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

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Date:	Tue, 14 Apr 2015 12:25:45 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	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}

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...)

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,
Rusty.
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