On Fri, Jan 11, 2013 at 06:23:44PM +0800, David Xu wrote: > On 2013/01/11 17:54, Konstantin Belousov wrote: > > On Mon, Jan 07, 2013 at 08:22:35PM +0200, Konstantin Belousov wrote: > >> Below is the forward of the patch for which I failed to obtain a private > >> review. Might be, the list generates more responses. > >> > >> Our rtld has a performance bootleneck, typically exposed by the images > >> with the lot of the run-time relocation processing, and by the C++ > >> exception handling. We block the signals delivery during the rtld > >> performing the relocations, as well as for the dl_iterate_phdr(3) (the > >> later is used for finding the dwarf unwinding tables). > >> > >> The signal blocking is needed to allow the rtld to resolve the symbols > >> for the signal handlers in the safe way, but also causes 2 syscalls > >> overhead per each rtld entry. > >> > >> The proposed approach allows to shave off those two syscalls, doubling > >> the FreeBSD performance for the (silly) throw/catch C++ microbenchmark. > >> > >> Date: Mon, 13 Aug 2012 15:26:00 +0300 > >> From: Konstantin Belousov <kostik...@gmail.com> > >> > >> ... > >> > >> The basic idea is to implement sigprocmask() as single write into usermode > >> address. If kernel needs to calculate the signal mask for current thread, > >> it takes into the consideration non-zero value of the word at the agreed > >> address. Also, usermode is informed about signals which were put on hold > >> due to fast sigblock active. > >> > >> As I said, on my measurements in microbenchmark that did throw/catch in > >> a loop, I see equal user and system time spent for unpatched system, and > >> same user time with zero system time on patched system. > >> > >> Patch can be improved further, e.g. it would be nice to allow rtld to fall > >> back to sigprocmask(2) if kernel does not support fast sigblock, to prevent > >> flag day. Also, the mask enforced by fast sigblock can be made > >> configurable. > >> > >> Note that libthr already blocks signals by catching them, and not using > >> rtld > >> service in the first line handler. I tried to make the change in the spirit > >> of libthr interceptors, but handoff to libthr appears too complicated to > >> work. In fact, libthr can be changed to start using fast sigblock instead > >> of wrapping sigaction, but this is out of scope of the proposal right now. > >> > >> Please comment. > > > > So there were no overly negative comments, and thanks to Alfred and David > > for useful notes. > > > > The patch at > > http://people.freebsd.org/~kib/misc/rtld-sigblock.2.patch > > is the commit candidate. Now kernel informs rtld about supprt for > > fast_sigblock with new auxv flag. Rtld can operate on old (and possibly > > future) kernels without fast_sigblock, rtld checks the auxv for > > presence of the ELF_BSDF_FASTSIGBLK flag before use. > > > > The patch looks fine to me. Sorry, I still have a question: > if I found a process does not responsing to a signal, now should we > check both its signal mask and fast block value ? but is there such > a tool available ? for example, in gdb.
Fair enough. I added the facility to export the address of the fast sigblock word to the tools, and implemented the procstat extension to print the address with -j. You can indeed use gdb to look up the content of the word when debugging, after getting it address from procstat. http://people.freebsd.org/~kib/misc/rtld-sigblock.3.patch
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