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 20:49:51 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ingo Molnar <mingo@...e.hu>
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]

On Tuesday 19 August 2008 20:24, Ingo Molnar wrote:
> * 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.

So it would magically use a different IPI vector, magically be callable
from interrupt context, have a different function type etc.? Why would
we do such a thing? If it doesn't walk like a duck and doesn't quack
like a duck, why should we call it a duck? ;)


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

I think the right way is just to keep the "special" classes of IPI-ish
functions (I guess reschedule and TLB flush importantly). The arch code
is still free to implement these using their call function or a special
vector or whatever.
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