Peter,
On Wed, Jul 29, 2009 at 2:19 PM, Peter Zijlstra<[email protected]> wrote:
> On Mon, 2009-07-27 at 18:51 +0200, stephane eranian wrote:
>> I believe there is a problem with the current perf_counters (PCL)
>> code for self-sampling threads. The problem is related to sample
>> notifications via signal.
>>
>> PCL (just like perfmon) is using SIGIO, an asynchronous signal,
>> to notify user applications of the availability of data in the event
>> buffer.
>>
>> 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.
>
> This signal stuff makes my head spin a little, however:
>
> 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. Consequently, it may be necessary to pass F_SETOWN the
> result of gettid(2) instead of get‐ pid(2) to get sensible results
> when F_SETSIG is used. (In current Linux threading
> implementations, a main thread’s thread ID is the same as its process
> ID. This means that a single-threaded program can equally use
> gettid(2) or getpid(2) in this scenario.) Note, how‐ ever, that
> the statements in this paragraph do not apply to the SIGURG signal
> generated for out-of-band data on a socket: this signal is always
> sent to either a process or a process group, depending on the value
> given to F_SETOWN. Note also that Linux imposes a limit on the
> number of real-time signals that may be queued to a process (see
> getrlimit(2) and signal(7)) and if this limit is reached, then the
> kernel reverts to delivering SIGIO, and this signal is delivered
> to the entire process rather than to a specific thread.
>
>
> 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.
>
Yes, I remember that manpage. I got the same impression and in fact that is
what I document in some of my test programs. So you read this right.
> However,
>
> kill_fasync(SIGIO)
> __kill_fasync()
> send_sigio()
> /* if pid_type is a PIDTYPE_PID and pid a TID this should
> only iterate the one thread, I think */
> do_each_pid_task() {
> send_sigio_to_task();
> } while_each_pid_task();
>
> where:
>
> send_sigio_to_task()
> group_send_sig_info()
> __group_send_sig_info()
> send_signal(.group = 1) /* uh-ow trouble */
> __send_signal()
> if (group)
> pending = &t->signal->shared_pending
>
> which will result in the signal being send to the whole process anyway.
>
Exactly! That is the code path and this is why this does not work as
expected. Nowhere along that path is there special casing for that
F_SETOWN of tid vs. pid. kill_fasync() implies group.
>
> Now I was considering teaching send_sigio_to_task() to use
> specific_send_sig_info() when fown->pid != fown->group_leader->pid or
> something, but I'm not sure that won't break anything.
>
Yes, that's the problem with touching this. I don't know if this will break
things. That's why I was suggested creating a parallel code path which
does what we want without modifying the existing path. Unless you know
some signal expert at redhat or elsewhere.
> Alternatively, I've missed a detail and I either read the manpage wrong,
> or the code, or both of them.
>
The code does not correspond to the manpage. Not clear which one
is correct though. This F_SETOWN trick looks very Linux specific.
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