Messages in this thread | | | Subject | Re: perf_counters issue with self-sampling threads | From | Peter Zijlstra <> | Date | Wed, 29 Jul 2009 14:19:08 +0200 |
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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.
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.
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.
Alternatively, I've missed a detail and I either read the manpage wrong, or the code, or both of them.
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