWN says:
>
> If a non-zero value is given to F_SETSIG  in  a  multi‐ threaded
> process running with a threading library that supports thread groups
> (e.g., NPTL),  then  a  positive value  given  to  F_SETOWN  has  a
> different  meaning: instead of being a process ID identifying a whole
> pro‐ cess,  it  is a thread ID identifying a specific thread within a
> process.

Heh. Definitely this is not what Linux does ;)

> Which seems to imply that when we feed fcntl(F_SETOWN) a TID instead of
> a PID it should deliver SIGIO to the thread instead of the whole process
> -- which, to me, seems a sane semantic.

I am not sure I understand the man above... But to me it looks like we
should always send a private signal when fown->signum != 0 ?

The change should be simple, but as you pointed out we can break things.

Oleg.