Re: [Lse-tech] Re: CPU affinity & IPI latency (FIX)_

[Hubertus Franke <frankeh@watson.ibm.com>]

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David, thanks for pointing this out. Ofcourse, it has be reset at that point.
Since it is still running, it actually proves that priority overwrites for
reservations work.
Did you have a chance to run it on the offending app. I don't have that one,
hence I only tried on kernel builds etc.
Here is the update patch.

-- Hubertus (frankeh@us.ibm.com)

> Davide Libenzi <davidel@xmailserver.org>@lists.sourceforge.net on
> 07/17/2001 02:00:45 PM
>
> Sent by:  lse-tech-admin@lists.sourceforge.net
>
> To:   Hubertus Frnake <frankeh@watson.ibm.com>
> cc:   linux-kernel@vger.kernel.org, lse-tech@lists.sourceforge.net,
>       ak@suse.de
> Subject:  Re: [Lse-tech] Re: CPU affinity & IPI latency (FIX)_
>
> On 17-Jul-2001 Hubertus Frnake wrote:
> > In an attempt to inline the code, somehow the tabs got lost. So here is
> the
> > attached correct patch fo 2.4.5. Please try and let me know whether you
> > see your problems disappear and/or others arise.
> > The sketchy writeup is still the same.
>
> Did You tried the patch ?
> Maybe this could help :
>
> +       next = cpu_resched(this_cpu);
> +       if (next) {
> +               cpu_resched(this_cpu) = NULL;
> -               next = p;
> +               goto found_next;
> +       }
>
> - Davide
>
> _______________________________________________
> Lse-tech mailing list
> Lse-tech@lists.sourceforge.net
> http://lists.sourceforge.net/lists/listinfo/lse-tech

[-- Attachment #2: cpuaff-patch --]
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diff -uwrbBN linux-2.4.5-van/kernel/sched.c linux-2.4.5-ca/kernel/sched.c
--- linux-2.4.5-van/kernel/sched.c	Fri Apr 20 21:26:16 2001
+++ linux-2.4.5-ca/kernel/sched.c	Tue Jul 17 14:02:13 2001
@@ -97,12 +97,14 @@
 static union {
 	struct schedule_data {
 		struct task_struct * curr;
+		struct task_struct * resched;
 		cycles_t last_schedule;
 	} schedule_data;
 	char __pad [SMP_CACHE_BYTES];
-} aligned_data [NR_CPUS] __cacheline_aligned = { {{&init_task,0}}};
+} aligned_data [NR_CPUS] __cacheline_aligned = { {{&init_task,0,0}}};
 
 #define cpu_curr(cpu) aligned_data[(cpu)].schedule_data.curr
+#define cpu_resched(cpu) aligned_data[(cpu)].schedule_data.resched
 #define last_schedule(cpu) aligned_data[(cpu)].schedule_data.last_schedule
 
 struct kernel_stat kstat;
@@ -208,7 +210,7 @@
 {
 #ifdef CONFIG_SMP
 	int this_cpu = smp_processor_id();
-	struct task_struct *tsk, *target_tsk;
+	struct task_struct *tsk, *target_tsk, *rtsk;
 	int cpu, best_cpu, i, max_prio;
 	cycles_t oldest_idle;
 
@@ -219,7 +221,9 @@
 	best_cpu = p->processor;
 	if (can_schedule(p, best_cpu)) {
 		tsk = idle_task(best_cpu);
-		if (cpu_curr(best_cpu) == tsk) {
+		if ((cpu_curr(best_cpu) == tsk) &&
+		    (cpu_resched(best_cpu) == NULL))
+		{
 			int need_resched;
 send_now_idle:
 			/*
@@ -244,13 +248,24 @@
 	 */
 	oldest_idle = (cycles_t) -1;
 	target_tsk = NULL;
+	best_cpu = 0;
 	max_prio = 1;
 
 	for (i = 0; i < smp_num_cpus; i++) {
 		cpu = cpu_logical_map(i);
 		if (!can_schedule(p, cpu))
 			continue;
+		/* first check whether there is an resched IPI
+		 * reservation for that cpu. If so consider priority
+		 * of the reservation instead of current.
+		 * We do not have to set the need_resched flag again
+		 * for the currently running task. It must have been
+		 * signalled before
+		 */
+		tsk = cpu_resched(cpu);
+		if (tsk == NULL)
 		tsk = cpu_curr(cpu);
+
 		/*
 		 * We use the first available idle CPU. This creates
 		 * a priority list between idle CPUs, but this is not
@@ -268,19 +283,30 @@
 				if (prio > max_prio) {
 					max_prio = prio;
 					target_tsk = tsk;
+					best_cpu = cpu;
 				}
 			}
 		}
 	}
 	tsk = target_tsk;
 	if (tsk) {
+		rtsk = cpu_resched(best_cpu);
+		if (rtsk) {
+			rtsk->has_cpu = 0; /* return rtsk to scheduable */
+			tsk->has_cpu  = 1; /* can't schedule this one no more*/
+			cpu_resched(best_cpu) = tsk;
+			return;
+		}
 		if (oldest_idle != -1ULL) {
 			best_cpu = tsk->processor;
 			goto send_now_idle;
 		}
 		tsk->need_resched = 1;
-		if (tsk->processor != this_cpu)
-			smp_send_reschedule(tsk->processor);
+		if (tsk->processor != this_cpu) {
+			tsk->has_cpu  = 1; 
+			cpu_resched(best_cpu) = tsk;
+			smp_send_reschedule(best_cpu);
+		}
 	}
 	return;
 		
@@ -578,6 +604,16 @@
 	 */
 
 repeat_schedule:
+	/* we check whether we have a resched_IPI reservation:
+	 * if so simply select the reserving task and next and
+	 * go to switch to it.
+	 */
+	next = cpu_resched(this_cpu);
+	if (next) {
+		cpu_resched(this_cpu) = NULL;
+		next = p;
+		goto found_next;
+	}
 	/*
 	 * Default process to select..
 	 */
@@ -604,6 +640,7 @@
 	 * switching to the next task, save this fact in
 	 * sched_data.
 	 */
+found_next:
 	sched_data->curr = next;
 #ifdef CONFIG_SMP
  	next->has_cpu = 1;