linux-kernel - Re: [PATCH v5 10/10] module: Rework module_addr

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Date:	Tue, 14 Apr 2015 08:42:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Ingo Molnar <mingo@...nel.org>, mathieu.desnoyers@...icios.com,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	andi@...stfloor.org, rostedt@...dmis.org, tglx@...utronix.de,
	laijs@...fujitsu.com, linux@...izon.com
Subject: Re: [PATCH v5 10/10] module: Rework module_addr_{min,max}

On Tue, Apr 14, 2015 at 12:25:45PM +0930, Rusty Russell wrote:
> Ingo Molnar <mingo@...nel.org> writes:
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> >
> >> __module_address() does an initial bound check before doing the 
> >> {list/tree} iteration to find the actual module. The bound variables 
> >> are nowhere near the mod_tree cacheline, in fact they're nowhere 
> >> near one another.
> >> 
> >> module_addr_min lives in .data while module_addr_max lives in .bss 
> >> (smarty pants GCC thinks the explicit 0 assignment is a mistake).
> >> 
> >> Rectify this by moving the two variables into a structure together 
> >> with the latch_tree_root to guarantee they all share the same 
> >> cacheline and avoid hitting two extra cachelines for the lookup.
> >> 
> >> While reworking the bounds code, move the bound update from 
> >> allocation to insertion time, this avoids updating the bounds for a 
> >> few error paths.
> >
> >> +static struct mod_tree_root {
> >> +	struct latch_tree_root root;
> >> +	unsigned long addr_min;
> >> +	unsigned long addr_max;
> >> +} mod_tree __cacheline_aligned = {
> >> +	.addr_min = -1UL,
> >> +};
> >> +
> >> +#define module_addr_min mod_tree.addr_min
> >> +#define module_addr_max mod_tree.addr_max
> 
> Nice catch.
> 
> Does the min/max comparison still win us anything?  (I'm guessing yes...)

Yep, while a tree iteration is much faster than the linear thing it is
still quite a bit slower than two simple compares.

> In general, I'm happy with this series.  Assume you want another
> go-round for Ingo's tweaks, then I'll take them for 4.2.

Thanks!
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