process 0 ("swapper")

process 0 ("swapper")

[Kevin Wilson]

Hi,
I see that the idle process (process 0) , is called "swapper".


init_idle() in kernel/sched/core.c:
...
#if defined(CONFIG_SMP)
	sprintf(idle->comm, "%s/%d", INIT_TASK_COMM, cpu);
#endif
...
#define INIT_TASK_COMM "swapper" in include/linux/init_task.h

Two questions:
1) why does this  #if defined(CONFIG_SMP) here ? shouldn't we name
   it swapper also in non SMP mode ?

2) Is there a reason that we do not see it with ps aux  (and also not
under proc; there is no /proc/0).
   - 1) no process 0; processes start from "1".
   - 2)  following command gives nothing:
        ps aux | grep  -i swapper


rgs,
Kevin

process 0 ("swapper")

[Valdis.Kletnieks at vt.edu]

On Fri, 15 Feb 2013 22:24:29 +0200, Kevin Wilson said:

> #if defined(CONFIG_SMP)
> 	sprintf(idle->comm, "%s/%d", INIT_TASK_COMM, cpu);
> #endif
> ...
> #define INIT_TASK_COMM "swapper" in include/linux/init_task.h
>
> Two questions:
> 1) why does this  #if defined(CONFIG_SMP) here ? shouldn't we name
>    it swapper also in non SMP mode ?

That code is entered once per CPU as the CPU is started up.  And that %d
doesn't make much sense in single-processor mode.

> 2) Is there a reason that we do not see it with ps aux  (and also not
> under proc; there is no /proc/0).

/bin/ps just pretty-prints stuff out of /proc - so you don't see it with
ps because you don't have a /proc/0.  And that's because in fs/proc/base.c
we see this code in proc_pid_lookup:

        tgid = name_to_int(dentry);
        if (tgid == ~0U)
                goto out;

In other words, if you ask for pid 0, it bails and doesn't return anything.

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process 0 ("swapper")

[Kevin Wilson]

Hi,

> we see this code in proc_pid_lookup:
>
>         tgid = name_to_int(dentry);
>         if (tgid == ~0U)
>                 goto out;
>
> In other words, if you ask for pid 0, it bails and doesn't return anything.

Are you sure that this is what it cjecks?

 ~0U is not 0 but -1;

for example, try:
	printf("%d\n",~0U );
	printf("%x\n",~0U );	
	printf("%x\n",-1 );
and you get:
-1
ffffffff
ffffffff
rgs
Kevin

process 0 ("swapper")

[anish kumar]

On Sat, 2013-02-16 at 18:48 +0200, Kevin Wilson wrote:
> Hi,
> 
> > we see this code in proc_pid_lookup:
> >
> >         tgid = name_to_int(dentry);
> >         if (tgid == ~0U)
> >                 goto out;
It is the error case when name can't be converted to integer i.e.
name is wrong.
> >
> > In other words, if you ask for pid 0, it bails and doesn't return anything.
> 
> Are you sure that this is what it cjecks?
> 
>  ~0U is not 0 but -1;
right
> 
> for example, try:
> 	printf("%d\n",~0U );
> 	printf("%x\n",~0U );	
> 	printf("%x\n",-1 );
> and you get:
> -1
> ffffffff
> ffffffff
> rgs
> Kevin
> 
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

process 0 ("swapper")

[Valdis.Kletnieks at vt.edu]

On Sat, 16 Feb 2013 18:48:52 +0200, Kevin Wilson said:

>  ~0U is not 0 but -1;

-ENOCAFFEINE.

You'd think that after having done kernel-level C programming since the days
of SunOS 3.1.5 and BSD 4.2 I'd k   know better. ;)
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