From owner-freebsd-net  Thu Nov 29  4:26:40 2001
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Date: Thu, 29 Nov 2001 23:26:20 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Luigi Rizzo <rizzo@aciri.org>
Cc: <net@FreeBSD.ORG>
Subject: Re: Revised polling code for STABLE
In-Reply-To: <20011126115704.L88153@iguana.aciri.org>
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> On Mon, 26 Nov 2001, Luigi Rizzo wrote:
>
> > [Bcc to -stable in case someone is interested there]
> >
> > Attached you can find the latest version of the polling code
> > for STABLE (which is useful to make boxes much more robust
> > to attacks and have much better responsiveness under [over]load).
> > ...
> Index: sys/i386/i386/swtch.s
> ===================================================================
> RCS file: /home/xorpc/u2/freebsd/src/sys/i386/i386/swtch.s,v
> retrieving revision 1.89.2.4
> diff -u -r1.89.2.4 swtch.s
> --- sys/i386/i386/swtch.s	2001/07/26 02:29:10	1.89.2.4
> +++ sys/i386/i386/swtch.s	2001/10/26 21:54:19
> @@ -246,6 +246,17 @@
>  	call	_procrunnable
>  	testl	%eax,%eax
>  	CROSSJUMP(jnz, sw1a, jz)
> +#ifdef	XORP_ETHER_POLLING
> +	incl	_idle_done
> +	pushl	_poll_burst
> +	call	_ether_poll
> +	addl	$4,%esp
> +	sti
> +	nop
> +	cli
> +	testl   %eax, %eax
> +	jnz     idle_loop
> +#endif
>  	call	_vm_page_zero_idle
>  	testl	%eax, %eax
>  	jnz	idle_loop

This has some serious bugs:
(1) ether_poll() is called with interrupts disabled.  This breaks fast
    interrupt handlers.  ether_poll() uses splimp() so it is apparently
    not intended to run with priority higher than splhigh().
(2) The sti/cli stuff belongs in ether_poll(), and the test of the return
    code of ether_poll() only works because ether_poll() always returns 1.
    vm_page_zero_idle() and the hlt instruction are never reached and the
    system spins calling ether_poll() from idle until a process becomes
    runnable.  ether_poll() needs quite different handling than
    vm_page_zero_idle() since it can reasonably find something to do
    whenever an interface is up, while vm_page_zero_idle() soon runs out
    of things to do if it is called a lot.

> Index: sys/kern/kern_clock.c

It shouldn't be necessary to touch this file.  Everything involving
"ether" certainly doesn't belong here.  Everything related to periodic
polling can be done, probably more efficiently, using schedsoft*() or
timeout().  E.g., schedsoftnet() arranges for netisrs to be checked
for at the next clock interrupt.  But this can now be done almost as
well (probably better in -current) using an ordinary timeout.

Bruce


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