Re: [RFC][PATCH] signals: don't copy siginfo_t on dequeue

[Ingo Molnar <mingo@elte.hu>]

* Vegard Nossum <vegard.nossum@gmail.com> wrote:

> >From 60fc9a464377159ab807aec63277d4970019d631 Mon Sep 17 00:00:00 2001
> From: Vegard Nossum <vegard.nossum@gmail.com>
> Date: Thu, 26 Feb 2009 19:17:58 +0100
> Subject: [PATCH] signals: don't copy siginfo_t on dequeue
> 
> Instead of copying the siginfo_t whenever a signal is dequeued, just
> get the pointer to the struct sigqueue, which can be freed by the
> caller when the signal has been delivered.
> 
> We can save kernel text (x86, 32-bit):
> 
> $ scripts/bloat-o-meter vmlinux-unpatched vmlinux
> add/remove: 0/0 grow/shrink: 3/7 up/down: 81/-538 (-457)
> function                                     old     new   delta
> get_signal_to_deliver                        871     922     +51
> release_console_sem                          459     481     +22
> generate_resume_trace                        611     619      +8
> send_sigqueue                                257     253      -4
> vma_adjust                                  1101    1093      -8
> sys_rt_sigtimedwait                          548     531     -17
> dequeue_signal                               415     372     -43
> __dequeue_signal                             388     259    -129
> signalfd_read                               1290    1139    -151
> do_notify_resume                            2216    2030    -186
> 
> And we reduce stack pressure; In handle_signal() (in x86 code), we
> replace a siginfo_t (128 bytes) with a pointer (8 bytes on x86_64),
> and the same in signalfd_read().
> 
> There is a slight slowdown (2.02% relative increase in CPU time):
> 
> 		unpatched	patched
> ----------------------------------------
> mean:		3.078500	3.140800
> stddev:	0.074624	0.168989
> 
> (Numbers are: CPU time in seconds, for two processes to 
> ping-pong in total 655360 SIGUSR1/SIGUSR2 signals between each 
> other. This was repeated 100 times for each kernel.)

hm, does this SIGUSR1/SIGUSR2 test actually make use siginfo?

I.e. shouldnt we have seen a speedup, due to not having to copy 
the siginfo structure?

	Ingo