Re: [RFC PATCH 09/18] kthread: Make it easier to correctly sleep in iterant kthreads

[Petr Mladek]

On Fri 2015-06-05 18:10:21, Peter Zijlstra wrote:
> On Fri, Jun 05, 2015 at 05:01:08PM +0200, Petr Mladek wrote:
> > Many kthreads go into an interruptible sleep when there is nothing
> > to do. They should check if anyone did not requested the kthread
> > to terminate, freeze, or park in the meantime. It is easy to do
> > it a wrong way.
> 
> INTERRUPTIBLE is the wrong state to idle in for kthreads, use
> TASK_IDLE.
> 
> ---
> 
> commit 80ed87c8a9ca0cad7ca66cf3bbdfb17559a66dcf
> Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date:   Fri May 8 14:23:45 2015 +0200
> 
>     sched/wait: Introduce TASK_NOLOAD and TASK_IDLE
>     
>     Currently people use TASK_INTERRUPTIBLE to idle kthreads and wait for
>     'work' because TASK_UNINTERRUPTIBLE contributes to the loadavg. Having
>     all idle kthreads contribute to the loadavg is somewhat silly.
>     
>     Now mostly this works OK, because kthreads have all their signals
>     masked. However there's a few sites where this is causing problems and
>     TASK_UNINTERRUPTIBLE should be used, except for that loadavg issue.
>     
>     This patch adds TASK_NOLOAD which, when combined with
>     TASK_UNINTERRUPTIBLE avoids the loadavg accounting.
>     
>     As most of imagined usage sites are loops where a thread wants to
>     idle, waiting for work, a helper TASK_IDLE is introduced.

Just to be sure. Do you suggest to use TASK_IDLE everywhere in
kthreads or only when the uninterruptible sleep is really needed?

IMHO, we should not use TASK_IDLE in freezable kthreads because
it would break freezing. Well, we could freezable_schedule() but only
on locations where it is safe to get freezed. Anyway, we need to
be careful here.

BTW: What is the preferred way of freezing, please? Is it better
to end up in the fridge or is it fine to call freezer_do_not_count();
or set PF_NOFREEZE when it is safe?

The fridge looks more clean to me but in this case we should avoid
uninterruptible sleep as much as possible.


Best Regards,
Petr

>     Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>     Cc: Julian Anastasov <ja@xxxxxx>
>     Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>     Cc: NeilBrown <neilb@xxxxxxx>
>     Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
>     Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>     Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>     Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index dd07ac03f82a..7de815c6fa78 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -218,9 +218,10 @@ print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq);
>  #define TASK_WAKEKILL		128
>  #define TASK_WAKING		256
>  #define TASK_PARKED		512
> -#define TASK_STATE_MAX		1024
> +#define TASK_NOLOAD		1024
> +#define TASK_STATE_MAX		2048
>  
> -#define TASK_STATE_TO_CHAR_STR "RSDTtXZxKWP"
> +#define TASK_STATE_TO_CHAR_STR "RSDTtXZxKWPN"
>  
>  extern char ___assert_task_state[1 - 2*!!(
>  		sizeof(TASK_STATE_TO_CHAR_STR)-1 != ilog2(TASK_STATE_MAX)+1)];
> @@ -230,6 +231,8 @@ extern char ___assert_task_state[1 - 2*!!(
>  #define TASK_STOPPED		(TASK_WAKEKILL | __TASK_STOPPED)
>  #define TASK_TRACED		(TASK_WAKEKILL | __TASK_TRACED)
>  
> +#define TASK_IDLE		(TASK_UNINTERRUPTIBLE | TASK_NOLOAD)
> +
>  /* Convenience macros for the sake of wake_up */
>  #define TASK_NORMAL		(TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE)
>  #define TASK_ALL		(TASK_NORMAL | __TASK_STOPPED | __TASK_TRACED)
> @@ -245,7 +248,8 @@ extern char ___assert_task_state[1 - 2*!!(
>  			((task->state & (__TASK_STOPPED | __TASK_TRACED)) != 0)
>  #define task_contributes_to_load(task)	\
>  				((task->state & TASK_UNINTERRUPTIBLE) != 0 && \
> -				 (task->flags & PF_FROZEN) == 0)
> +				 (task->flags & PF_FROZEN) == 0 && \
> +				 (task->state & TASK_NOLOAD) == 0)
>  
>  #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
>  
> diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> index 30fedaf3e56a..d57a575fe31f 100644
> --- a/include/trace/events/sched.h
> +++ b/include/trace/events/sched.h
> @@ -147,7 +147,8 @@ TRACE_EVENT(sched_switch,
>  		  __print_flags(__entry->prev_state & (TASK_STATE_MAX-1), "|",
>  				{ 1, "S"} , { 2, "D" }, { 4, "T" }, { 8, "t" },
>  				{ 16, "Z" }, { 32, "X" }, { 64, "x" },
> -				{ 128, "K" }, { 256, "W" }, { 512, "P" }) : "R",
> +				{ 128, "K" }, { 256, "W" }, { 512, "P" },
> +				{ 1024, "N" }) : "R",
>  		__entry->prev_state & TASK_STATE_MAX ? "+" : "",
>  		__entry->next_comm, __entry->next_pid, __entry->next_prio)
>  );
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