linux-kernel - Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions

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Message-ID: <473cf88b25731a3729a3566abbebd0b6@codeaurora.org>
Date:   Thu, 19 Apr 2018 14:48:42 +0800
From:   yuankuiz@...eaurora.org
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andy Whitcroft <apw@...onical.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pm-owner@...r.kernel.org
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool
 member definitions

On 2018-04-19 01:16 PM, Julia Lawall wrote:
> On Wed, 18 Apr 2018, Joe Perches wrote:
> 
>> On Thu, 2018-04-19 at 06:40 +0200, Julia Lawall wrote:
>> >
>> > On Wed, 18 Apr 2018, Joe Perches wrote:
>> >
>> > > On Tue, 2018-04-17 at 17:07 +0800, yuankuiz@...eaurora.org wrote:
>> > > > Hi julia,
>> > > >
>> > > > On 2018-04-15 05:19 AM, Julia Lawall wrote:
>> > > > > On Wed, 11 Apr 2018, Joe Perches wrote:
>> > > > >
>> > > > > > On Thu, 2018-04-12 at 08:22 +0200, Julia Lawall wrote:
>> > > > > > > On Wed, 11 Apr 2018, Joe Perches wrote:
>> > > > > > > > On Wed, 2018-04-11 at 09:29 -0700, Andrew Morton wrote:
>> > > > > > > > > We already have some 500 bools-in-structs
>> > > > > > > >
>> > > > > > > > I got at least triple that only in include/
>> > > > > > > > so I expect there are at probably an order
>> > > > > > > > of magnitude more than 500 in the kernel.
>> > > > > > > >
>> > > > > > > > I suppose some cocci script could count the
>> > > > > > > > actual number of instances.  A regex can not.
>> > > > > > >
>> > > > > > > I got 12667.
>> > > > > >
>> > > > > > Could you please post the cocci script?
>> > > > > >
>> > > > > > > I'm not sure to understand the issue.  Will using a bitfield help if there
>> > > > > > > are no other bitfields in the structure?
>> > > > > >
>> > > > > > IMO, not really.
>> > > > > >
>> > > > > > The primary issue is described by Linus here:
>> > > > > > https://lkml.org/lkml/2017/11/21/384
>> > > > > >
>> > > > > > I personally do not find a significant issue with
>> > > > > > uncontrolled sizes of bool in kernel structs as
>> > > > > > all of the kernel structs are transitory and not
>> > > > > > written out to storage.
>> > > > > >
>> > > > > > I suppose bool bitfields are also OK, but for the
>> > > > > > RMW required.
>> > > > > >
>> > > > > > Using unsigned int :1 bitfield instead of bool :1
>> > > > > > has the negative of truncation so that the uint
>> > > > > > has to be set with !! instead of a simple assign.
>> > > > >
>> > > > > At least with gcc 5.4.0, a number of structures become larger with
>> > > > > unsigned int :1. bool:1 seems to mostly solve this problem.  The
>> > > > > structure
>> > > > > ichx_desc, defined in drivers/gpio/gpio-ich.c seems to become larger
>> > > > > with
>> > > > > both approaches.
>> > > >
>> > > > [ZJ] Hopefully, this could make it better in your environment.
>> > > >       IMHO, this is just for double check.
>> > >
>> > > I doubt this is actually better or smaller code.
>> > >
>> > > Check the actual object code using objdump and the
>> > > struct alignment using pahole.
>> >
>> > I didn't have a chance to try it, but it looks quite likely to result in a
>> > smaller data structure based on the other examples that I looked at.
>> 
>> I _really_ doubt there is any difference in size between the
>> below in any architecture
>> 
>> struct foo {
>> 	int bar;
>> 	bool baz:1;
>> 	int qux;
>> };
>> 
>> and
>> 
>> struct foo {
>> 	int bar;
>> 	bool baz;
>> 	int qux;
>> };
>> 
>> Where there would be a difference in size is
>> 
>> struct foo {
>> 	int bar;
>> 	bool baz1:1;
>> 	bool baz2:1;
>> 	int qux;
>> };
>> 
>> and
>> 
>> struct foo {
>> 	int bar;
>> 	bool baz1;
>> 	bool baz2;
>> 
>> int qux;
>> };
> 
> In the situation of the example there are two bools together in the 
> middle
> of the structure and one at the end.  Somehow, even converting to 
> bool:1
> increases the size.  But it seems plausible that putting all three 
> bools
> together and converting them all to :1 would reduce the size.  I don't
> know.  The size increase (more than 8 bytes) seems out of proportion 
> for 3
> bools.
[ZJ] Typically, addition saving is due for difference padding.
> 
> I was able to check around 3000 structures that were not declared with 
> any
> attributes, that don't declare named types internally, and that are
> compiled for x86.  Around 10% become smaller whn using bool:1, 
> typically
> by at most 8 bytes.
> 
> julia
> 
>> 
>>