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

* 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


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