Re: [PATCH] minor sched-E1 tweaks and questions

Rusty Russell (
Thu, 10 Jan 2002 14:48:42 +1100

In message <Pine.LNX.4.33.0201091126520.2276-100000@localhost.localdomain> you
> > Q. How can this happen in expire_task():
> > if (p->array != rq->active) {
> > p->need_resched = 1;
> > return;
> > }
> if a task gets delayed by some really heavy IRQ load and the timer
> interrupt hits the task twice.

Hmm... still don't see it. update_process_times() surely doesn't
re-enter? And another CPU cannot load_balance() p (== current) away
from us.

Another question:

if (likely(prev != next)) {
rq->curr = next;
next->cpu = prev->cpu;
context_switch(prev, next);
* The runqueue pointer might be from another CPU
* if the new task was last running on a different
* CPU - thus re-load it.
rq = this_rq();

I do not understand this comment. How can rq (ie. smp_processor_id())
change? Nothing sleeps here, and if it DID change, the
spin_unlock_irq() would be wrong...

> i've taken your fixes, please double-check the next patch whether all of
> them are correctly applied.

Um, missed one:

static struct runqueue {
- int cpu;
spinlock_t lock;
- unsigned long nr_running, nr_switches, last_rt_event;
+ unsigned long nr_running, nr_switches;
task_t *curr, *idle;
prio_array_t *active, *expired, arrays[2];
- char __pad [SMP_CACHE_BYTES];
+ int prev_nr_running[NR_CPUS];
} runqueues [NR_CPUS] __cacheline_aligned;

You want each entry in the array to be aligned, not the whole array!
You need to define the struct runqueue to be the cacheline aligned
(using ____cacheline_aligned since it's a type), THEN put the
__cacheline_aligned after the array declaration so it gets put in the
aligned section:

struct runqueue {
spinlock_t lock;
unsigned long nr_running, nr_switches;
task_t *curr, *idle;
prio_array_t *active, *expired, arrays[2];
int prev_nr_running[NR_CPUS];
} ____cacheline_aligned;

static struct runqueue runqueues [NR_CPUS] __cacheline_aligned;

This is why my __per_cpu patch was invented 8)

  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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