From: Julia Lawall <julia.lawall@lip6.fr>
To: Joe Perches <joe@perches.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>,
yuankuiz@codeaurora.org,
Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
"Rafael J. Wysocki" <rafael@kernel.org>,
Andy Whitcroft <apw@canonical.com>,
Linux PM <linux-pm@vger.kernel.org>,
"Rafael J. Wysocki" <rjw@rjwysocki.net>,
Frederic Weisbecker <fweisbec@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
paulmck@linux.vnet.ibm.com, Ingo Molnar <mingo@kernel.org>,
Len Brown <len.brown@intel.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-pm-owner@vger.kernel.org
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions
Date: Thu, 19 Apr 2018 07:16:11 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.20.1804190705580.2830@hadrien> (raw)
In-Reply-To: <b17a1d0de54ca0db12bd38a0b7fb20a5befb6778.camel@perches.com>
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@codeaurora.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.
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
>
>
next prev parent reply other threads:[~2018-04-19 5:16 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-10 7:33 Subject: [PATCH] [PATCH] time: tick-sched: use bool for tick_stopped yuankuiz
2018-04-10 7:45 ` yuankuiz
2018-04-10 8:51 ` yuankuiz
2018-04-10 8:54 ` yuankuiz
2018-04-10 7:55 ` Subject: [PATCH] " Thomas Gleixner
2018-04-10 8:12 ` yuankuiz
2018-04-10 8:00 ` Rafael J. Wysocki
2018-04-10 8:15 ` yuankuiz
2018-04-10 9:10 ` Thomas Gleixner
2018-04-10 10:07 ` yuankuiz
2018-04-10 11:06 ` Thomas Gleixner
2018-04-10 14:08 ` yuankuiz
2018-04-10 14:49 ` yuankuiz
2018-04-10 23:09 ` yuankuiz
2018-04-10 23:20 ` yuankuiz
2018-04-20 1:47 ` yuankuiz
2018-04-20 6:44 ` yuankuiz
2018-04-20 19:24 ` Joe Perches
2018-04-25 7:01 ` yuankuiz
2018-04-10 11:26 ` Peter Zijlstra
2018-04-10 12:07 ` Thomas Gleixner
2018-04-10 12:26 ` Peter Zijlstra
2018-04-10 12:33 ` Subject: [PATCH] " Peter Zijlstra
2018-04-10 15:14 ` Joe Perches
2018-04-10 16:30 ` Peter Zijlstra
2018-04-10 15:41 ` [PATCH] checkpatch: whinge about bool bitfields Joe Perches
2018-04-10 18:19 ` [PATCH] checkpatch: Add a --strict test for structs with bool member definitions Joe Perches
2018-04-10 21:39 ` Andrew Morton
2018-04-10 21:53 ` Joe Perches
2018-04-10 22:00 ` Andrew Morton
2018-04-11 8:15 ` Peter Zijlstra
2018-04-11 16:29 ` Andrew Morton
2018-04-11 16:51 ` Joe Perches
2018-04-12 6:22 ` Julia Lawall
2018-04-12 6:42 ` Joe Perches
2018-04-12 7:03 ` Julia Lawall
2018-04-12 8:13 ` Peter Zijlstra
2018-04-14 21:19 ` Julia Lawall
2018-04-17 9:07 ` yuankuiz
2018-04-18 18:38 ` Joe Perches
2018-04-19 4:40 ` Julia Lawall
2018-04-19 4:51 ` Joe Perches
2018-04-19 5:16 ` Julia Lawall [this message]
2018-04-19 6:48 ` yuankuiz
2018-04-19 10:42 ` yuankuiz
2018-04-20 1:31 ` yuankuiz
2018-04-11 17:00 ` Peter Zijlstra
2018-04-12 7:47 ` Ingo Molnar
2018-04-12 8:11 ` Peter Zijlstra
2018-04-12 9:35 ` Andrea Parri
2018-04-12 11:50 ` Peter Zijlstra
2018-04-12 12:01 ` Joe Perches
2018-04-12 12:08 ` Peter Zijlstra
2018-04-12 12:38 ` Joe Perches
2018-04-12 16:47 ` Andrew Morton
2018-04-12 11:52 ` Kalle Valo
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