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SubjectRe: [PATCH] SMP signal latency fix up.
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On Thu, 2003-11-06 at 17:39, Mark Gross wrote:
> On Thu, 2003-11-06 at 15:20, Linus Torvalds wrote:
> > On 6 Nov 2003, Mark Gross wrote:
> > >
> > > Running on SMP and the 2.6.0-test9 kernel, it takes about 10000 * 1/HZ seconds. Running this
> > > command with maxcpus=1 the command finishes in fraction of a second. Under SMP the
> > > signal delivery isn't kicking the task if its in the run state on the other CPU.
> >
> > It looks like the "wake_up_process_kick()" interface is just broken. As it
> > stands now, it's literally _designed_ to kick the process only when it
> > wakes it up, which is silly and wrong. It makes no sense to kick a process
> > that we just woke up, because it _will_ react immediately anyway.
> >
> > We literally want to kick only processes that didn't need waking up, and
> > the current interface is totally unsuitable for that.
> >
> > > The following patch has been tested and seems to fix the problem.
> > > I'm confident about the change to sched.c actualy fixes a cut and paste bug.
> >
> > Naah, it's a thinko, the code shouldn't be like that at all.
> >
> > There's only one user of the "wake_up_process_kick()" thing, and that one
> > user really wants to kick the process totally independently of waking it
> > up. Which just implies that we should just have a _regular_
> > "wake_up_process()" there, and a _separate_ "kick_process()" thing.
> >
> > So I've got a feeling that
> > - we should remove the "kick" argument from "try_to_wake_up()"
> > - the signal wakeup case should instead do a _regular_ wakeup.
> > - we should kick the process if the wakeup _fails_.
> >
> > Ie signal wakeup should most likely look something like
> >
> >
> > inline void signal_wake_up(struct task_struct *t, int resume)
> > {
> > int woken;
> > unsigned int mask;
> >
> > set_tsk_thread_flag(t,TIF_SIGPENDING);
> > mask = TASK_INTERRUPTIBLE;
> > if (resume)
> > mask |= TASK_STOPPED;
> > woken = 0;
> > if (t->state & mask)
> > woken = wake_up_state(p, mask);
> > if (!woken)
> > kick_process(p);
> > }
> >
> > where the "kick_process()" thing does the "is the task running on some
> > other CPU and if so send it a reschedule event to make it react" thing.
> >
> > Ingo?
> >
> > Linus
>
>
> Are you thinking about something like this?
>
> It seems to work. I dropped the "task_running" test from
> smp_process_kick intentionaly. As well as the movement of the success
> flag. I hope its not too wrong.
>
> It seems to work on a HT P4 desktop and a dual PIII box.
>
> --mgross
Evolution messed up my patch. Retry:

diff -urN -X dontdiff linux-2.6.0-test9/include/linux/sched.h /opt/linux-2.6.0-test9/include/linux/sched.h
--- linux-2.6.0-test9/include/linux/sched.h 2003-10-25 11:42:56.000000000 -0700
+++ /opt/linux-2.6.0-test9/include/linux/sched.h 2003-11-06 14:44:22.000000000 -0800
@@ -573,7 +573,7 @@

extern int FASTCALL(wake_up_state(struct task_struct * tsk, unsigned int state));
extern int FASTCALL(wake_up_process(struct task_struct * tsk));
-extern int FASTCALL(wake_up_process_kick(struct task_struct * tsk));
+extern void FASTCALL(smp_process_kick(struct task_struct * tsk));
extern void FASTCALL(wake_up_forked_process(struct task_struct * tsk));
extern void FASTCALL(sched_exit(task_t * p));

diff -urN -X dontdiff linux-2.6.0-test9/kernel/sched.c /opt/linux-2.6.0-test9/kernel/sched.c
--- linux-2.6.0-test9/kernel/sched.c 2003-10-25 11:44:29.000000000 -0700
+++ /opt/linux-2.6.0-test9/kernel/sched.c 2003-11-06 14:58:29.000000000 -0800
@@ -585,7 +585,7 @@
*
* returns failure only if the task is already active.
*/
-static int try_to_wake_up(task_t * p, unsigned int state, int sync, int kick)
+static int try_to_wake_up(task_t * p, unsigned int state, int sync)
{
unsigned long flags;
int success = 0;
@@ -624,13 +624,8 @@
if (TASK_PREEMPTS_CURR(p, rq))
resched_task(rq->curr);
}
- success = 1;
}
-#ifdef CONFIG_SMP
- else
- if (unlikely(kick) && task_running(rq, p) && (task_cpu(p) != smp_processor_id()))
- smp_send_reschedule(task_cpu(p));
-#endif
+ success = 1;
p->state = TASK_RUNNING;
}
task_rq_unlock(rq, &flags);
@@ -640,19 +635,22 @@

int wake_up_process(task_t * p)
{
- return try_to_wake_up(p, TASK_STOPPED | TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE, 0, 0);
+ return try_to_wake_up(p, TASK_STOPPED | TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE, 0);
}

EXPORT_SYMBOL(wake_up_process);

-int wake_up_process_kick(task_t * p)
+void smp_process_kick(task_t * p)
{
- return try_to_wake_up(p, TASK_STOPPED | TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE, 0, 1);
+#ifdef CONFIG_SMP
+ if (task_cpu(p) != smp_processor_id())
+ smp_send_reschedule(task_cpu(p));
+#endif
}

int wake_up_state(task_t *p, unsigned int state)
{
- return try_to_wake_up(p, state, 0, 0);
+ return try_to_wake_up(p, state, 0);
}

/*
@@ -1624,7 +1622,7 @@
int default_wake_function(wait_queue_t *curr, unsigned mode, int sync)
{
task_t *p = curr->task;
- return try_to_wake_up(p, mode, sync, 0);
+ return try_to_wake_up(p, mode, sync);
}

EXPORT_SYMBOL(default_wake_function);
diff -urN -X dontdiff linux-2.6.0-test9/kernel/signal.c /opt/linux-2.6.0-test9/kernel/signal.c
--- linux-2.6.0-test9/kernel/signal.c 2003-10-25 11:43:27.000000000 -0700
+++ /opt/linux-2.6.0-test9/kernel/signal.c 2003-11-06 15:05:41.945237624 -0800
@@ -538,6 +538,7 @@
inline void signal_wake_up(struct task_struct *t, int resume)
{
unsigned int mask;
+ int woken;

set_tsk_thread_flag(t,TIF_SIGPENDING);

@@ -551,10 +552,14 @@
mask = TASK_INTERRUPTIBLE;
if (resume)
mask |= TASK_STOPPED;
+ woken = 0;
if (t->state & mask) {
- wake_up_process_kick(t);
- return;
+ woken = wake_up_process(t);
}
+#ifdef CONFIG_SMP
+ if (!woken)
+ smp_process_kick(t);
+#endif
}

/*

diff -urN -X dontdiff linux-2.6.0-test9/include/linux/sched.h /opt/linux-2.6.0-test9/include/linux/sched.h
--- linux-2.6.0-test9/include/linux/sched.h 2003-10-25 11:42:56.000000000 -0700
+++ /opt/linux-2.6.0-test9/include/linux/sched.h 2003-11-06 14:44:22.000000000 -0800
@@ -573,7 +573,7 @@

extern int FASTCALL(wake_up_state(struct task_struct * tsk, unsigned int state));
extern int FASTCALL(wake_up_process(struct task_struct * tsk));
-extern int FASTCALL(wake_up_process_kick(struct task_struct * tsk));
+extern void FASTCALL(smp_process_kick(struct task_struct * tsk));
extern void FASTCALL(wake_up_forked_process(struct task_struct * tsk));
extern void FASTCALL(sched_exit(task_t * p));

diff -urN -X dontdiff linux-2.6.0-test9/kernel/sched.c /opt/linux-2.6.0-test9/kernel/sched.c
--- linux-2.6.0-test9/kernel/sched.c 2003-10-25 11:44:29.000000000 -0700
+++ /opt/linux-2.6.0-test9/kernel/sched.c 2003-11-06 14:58:29.000000000 -0800
@@ -585,7 +585,7 @@
*
* returns failure only if the task is already active.
*/
-static int try_to_wake_up(task_t * p, unsigned int state, int sync, int kick)
+static int try_to_wake_up(task_t * p, unsigned int state, int sync)
{
unsigned long flags;
int success = 0;
@@ -624,13 +624,8 @@
if (TASK_PREEMPTS_CURR(p, rq))
resched_task(rq->curr);
}
- success = 1;
}
-#ifdef CONFIG_SMP
- else
- if (unlikely(kick) && task_running(rq, p) && (task_cpu(p) != smp_processor_id()))
- smp_send_reschedule(task_cpu(p));
-#endif
+ success = 1;
p->state = TASK_RUNNING;
}
task_rq_unlock(rq, &flags);
@@ -640,19 +635,22 @@

int wake_up_process(task_t * p)
{
- return try_to_wake_up(p, TASK_STOPPED | TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE, 0, 0);
+ return try_to_wake_up(p, TASK_STOPPED | TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE, 0);
}

EXPORT_SYMBOL(wake_up_process);

-int wake_up_process_kick(task_t * p)
+void smp_process_kick(task_t * p)
{
- return try_to_wake_up(p, TASK_STOPPED | TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE, 0, 1);
+#ifdef CONFIG_SMP
+ if (task_cpu(p) != smp_processor_id())
+ smp_send_reschedule(task_cpu(p));
+#endif
}

int wake_up_state(task_t *p, unsigned int state)
{
- return try_to_wake_up(p, state, 0, 0);
+ return try_to_wake_up(p, state, 0);
}

/*
@@ -1624,7 +1622,7 @@
int default_wake_function(wait_queue_t *curr, unsigned mode, int sync)
{
task_t *p = curr->task;
- return try_to_wake_up(p, mode, sync, 0);
+ return try_to_wake_up(p, mode, sync);
}

EXPORT_SYMBOL(default_wake_function);
diff -urN -X dontdiff linux-2.6.0-test9/kernel/signal.c /opt/linux-2.6.0-test9/kernel/signal.c
--- linux-2.6.0-test9/kernel/signal.c 2003-10-25 11:43:27.000000000 -0700
+++ /opt/linux-2.6.0-test9/kernel/signal.c 2003-11-06 15:05:41.945237624 -0800
@@ -538,6 +538,7 @@
inline void signal_wake_up(struct task_struct *t, int resume)
{
unsigned int mask;
+ int woken;

set_tsk_thread_flag(t,TIF_SIGPENDING);

@@ -551,10 +552,14 @@
mask = TASK_INTERRUPTIBLE;
if (resume)
mask |= TASK_STOPPED;
+ woken = 0;
if (t->state & mask) {
- wake_up_process_kick(t);
- return;
+ woken = wake_up_process(t);
}
+#ifdef CONFIG_SMP
+ if (!woken)
+ smp_process_kick(t);
+#endif
}

/*
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