linux-kernel - Re: [PATCH] FRV: do_gettimeofday() should no longer use tickadj

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Message-ID: <17274.1157553962@warthog.cambridge.redhat.com>
Date:	Wed, 06 Sep 2006 15:46:02 +0100
From:	David Howells <dhowells@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Howells <dhowells@...hat.com>,
	john stultz <johnstul@...ibm.com>,
	Adrian Bunk <bunk@...sta.de>, Andrew Morton <akpm@...l.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Jeff Garzik <jeff@...zik.org>,
	netdev@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH] FRV: do_gettimeofday() should no longer use tickadj 

Ingo Molnar <mingo@...e.hu> wrote:

> we'll get rid of that pt_regs thing centrally, from all drivers at once 
> - there's upstream buy-in for that already, and Thomas already generated 
> a test-patch for that a few months ago. But it's not a big issue right 
> now.

Yay!  Can you give me a pointer to the patch?

> this shouldnt be a big issue either - we use indirect jumps all around 
> the kernel.

Yes, I know.  I'm sometimes concerned at just how fast indirect jumps (and even
direct calls) are proliferating.  Look at the read syscall path for something
like ext3 these days: it's like a pile of spaghetti.  That seems particularly
true of direct-IO where it seems to weave in and out of core code and the
filesystem as it goes down.  I'm also concerned about stack usage.

> CPUs are either smart enough to predict it

I was told a while back (2002?) not to use indirect pointers for some stuff
because CPUs _couldn't_ predict it.  Maybe this has changed in modern CPUs.

> >  (3) ACK'ing and controlling interrupts has to be done by groups.
> 
> please be more specific,

Under some circumstances I can work out which sources have triggered which
interrupts (there are various off-CPU FPGAs which implement auxiliary PICs that
do announce their sources), but the aux-PIC channels are grouped together upon
delivery to the CPU PIC, so some of the ACK'ing has to be done at the group
level.

> how is this not possible via genirq?

How is it possible with genirq?

Unless I tie all the grouped sources together into one virtual IRQ line, this
doesn't appear to be possible.  But doing that I might then also have a mixed
set of "flow" types in any particular IRQ.

> >  (4) No account is taken of interrupt priority.
> 
> hm, i'm not sure what you mean - could you be more specific?

The FRV CPU, like many others, supports interrupt prioritisation.  A particular
interrupt level is set in the PSR, and any interrupt of a higher priority can
interrupt.  do_IRQ() can then do the interrupt processing in the interrupt
level of the interrupt that invoked it, thus permitting higher priority
interrupts to still happen.

> but ... somehow the current FRV code does figure out which IRQ source 
> fired, right?

Not always; sometimes it has to fall back to polling the drivers unfortunately.

Btw why are we using IRQ_INPROGRESS, IRQ_DISABLED, IRQ_PENDING and friends?
They would appear unnecessary.

David
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