* Re: [Lse-tech] Re: CPU affinity & IPI latency (updated patch)
[not found] <OF67655A11.398205F1-ON85256A8E.004D7BD7@pok.ibm.com>
@ 2001-07-19 14:25 ` Hubertus Franke
0 siblings, 0 replies; only message in thread
From: Hubertus Franke @ 2001-07-19 14:25 UTC (permalink / raw)
To: davidel; +Cc: linux-kernel, lse-tech
[-- Attachment #1: Type: text/plain, Size: 3236 bytes --]
David, thanks for talking this through. I attach the latest patch that
I think corrects the problems identified earlier and should serve as the base
discussion for our discussion.
> Sent by: davidel@ewok.dev.mcafeelabs.com
>
> To: Hubertus Franke/Watson/IBM@IBMUS
> cc: mkravetz@sequent.com, ak@suse.de
> Subject: Re: [Lse-tech] Re: CPU affinity & IPI latency (FIX)_
>
> On 18-Jul-2001 Hubertus Franke wrote:
> >
> > The only reason one might want to put it outside is to force a shorter
> > schedule function, as this one only takes cpu_schedule(cpu) rather
> > then running the whole list.
> > I moved it inside the function.
>
> Maybe we're talking about two different things :)
> What's the line number you refer to ?
I am talking about this line in the fast track of reschedule_idle().
The current thing that we seem to agree on is
this (and that's in the atached whole patch)
@@ -229,8 +233,11 @@
*/
need_resched = tsk->need_resched;
tsk->need_resched = 1;
- if ((best_cpu != this_cpu) && !need_resched)
+ if ((best_cpu != this_cpu) && !need_resched) {
+ p->has_cpu = 1;
+ cpu_resched(best_cpu) = p;
smp_send_reschedule(best_cpu);
+ }
return;
}
}
What could be done is the following. If we haven't
issued a need_resched event yet, then lock down <p>
to the best_cpu and if necessary issue an IPI.
This would ensure due to the fast check in schedule()
that we wouldn't run through the whole list in this
particular case either.
@@ -229,8 +233,11 @@
*/
need_resched = tsk->need_resched;
tsk->need_resched = 1;
- if ((best_cpu != this_cpu) && !need_resched)
+ if (!need_resched) {
+ p->has_cpu = 1;
+ cpu_resched(best_cpu) = p;
+ if (best_cpu != this_cpu)
smp_send_reschedule(best_cpu);
+ }
return;
}
}
>
> >
> > In general is this something we should push for in the kernel scheduler
> > or should your quote be taken as the remedy for this problem ?
> > "If You want to have a better reschedule latency use the poll idle."
>
> The poll idle is power expensive but it has a better latency.
> Right now is boot command line settable.
>
> - Davide
I know, my question was, whether you think this whole discussion is useless
because somebody can always switch over to poll and should do so if he/she
has a problem with the current implementation ?
Also somebody contacted me with concerns to hot-plug cpus. Not a problem
One merely needs to do something like this before yanking the cpu.
if (cpu_resched(yankcpu)) {
cpu_resched(yankcpu)->has_cpu = 0;
cpu_resched(yankcpu) = NULL;
}
and the temp-bound task is free to be scheduable from anywhere else again.
-- Hubertus Franke
(frankeh@us.ibm.com)
[-- Attachment #2: cpuaff-patch --]
[-- Type: text/plain, Size: 3868 bytes --]
diff -uwrbBN linux-2.4.5-van/Makefile linux-2.4.5-ca/Makefile
--- linux-2.4.5-van/Makefile Fri May 25 12:51:33 2001
+++ linux-2.4.5-ca/Makefile Tue Jul 17 06:07:50 2001
@@ -1,7 +1,7 @@
VERSION = 2
PATCHLEVEL = 4
SUBLEVEL = 5
-EXTRAVERSION =
+EXTRAVERSION = -ca
KERNELRELEASE=$(VERSION).$(PATCHLEVEL).$(SUBLEVEL)$(EXTRAVERSION)
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 Wed Jul 18 17:01:36 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:
/*
@@ -229,8 +233,11 @@
*/
need_resched = tsk->need_resched;
tsk->need_resched = 1;
- if ((best_cpu != this_cpu) && !need_resched)
+ if ((best_cpu != this_cpu) && !need_resched) {
+ p->has_cpu = 1;
+ cpu_resched(best_cpu) = p;
smp_send_reschedule(best_cpu);
+ }
return;
}
}
@@ -244,13 +251,24 @@
*/
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 +286,30 @@
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 */
+ p->has_cpu = 1; /* can't schedule this one no more*/
+ cpu_resched(best_cpu) = p;
+ 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) {
+ p->has_cpu = 1;
+ cpu_resched(best_cpu) = p;
+ smp_send_reschedule(best_cpu);
+ }
}
return;
@@ -578,6 +607,15 @@
*/
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;
+ goto found_next;
+ }
/*
* Default process to select..
*/
@@ -604,6 +642,7 @@
* switching to the next task, save this fact in
* sched_data.
*/
+found_next:
sched_data->curr = next;
#ifdef CONFIG_SMP
next->has_cpu = 1;
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