linux-kernel - Re: Subject: [PATCH 1/2] x86: get back 15 vectors

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Message-ID: <4B4273D4.2080709@zytor.com>
Date:	Mon, 04 Jan 2010 15:03:48 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jesse Brandeburg <jesse.brandeburg@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	NetDEV list <netdev@...r.kernel.org>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: Subject: [PATCH 1/2] x86: get back 15 vectors

On 01/04/2010 02:01 PM, Yinghai Lu wrote:
>>
>>>  /*
>>> + * First APIC vector available to drivers: (vectors 0x30-0xee) we
>>> + * start at 0x31 to spread out vectors evenly between priority
>>> + * levels. (0x80 is the syscall vector)
>>> + */
>>> +#define FIRST_DEVICE_VECTOR		(IRQ15_VECTOR + 2)
>>> +
>>
>> We really should fix that so we can do +1 here instead of +2; that
>> presumably means fixing the logic so we do something smarter than just
>> jump over 0x80.
> 
> we already use used_vectors to skip 0x80. so we could change that to +1?
> 

Yes, but the problem is that we *skip* 0x80, which leads to suboptimal
allocation on systems with only a handful of vectors.

The easy solution to accomplishing what we want without wasting vector
0x30 is obviously to start allocation at 0x31, but not by artificially
limiting the vector space; see the attached patch.

For what it's worth, this code(__assign_irq_vector() in
arch/x86/kernel/apic/io_apic.c) has me somewhat confused about the use
of the constant 8:

		vector += 8;

The only justification that I can immediately think of is to try to
assign exactly two sources to each priority level (since early APICs
started losing interrupts with more than two sources per priority level.)

This is ancient code -- predates not just the git but the bk history --
and as such I would assume that that is the motivation.

	-hpa

View attachment "0001-x86-irq-Don-t-waste-a-vector-to-improve-vector-sprea.patch" of type "text/x-patch" (2373 bytes)