linux-kernel - Re: [PATCH 0 of 9] x86/smp function calls: convert x86 tlb flushes to use function calls [POST 2]

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Date:	Tue, 19 Aug 2008 12:24:30 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	Andi Kleen <andi@...stfloor.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH 0 of 9] x86/smp function calls: convert x86 tlb flushes
	to use function calls [POST 2]


* Nick Piggin <nickpiggin@...oo.com.au> wrote:

> > At least we could/should perhaps standardize/generalize all the 
> > 'specific' IPI handlers into the smp_function_call() framework: if 
> > function address equals to a pre-cooked IPI entry point we could 
> > call that function without a kmalloc. As these are all hardwired, 
> > __builtin_is_constant_p() could come to the help as well. Hm?
> 
> No, it's not just the function call but also payload, list entry for 
> queue, scoreboard of CPUs have processed it, a lock, etc etc etc.
> 
> smp_call_function is *always* going to be heavier than a hard wired 
> special case, no matter how it is implemented. For such low level 
> performance critical functionality, I miss the days when people were 
> rabid about saving every cycle rather than every line of code ;)

no, i was thinking about really high level hardwiring, i.e. hardwiring 
the _function pointer_ knowledge into smp_function_call().

for example for the reschedule IPI, it would be hardwired on x86 to just 
call into the special IPI handler, via:

  smp_call_function_mask(target_mask, smp_send_reschedule, NULL, 0);

Exactly same cost and call sequence as a direct hardwired-to-IPI 
function call (and the same underlying mechanism) - just consolidated 
around a single cross-call API.

Same for all the other special cross-CPU handlers. That way some 
architectures would hardwire it, some wouldnt, etc.

	Ingo
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