linux-kernel - Re: signals logged / [RFC] log out-of-virtual-memory events

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Message-ID: <Pine.LNX.4.61.0705202235430.13923@yvahk01.tjqt.qr>
Date:	Sun, 20 May 2007 22:38:08 +0200 (MEST)
From:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
To:	Folkert van Heusden <folkert@...heusden.com>
cc:	Stephen Hemminger <shemminger@...ux-foundation.org>,
	Eric Dumazet <dada1@...mosbay.com>, Andi Kleen <ak@...e.de>,
	Rik van Riel <riel@...hat.com>,
	righiandr@...rs.sourceforge.net,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: signals logged / [RFC] log out-of-virtual-memory events


On May 20 2007 18:12, Folkert van Heusden wrote:
>> >  
>> > +	if (unlikely(sig == SIGQUIT || sig == SIGILL  || sig == SIGTRAP ||
>> > +	    sig == SIGABRT || sig == SIGBUS  || sig == SIGFPE  ||
>> > +	    sig == SIGSEGV || sig == SIGXCPU || sig == SIGXFSZ ||
>> > +	    sig == SIGSYS  || sig == SIGSTKFLT))
>> > +	{
>> > +		printk(KERN_WARNING "Sig %d send to %d owned by %d.%d (%s)\n",
>> > +			sig, t->pid, t->uid, t->gid, t->comm);
>> > +	}
>> > +
>> >  	/*
>> >  	 * fast-pathed signals for kernel-internal things like SIGSTOP
>> >  	 * or SIGKILL.
>> 
>> Would turning that into a switch() generate better code.

Yes, this time.

>Doubt it: in the worst case you still nee to check for each possibility.
>Furthermore a.f.a.i.k. with switch you cannot do 'unlinkely()'.

With if(), it generates a ton of "CMP, JE" instructions.
With switch(), I would assume gcc transforms it into using
a jump table (aka "JMP [table+sig]")

I tried it: with switch(), gcc transforms this into a
bitmap comparison ("MOV eax, 1; SHL eax, sig; TEST eax, 0x830109f8"),
which seems even cheaper than a jump table.


	Jan
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