i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

[Dirk Behme]

Hi,

working on a i.MX6 Solo SoC (one Cortex A9 core) we are usually booting 
with maxcpus=1 a kernel built with CONFIG_NR_CPUS=4. This is done to be 
able to boot the same kernel on the Solo and Quad version of that SoC.

Now, we tried to switch from 'maxcpus=1' to 'nosmp'. With 'nosmp' we get 
the warning [1] in the boot messages. This isn't there with 'maxcpus=1'.

Is it a good idea to use 'nosmp' instead of 'maxcpus=1' on a single 
Cortex A9 SoC?

Looking into the warning, it's the result of setup_max_cpus being 0 in 
case of 'nosmp' (and being 1 in case of 'maxcpus=1'). So with 'nosmp' 
the function local_timer_register()

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=arch/arm/kernel/smp.c;h=84f4cbf652e58b3acfea846ad1cad9ddbee72d95;hb=refs/heads/master#l514

returns -ENXIO (== -6).

Is this the intended behaviour for 'nosmp' resulting in below boot warning?

The call stack is

    imx6q_timer_init()
-> twd_local_timer_of_register()()
-> twd_local_timer_common_register()
-> local_timer_register()  (returns -ENXIO due to setup_max_cpus == 0)

Best regards

Dirk

[1]
...
sched_clock: 32 bits at 49MHz, resolution 20ns, wraps every 86767ms
------------[ cut here ]------------
WARNING: at arch/arm/kernel/smp_twd.c:345 
twd_local_timer_of_register+0x7c/0x90()
twd_local_timer_of_register failed (-6)
Modules linked in:
Backtrace:
[<80011f14>] (dump_backtrace+0x0/0x10c) from [<8044dd30>] 
(dump_stack+0x18/0x1c)
  r7:805e9f58 r6:805ba84c r5:80539331 r4:00000159
[<8044dd18>] (dump_stack+0x0/0x1c) from [<80020fbc>] 
(warn_slowpath_common+0x54/0x6c)
[<80020f68>] (warn_slowpath_common+0x0/0x6c) from [<80021078>] 
(warn_slowpath_fmt+0x38/0x40)
  r9:412fc09a r8:8fffffff r7:ffffffff r6:00000001 r5:80633b8c
r4:80b32da8
[<80021040>] (warn_slowpath_fmt+0x0/0x40) from [<805ba84c>] 
(twd_local_timer_of_register+0x7c/0x90)
  r3:fffffffa r2:8053934b
[<805ba7d0>] (twd_local_timer_of_register+0x0/0x90) from [<805c0bec>] 
(imx6q_timer_init+0x18/0x4c)
  r5:80633800 r4:8053b701
[<805c0bd4>] (imx6q_timer_init+0x0/0x4c) from [<805ba4e8>] 
(time_init+0x28/0x38)
  r5:80633800 r4:805dc0f4
[<805ba4c0>] (time_init+0x0/0x38) from [<805b6854>] 
(start_kernel+0x1a0/0x310)
[<805b66b4>] (start_kernel+0x0/0x310) from [<10008044>] (0x10008044)
  r8:1000406a r7:805f3f8c r6:805dc0c4 r5:805f0518 r4:10c5387d
---[ end trace 1b75b31a2719ed1c ]---
CPU identified as i.MX6S/DL, silicon rev 1.0
...

i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

[Russell King - ARM Linux]

On Thu, Feb 14, 2013 at 03:10:43PM +0100, Dirk Behme wrote:
> working on a i.MX6 Solo SoC (one Cortex A9 core) we are usually booting  
> with maxcpus=1 a kernel built with CONFIG_NR_CPUS=4. This is done to be  
> able to boot the same kernel on the Solo and Quad version of that SoC.

And what happens if you compile a kernel without SMP support and boot
it on this SoC?

i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

[Fabio Estevam]

Hi Dirk,

On Thu, Feb 14, 2013 at 12:10 PM, Dirk Behme <dirk.behme@de.bosch.com> wrote:
> Hi,
>
> working on a i.MX6 Solo SoC (one Cortex A9 core) we are usually booting with
> maxcpus=1 a kernel built with CONFIG_NR_CPUS=4. This is done to be able to
> boot the same kernel on the Solo and Quad version of that SoC.
>
> Now, we tried to switch from 'maxcpus=1' to 'nosmp'. With 'nosmp' we get the
> warning [1] in the boot messages. This isn't there with 'maxcpus=1'.

maxcpus=1 and nosmp are not equivalent as per the comment from kernel/smp.c:

/*
 * Setup routine for controlling SMP activation
 *
 * Command-line option of "nosmp" or "maxcpus=0" will disable SMP
 * activation entirely (the MPS table probe still happens, though).
 *
 * Command-line option of "maxcpus=<NUM>", where <NUM> is an integer
 * greater than 0, limits the maximum number of CPUs activated in
 * SMP mode to <NUM>.
 */

void __weak arch_disable_smp_support(void) { }

i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

[Dirk Behme]

On 14.02.2013 15:40, Fabio Estevam wrote:
> Hi Dirk,
> 
> On Thu, Feb 14, 2013 at 12:10 PM, Dirk Behme <dirk.behme@de.bosch.com> wrote:
>> Hi,
>>
>> working on a i.MX6 Solo SoC (one Cortex A9 core) we are usually booting with
>> maxcpus=1 a kernel built with CONFIG_NR_CPUS=4. This is done to be able to
>> boot the same kernel on the Solo and Quad version of that SoC.
>>
>> Now, we tried to switch from 'maxcpus=1' to 'nosmp'. With 'nosmp' we get the
>> warning [1] in the boot messages. This isn't there with 'maxcpus=1'.
> 
> maxcpus=1 and nosmp are not equivalent as per the comment from kernel/smp.c:
> 
> /*
>  * Setup routine for controlling SMP activation
>  *
>  * Command-line option of "nosmp" or "maxcpus=0" will disable SMP
>  * activation entirely (the MPS table probe still happens, though).
>  *
>  * Command-line option of "maxcpus=<NUM>", where <NUM> is an integer
>  * greater than 0, limits the maximum number of CPUs activated in
>  * SMP mode to <NUM>.
>  */

Yes, understood that both options are not equal.

But should both options work on a single ARM Cortex A9 SoC without a 
warning? Or should one of the both options be preferred on a single ARM 
Cortex A9 SoC?

Thanks

Dirk

i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

[Dirk Behme]

On 14.02.2013 15:37, Russell King - ARM Linux wrote:
> On Thu, Feb 14, 2013 at 03:10:43PM +0100, Dirk Behme wrote:
>> working on a i.MX6 Solo SoC (one Cortex A9 core) we are usually booting  
>> with maxcpus=1 a kernel built with CONFIG_NR_CPUS=4. This is done to be  
>> able to boot the same kernel on the Solo and Quad version of that SoC.
> 
> And what happens if you compile a kernel without SMP support and boot
> it on this SoC?

- Case 1:

Kernel command line: ... nosmp ...
$ cat .config | grep SMP
CONFIG_USE_GENERIC_SMP_HELPERS=y
CONFIG_GENERIC_SMP_IDLE_THREAD=y
CONFIG_HAVE_SMP=y
CONFIG_SMP=y
CONFIG_SMP_ON_UP=y

=> Gives warning [1] as reported


- Case 2:

Kernel command line: ... nosmp ...
$ cat .config | grep SMP
CONFIG_BROKEN_ON_SMP=y
CONFIG_GENERIC_SMP_IDLE_THREAD=y
CONFIG_HAVE_SMP=y
# CONFIG_SMP is not set

=> Boots without warning


Best regards

Dirk


[1]

...
sched_clock: 32 bits at 49MHz, resolution 20ns, wraps every 86767ms
------------[ cut here ]------------
WARNING: at arch/arm/kernel/smp_twd.c:345 
twd_local_timer_of_register+0x7c/0x90()
twd_local_timer_of_register failed (-6)
Modules linked in:
Backtrace:
[<80011f14>] (dump_backtrace+0x0/0x10c) from [<8044dd30>] 
(dump_stack+0x18/0x1c)
  r7:805e9f58 r6:805ba84c r5:80539331 r4:00000159
[<8044dd18>] (dump_stack+0x0/0x1c) from [<80020fbc>] 
(warn_slowpath_common+0x54/0x6c)
[<80020f68>] (warn_slowpath_common+0x0/0x6c) from [<80021078>] 
(warn_slowpath_fmt+0x38/0x40)
  r9:412fc09a r8:8fffffff r7:ffffffff r6:00000001 r5:80633b8c
r4:80b32da8
[<80021040>] (warn_slowpath_fmt+0x0/0x40) from [<805ba84c>] 
(twd_local_timer_of_register+0x7c/0x90)
  r3:fffffffa r2:8053934b
[<805ba7d0>] (twd_local_timer_of_register+0x0/0x90) from [<805c0bec>] 
(imx6q_timer_init+0x18/0x4c)
  r5:80633800 r4:8053b701
[<805c0bd4>] (imx6q_timer_init+0x0/0x4c) from [<805ba4e8>] 
(time_init+0x28/0x38)
  r5:80633800 r4:805dc0f4
[<805ba4c0>] (time_init+0x0/0x38) from [<805b6854>] 
(start_kernel+0x1a0/0x310)
[<805b66b4>] (start_kernel+0x0/0x310) from [<10008044>] (0x10008044)
  r8:1000406a r7:805f3f8c r6:805dc0c4 r5:805f0518 r4:10c5387d
---[ end trace 1b75b31a2719ed1c ]---
CPU identified as i.MX6S/DL, silicon rev 1.0
...

i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

[Shawn Guo]

On Thu, Feb 14, 2013 at 02:37:52PM +0000, Russell King - ARM Linux wrote:
> On Thu, Feb 14, 2013 at 03:10:43PM +0100, Dirk Behme wrote:
> > working on a i.MX6 Solo SoC (one Cortex A9 core) we are usually booting  
> > with maxcpus=1 a kernel built with CONFIG_NR_CPUS=4. This is done to be  
> > able to boot the same kernel on the Solo and Quad version of that SoC.
> 
> And what happens if you compile a kernel without SMP support and boot
> it on this SoC?

Thanks Dirk for bringing this up on the list.  We have hit this fat
warning on FSL internal 3.5.7 kernel tree when booting SMP kernel with
nosmp on kernel cmdline.

As what Dirk confirmed, we do not see the warning with kernel built
with !SMP.

Russell,

The following is an immediate and less invasive change to lower the
warning level.  If you agree, I will post a patch and then put it
into patch system.

Shawn

diff --git a/arch/arm/kernel/smp_twd.c b/arch/arm/kernel/smp_twd.c
index c092115..f796b83 100644
--- a/arch/arm/kernel/smp_twd.c
+++ b/arch/arm/kernel/smp_twd.c
@@ -392,6 +392,6 @@ void __init twd_local_timer_of_register(void)
        err = twd_local_timer_common_register(np);

 out:
-       WARN(err, "twd_local_timer_of_register failed (%d)\n", err);
+       pr_warn("twd_local_timer_of_register failed (%d)\n", err);
 }
 #endif

i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

[Dirk Behme]

On 18.02.2013 03:50, Shawn Guo wrote:
> On Thu, Feb 14, 2013 at 02:37:52PM +0000, Russell King - ARM Linux wrote:
>> On Thu, Feb 14, 2013 at 03:10:43PM +0100, Dirk Behme wrote:
>>> working on a i.MX6 Solo SoC (one Cortex A9 core) we are usually booting  
>>> with maxcpus=1 a kernel built with CONFIG_NR_CPUS=4. This is done to be  
>>> able to boot the same kernel on the Solo and Quad version of that SoC.
>> And what happens if you compile a kernel without SMP support and boot
>> it on this SoC?
> 
> Thanks Dirk for bringing this up on the list.  We have hit this fat
> warning on FSL internal 3.5.7 kernel tree when booting SMP kernel with
> nosmp on kernel cmdline.
> 
> As what Dirk confirmed, we do not see the warning with kernel built
> with !SMP.
> 
> Russell,
> 
> The following is an immediate and less invasive change to lower the
> warning level.  If you agree, I will post a patch and then put it
> into patch system.
> 
> Shawn
> 
> diff --git a/arch/arm/kernel/smp_twd.c b/arch/arm/kernel/smp_twd.c
> index c092115..f796b83 100644
> --- a/arch/arm/kernel/smp_twd.c
> +++ b/arch/arm/kernel/smp_twd.c
> @@ -392,6 +392,6 @@ void __init twd_local_timer_of_register(void)
>         err = twd_local_timer_common_register(np);
> 
>  out:
> -       WARN(err, "twd_local_timer_of_register failed (%d)\n", err);
> +       pr_warn("twd_local_timer_of_register failed (%d)\n", err);
>  }
>  #endif

Any news on this? Time to move it to the patch system?

Thanks

Dirk

i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

[Shawn Guo]

On Thu, Feb 21, 2013 at 08:16:01AM +0100, Dirk Behme wrote:
> >diff --git a/arch/arm/kernel/smp_twd.c b/arch/arm/kernel/smp_twd.c
> >index c092115..f796b83 100644
> >--- a/arch/arm/kernel/smp_twd.c
> >+++ b/arch/arm/kernel/smp_twd.c
> >@@ -392,6 +392,6 @@ void __init twd_local_timer_of_register(void)
> >        err = twd_local_timer_common_register(np);
> >
> > out:
> >-       WARN(err, "twd_local_timer_of_register failed (%d)\n", err);
> >+       pr_warn("twd_local_timer_of_register failed (%d)\n", err);
> > }
> > #endif
> 
> Any news on this? Time to move it to the patch system?
> 
Thanks for reminding.  I will send it out today.

Shawn