Fast sigblock (AKA rtld speedup)

[Konstantin Belousov]

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 <kostikbel at 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.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 834 bytes
Desc: not available
URL: <http://lists.freebsd.org/pipermail/freebsd-arch/attachments/20130111/fd39a19e/attachment.sig>