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SubjectRe: [PATCH] tvec_bases too large for per-cpu data
Hopefully attached revised patch addresses all concerns mentioned (except that it intentionally continues to not use
alloc_percpu()).

Jan

>>> Andrew Morton <akpm@osdl.org> 23.01.06 11:57:02 >>>
"Jan Beulich" <JBeulich@novell.com> wrote:
>
> >>> Andrew Morton <akpm@osdl.org> 21.01.06 08:25:00 >>>
> >"Jan Beulich" <JBeulich@novell.com> wrote:
> >>
> >> The biggest arch-independent consumer is tvec_bases (over 4k on 32-bit
> >> archs,
> >> over 8k on 64-bit ones), which now gets converted to use dynamically
> >> allocated
> >> memory instead.
> >
> >ho hum, another pointer hop.
> >
> >Did you consider using alloc_percpu()?
>
> I did, but I saw drawbacks with that (most notably the fact that all instances are allocated at
> once, possibly wasting a lot of memory).

It's 4k for each cpu which is in the possible_map but which will never be
brought online. I don't think that'll be a lot of memory - are there
machines which have a lot of possible-but-not-really-there CPUs?

> >The patch does trickery in init_timers_cpu() which, from my reading, defers
> >the actual per-cpu allocation until the second CPU comes online.
> >Presumably because of some ordering issue which you discovered. Readers of
> >the code need to know what that issue was.
>
> No, I don't see any trickery there (on demand allocation in CPU_UP_PREPARE is being done
> elsewhere in very similar ways), and I also didn't see any ordering issues. Hence I also didn't
> see any need to explain this in detail.

There _must_ be ordering issues. Otherwise we'd just dynamically allocate
all the structs up-front and be done with it.

Presumably the ordering issue is that init_timers() is called before
kmem_cache_init(). That's non-obvious and should be commented.

> >And boot_tvec_bases will always be used for the BP, and hence one slot in
> >the per-cpu array will forever be unused. Until the BP is taken down and
> >brought back up, in which case it will suddenly start to use a dynamically
> >allocated structure.
>
> Why? Each slot is allocated at most once, the BP's is never allocated (it will continue to use the
> static one even when brought down and back up).

OK, I missed the `if (likely(!base))' test in there. Patch seems OK from
that POV and we now seem to know what the ordering problem is.
- The `#ifdef CONFIG_NUMA' in init_timers_cpu() seems to be unnecessary -
kmalloc_node() will use kmalloc() if !NUMA.

- The likely()s in init_timers_cpu() seems fairly pointless - it's not a
fastpath.

- We prefer to do this:

if (expr) {
...
} else {
...
}
and not

if (expr) {
...
}
else {
...
}

From: Jan Beulich <jbeulich@novell.com>

With internal Xen-enabled kernels we see the kernel's static per-cpu data area
exceed the limit of 32k on x86-64, and even native x86-64 kernels get fairly
close to that limit. I generally question whether it is reasonable to have
data structures several kb in size allocated as per-cpu data when the space
there is rather limited.
The biggest arch-independent consumer is tvec_bases (over 4k on 32-bit archs,
over 8k on 64-bit ones), which now gets converted to use dynamically allocated
memory instead.

Signed-Off-By: Jan Beulich <jbeulich@novell.com>

diff -Npru /home/jbeulich/tmp/linux-2.6.16-rc1/kernel/timer.c 2.6.16-rc1-per-cpu-tvec_bases/kernel/timer.c
--- /home/jbeulich/tmp/linux-2.6.16-rc1/kernel/timer.c 2006-01-27 15:10:49.000000000 +0100
+++ 2.6.16-rc1-per-cpu-tvec_bases/kernel/timer.c 2006-01-27 16:22:35.000000000 +0100
@@ -86,7 +86,8 @@ struct tvec_t_base_s {
} ____cacheline_aligned_in_smp;

typedef struct tvec_t_base_s tvec_base_t;
-static DEFINE_PER_CPU(tvec_base_t, tvec_bases);
+static DEFINE_PER_CPU(tvec_base_t *, tvec_bases);
+static tvec_base_t boot_tvec_bases;

static inline void set_running_timer(tvec_base_t *base,
struct timer_list *timer)
@@ -157,7 +158,7 @@ EXPORT_SYMBOL(__init_timer_base);
void fastcall init_timer(struct timer_list *timer)
{
timer->entry.next = NULL;
- timer->base = &per_cpu(tvec_bases, raw_smp_processor_id()).t_base;
+ timer->base = &per_cpu(tvec_bases, raw_smp_processor_id())->t_base;
}
EXPORT_SYMBOL(init_timer);

@@ -218,7 +219,7 @@ int __mod_timer(struct timer_list *timer
ret = 1;
}

- new_base = &__get_cpu_var(tvec_bases);
+ new_base = __get_cpu_var(tvec_bases);

if (base != &new_base->t_base) {
/*
@@ -258,7 +259,7 @@ EXPORT_SYMBOL(__mod_timer);
*/
void add_timer_on(struct timer_list *timer, int cpu)
{
- tvec_base_t *base = &per_cpu(tvec_bases, cpu);
+ tvec_base_t *base = per_cpu(tvec_bases, cpu);
unsigned long flags;

BUG_ON(timer_pending(timer) || !timer->function);
@@ -492,7 +493,7 @@ unsigned long next_timer_interrupt(void)
tvec_t *varray[4];
int i, j;

- base = &__get_cpu_var(tvec_bases);
+ base = __get_cpu_var(tvec_bases);
spin_lock(&base->t_base.lock);
expires = base->timer_jiffies + (LONG_MAX >> 1);
list = 0;
@@ -856,7 +857,7 @@ EXPORT_SYMBOL(xtime_lock);
*/
static void run_timer_softirq(struct softirq_action *h)
{
- tvec_base_t *base = &__get_cpu_var(tvec_bases);
+ tvec_base_t *base = __get_cpu_var(tvec_bases);

hrtimer_run_queues();
if (time_after_eq(jiffies, base->timer_jiffies))
@@ -1209,12 +1210,34 @@ asmlinkage long sys_sysinfo(struct sysin
return 0;
}

-static void __devinit init_timers_cpu(int cpu)
+static int __devinit init_timers_cpu(int cpu)
{
int j;
tvec_base_t *base;

- base = &per_cpu(tvec_bases, cpu);
+ base = per_cpu(tvec_bases, cpu);
+ if (!base) {
+ static char boot_done;
+
+ /* Cannot do allocation in init_timers as that runs before the
+ allocator initializes (and would waste memory if there are
+ more possible CPUs than will ever be installed/brought up). */
+ if (boot_done) {
+#ifdef CONFIG_NUMA
+ base = kmalloc_node(sizeof(*base), GFP_KERNEL, cpu_to_node(cpu));
+ if (!base)
+ /* Just in case, fall back to normal allocation. */
+#endif
+ base = kmalloc(sizeof(*base), GFP_KERNEL);
+ if (!base)
+ return -ENOMEM;
+ memset(base, 0, sizeof(*base));
+ } else {
+ base = &boot_tvec_bases;
+ boot_done = 1;
+ }
+ per_cpu(tvec_bases, cpu) = base;
+ }
spin_lock_init(&base->t_base.lock);
for (j = 0; j < TVN_SIZE; j++) {
INIT_LIST_HEAD(base->tv5.vec + j);
@@ -1226,6 +1249,7 @@ static void __devinit init_timers_cpu(in
INIT_LIST_HEAD(base->tv1.vec + j);

base->timer_jiffies = jiffies;
+ return 0;
}

#ifdef CONFIG_HOTPLUG_CPU
@@ -1248,8 +1272,8 @@ static void __devinit migrate_timers(int
int i;

BUG_ON(cpu_online(cpu));
- old_base = &per_cpu(tvec_bases, cpu);
- new_base = &get_cpu_var(tvec_bases);
+ old_base = per_cpu(tvec_bases, cpu);
+ new_base = get_cpu_var(tvec_bases);

local_irq_disable();
spin_lock(&new_base->t_base.lock);
@@ -1279,7 +1303,8 @@ static int __devinit timer_cpu_notify(st
long cpu = (long)hcpu;
switch(action) {
case CPU_UP_PREPARE:
- init_timers_cpu(cpu);
+ if (init_timers_cpu(cpu) < 0)
+ return NOTIFY_BAD;
break;
#ifdef CONFIG_HOTPLUG_CPU
case CPU_DEAD:
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