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

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 [off-list ref]
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.
    
    Signed-off-by: Peter Zijlstra (Intel) [off-list ref]
    Cc: Julian Anastasov [off-list ref]
    Cc: Linus Torvalds [off-list ref]
    Cc: NeilBrown [off-list ref]
    Cc: Oleg Nesterov [off-list ref]
    Cc: Peter Zijlstra [off-list ref]
    Cc: Thomas Gleixner [off-list ref]
    Signed-off-by: Ingo Molnar [off-list ref]

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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