From: Arnd Bergmann <arnd@arndb.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
Mark Brown <broonie@kernel.org>,
Liam Girdwood <lgirdwood@gmail.com>,
akpm@linux-foundation.org
Subject: Re: [PATCH] mutex: make mutex_lock_nested an inline function
Date: Tue, 13 Oct 2015 23:46:35 +0200 [thread overview]
Message-ID: <11817958.z9KtmeKzV7@wuerfel> (raw)
In-Reply-To: <20151013203812.GM17308@twins.programming.kicks-ass.net>
On Tuesday 13 October 2015 22:38:12 Peter Zijlstra wrote:
> On Tue, Oct 13, 2015 at 10:30:08PM +0200, Arnd Bergmann wrote:
> > The second argument of the mutex_lock_nested() helper is only
> > evaluated if CONFIG_DEBUG_LOCK_ALLOC is set. Otherwise we
> > get this build warning for the new regulator_lock_supply
> > function:
> >
> > drivers/regulator/core.c: In function 'regulator_lock_supply':
> > drivers/regulator/core.c:142:6: warning: unused variable 'i' [-Wunused-variable]
> >
> > To avoid the warning, this patch changes the definition of
> > mutex_lock_nested() to be static inline function rather than
> > a macro, which tells gcc that the variable is potentially
> > used.
>
> > -# define mutex_lock_nested(lock, subclass) mutex_lock(lock)
> > +static inline void mutex_lock_nested(struct mutex *lock, unsigned int subclass)
> > +{
> > + return mutex_lock(lock);
> > +}
>
> Can you verify that this results in an identical kernel?
>
> Having this a proper argument results in the compiler having to actually
> evaluate the expression resulting in @subclass, this might have side
> effects and generate code.
>
> A quick grep shows a large amount of trivial code that optimizers will
> still happily throw away, but it should be verified that this does not
> result in pointless code generation.
Indeed, I'm seeing a tiny code growth with ARM multi_v7_defconfig when
my patch is applied, as the image (according to size -A) grows from
13740187 bytes to 13740283, all of it in .text of two drivers (i2c-core
and three files of bluetooth.ko).
--- build/multi_v7_defconfig-before/vmlinux.o.size 2015-10-13 23:11:40.544389776 +0200
+++ build/multi_v7_defconfig/vmlinux.o.size 2015-10-13 23:08:00.151043811 +0200
@@ -1,6 +1,6 @@
build/multi_v7_defconfig/vmlinux.o :
section size addr
-.text 8219408 0
+.text 8219504 0
--- build/multi_v7_defconfig-before/net/bluetooth/bluetooth.ko.size 2015-10-13 23:11:40.704382038 +0200
+++ build/multi_v7_defconfig/net/bluetooth/bluetooth.ko.size 2015-10-13 23:07:58.639116862 +0200
@@ -1,7 +1,7 @@
build/multi_v7_defconfig/net/bluetooth/bluetooth.ko :
section size addr
.note.gnu.build-id 36 0
-.text 241512 0
+.text 241696 0
--- build/multi_v7_defconfig-before/drivers/i2c/i2c-core.o.size 2015-10-13 23:11:40.636385326 +0200
+++ build/multi_v7_defconfig/drivers/i2c/i2c-core.o.size 2015-10-13 23:07:53.403369830 +0200
@@ -1,6 +1,6 @@
build/multi_v7_defconfig/drivers/i2c/i2c-core.o :
section size addr
-.text 12112 0
+.text 12208 0
The code in question is
a)
static ssize_t
i2c_sysfs_delete_device(struct device *dev, struct device_attribute *attr,
const char *buf, size_t count)
{
...
mutex_lock_nested(&adap->userspace_clients_lock,
i2c_adapter_depth(adap));
...
}
and
b)
static inline void l2cap_chan_lock(struct l2cap_chan *chan)
{
mutex_lock_nested(&chan->lock, atomic_read(&chan->nesting));
}
The first one has a small size impact but no performance change as it is only
called during probe/release of i2c modules. The second one adds an extra
pointer access (due to the volatile keyword in atomic_read()) for every
caller of l2cap_chan_lock().
Arnd
WARNING: multiple messages have this Message-ID (diff)
From: arnd@arndb.de (Arnd Bergmann)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] mutex: make mutex_lock_nested an inline function
Date: Tue, 13 Oct 2015 23:46:35 +0200 [thread overview]
Message-ID: <11817958.z9KtmeKzV7@wuerfel> (raw)
In-Reply-To: <20151013203812.GM17308@twins.programming.kicks-ass.net>
On Tuesday 13 October 2015 22:38:12 Peter Zijlstra wrote:
> On Tue, Oct 13, 2015 at 10:30:08PM +0200, Arnd Bergmann wrote:
> > The second argument of the mutex_lock_nested() helper is only
> > evaluated if CONFIG_DEBUG_LOCK_ALLOC is set. Otherwise we
> > get this build warning for the new regulator_lock_supply
> > function:
> >
> > drivers/regulator/core.c: In function 'regulator_lock_supply':
> > drivers/regulator/core.c:142:6: warning: unused variable 'i' [-Wunused-variable]
> >
> > To avoid the warning, this patch changes the definition of
> > mutex_lock_nested() to be static inline function rather than
> > a macro, which tells gcc that the variable is potentially
> > used.
>
> > -# define mutex_lock_nested(lock, subclass) mutex_lock(lock)
> > +static inline void mutex_lock_nested(struct mutex *lock, unsigned int subclass)
> > +{
> > + return mutex_lock(lock);
> > +}
>
> Can you verify that this results in an identical kernel?
>
> Having this a proper argument results in the compiler having to actually
> evaluate the expression resulting in @subclass, this might have side
> effects and generate code.
>
> A quick grep shows a large amount of trivial code that optimizers will
> still happily throw away, but it should be verified that this does not
> result in pointless code generation.
Indeed, I'm seeing a tiny code growth with ARM multi_v7_defconfig when
my patch is applied, as the image (according to size -A) grows from
13740187 bytes to 13740283, all of it in .text of two drivers (i2c-core
and three files of bluetooth.ko).
--- build/multi_v7_defconfig-before/vmlinux.o.size 2015-10-13 23:11:40.544389776 +0200
+++ build/multi_v7_defconfig/vmlinux.o.size 2015-10-13 23:08:00.151043811 +0200
@@ -1,6 +1,6 @@
build/multi_v7_defconfig/vmlinux.o :
section size addr
-.text 8219408 0
+.text 8219504 0
--- build/multi_v7_defconfig-before/net/bluetooth/bluetooth.ko.size 2015-10-13 23:11:40.704382038 +0200
+++ build/multi_v7_defconfig/net/bluetooth/bluetooth.ko.size 2015-10-13 23:07:58.639116862 +0200
@@ -1,7 +1,7 @@
build/multi_v7_defconfig/net/bluetooth/bluetooth.ko :
section size addr
.note.gnu.build-id 36 0
-.text 241512 0
+.text 241696 0
--- build/multi_v7_defconfig-before/drivers/i2c/i2c-core.o.size 2015-10-13 23:11:40.636385326 +0200
+++ build/multi_v7_defconfig/drivers/i2c/i2c-core.o.size 2015-10-13 23:07:53.403369830 +0200
@@ -1,6 +1,6 @@
build/multi_v7_defconfig/drivers/i2c/i2c-core.o :
section size addr
-.text 12112 0
+.text 12208 0
The code in question is
a)
static ssize_t
i2c_sysfs_delete_device(struct device *dev, struct device_attribute *attr,
const char *buf, size_t count)
{
...
mutex_lock_nested(&adap->userspace_clients_lock,
i2c_adapter_depth(adap));
...
}
and
b)
static inline void l2cap_chan_lock(struct l2cap_chan *chan)
{
mutex_lock_nested(&chan->lock, atomic_read(&chan->nesting));
}
The first one has a small size impact but no performance change as it is only
called during probe/release of i2c modules. The second one adds an extra
pointer access (due to the volatile keyword in atomic_read()) for every
caller of l2cap_chan_lock().
Arnd
next prev parent reply other threads:[~2015-10-13 21:47 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-13 20:30 [PATCH] mutex: make mutex_lock_nested an inline function Arnd Bergmann
2015-10-13 20:30 ` Arnd Bergmann
2015-10-13 20:38 ` Peter Zijlstra
2015-10-13 20:38 ` Peter Zijlstra
2015-10-13 21:46 ` Arnd Bergmann [this message]
2015-10-13 21:46 ` Arnd Bergmann
2015-10-14 8:20 ` Peter Zijlstra
2015-10-14 8:20 ` Peter Zijlstra
2015-10-14 8:37 ` Peter Zijlstra
2015-10-14 8:37 ` Peter Zijlstra
2015-10-14 9:00 ` Arnd Bergmann
2015-10-14 9:00 ` Arnd Bergmann
2015-10-14 9:08 ` Peter Zijlstra
2015-10-14 9:08 ` Peter Zijlstra
2015-10-14 9:59 ` Mark Brown
2015-10-14 9:59 ` Mark Brown
2015-10-14 10:27 ` Mark Brown
2015-10-14 10:27 ` Mark Brown
2015-10-14 11:07 ` Peter Zijlstra
2015-10-14 11:07 ` Peter Zijlstra
2015-10-14 12:36 ` Mark Brown
2015-10-14 12:36 ` Mark Brown
2015-10-14 13:47 ` Peter Zijlstra
2015-10-14 13:47 ` Peter Zijlstra
2015-10-14 13:50 ` Peter Zijlstra
2015-10-14 13:50 ` Peter Zijlstra
2015-10-14 13:58 ` Ingo Molnar
2015-10-14 13:58 ` Ingo Molnar
2015-10-14 14:11 ` Mark Brown
2015-10-14 14:11 ` Mark Brown
2015-10-22 15:02 ` Arnd Bergmann
2015-10-22 15:02 ` Arnd Bergmann
2015-10-22 15:09 ` Peter Zijlstra
2015-10-22 15:09 ` Peter Zijlstra
2015-10-22 17:44 ` Russell King - ARM Linux
2015-10-22 17:44 ` Russell King - ARM Linux
2015-10-27 18:13 ` Ingo Molnar
2015-10-27 18:13 ` Ingo Molnar
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