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Subject[patch] real-time enhanced page allocator and throttling
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On Mon, 2003-08-04 at 22:55, Andrew Morton wrote:

> There is a very good argument for giving !SCHED_OTHER tasks
> "special treatment" in the VM.

Yes, there is.

Attached patch is against 2.6.0-test2-mm4. It does two main things:

- Let real-time tasks dip further into the reserves than
usual in __alloc_pages(). There are a lot of ways to
special case this. This patch just cuts z->pages_low in
half, before doing the incremental min thing, for
real-time tasks. I do not do anything in the low memory
slow path. We can be a _lot_ more aggressive if we want.
Right now, we just give real-time tasks a little help.

- Never ever call balance_dirty_pages() on a real-time
task. Where and how exactly we handle this is up for
debate. We could, for example, special case real-time
tasks inside balance_dirty_pages(). This would allow
us to perform some of the work (say, waking up pdflush)
but not other work (say, the active throttling). As it
stands now, we do the per-processor accounting in
balance_dirty_pages_ratelimited() but we never call
balance_dirty_pages(). Lots of approaches work. What
we want to do is never engage the real-time task in
forced writeback.

It compiles, it boots, and it does not crash. I have not tested whether
it prevents any starvation in real-time applications that are being
observed -- mostly because I am not sure if my approach is what you
want. There are multiple ways to handle the real-time task path. I
picked one. I do not know.

Robert Love


include/linux/sched.h | 4 +++-
kernel/sched.c | 1 -
mm/page-writeback.c | 6 +++++-
mm/page_alloc.c | 29 ++++++++++++++++++++---------
4 files changed, 28 insertions(+), 12 deletions(-)


diff -urN linux-2.6.0-test2-mm4/include/linux/sched.h linux/include/linux/sched.h
--- linux-2.6.0-test2-mm4/include/linux/sched.h 2003-08-05 14:53:47.000000000 -0700
+++ linux/include/linux/sched.h 2003-08-05 12:38:41.000000000 -0700
@@ -282,7 +282,9 @@
#define MAX_RT_PRIO MAX_USER_RT_PRIO

#define MAX_PRIO (MAX_RT_PRIO + 40)
-
+
+#define rt_task(p) ((p)->prio < MAX_RT_PRIO)
+
/*
* Some day this will be a full-fledged user tracking system..
*/
diff -urN linux-2.6.0-test2-mm4/kernel/sched.c linux/kernel/sched.c
--- linux-2.6.0-test2-mm4/kernel/sched.c 2003-08-05 14:53:47.000000000 -0700
+++ linux/kernel/sched.c 2003-08-05 12:38:29.000000000 -0700
@@ -199,7 +199,6 @@
#define this_rq() (cpu_rq(smp_processor_id())) /* not __get_cpu_var(runqueues)! */
#define task_rq(p) cpu_rq(task_cpu(p))
#define cpu_curr(cpu) (cpu_rq(cpu)->curr)
-#define rt_task(p) ((p)->prio < MAX_RT_PRIO)

/*
* Default context-switch locking:
diff -urN linux-2.6.0-test2-mm4/mm/page_alloc.c linux/mm/page_alloc.c
--- linux-2.6.0-test2-mm4/mm/page_alloc.c 2003-08-05 14:48:38.000000000 -0700
+++ linux/mm/page_alloc.c 2003-08-05 14:40:56.000000000 -0700
@@ -518,7 +518,8 @@
*
* Herein lies the mysterious "incremental min". That's the
*
- * min += z->pages_low;
+ * local_low = z->pages_low;
+ * min += local_low;
*
* thing. The intent here is to provide additional protection to low zones for
* allocation requests which _could_ use higher zones. So a GFP_HIGHMEM
@@ -536,10 +537,11 @@
unsigned long min;
struct zone **zones, *classzone;
struct page *page;
+ struct reclaim_state reclaim_state;
+ struct task_struct *p = current;
int i;
int cold;
int do_retry;
- struct reclaim_state reclaim_state;

if (wait)
might_sleep();
@@ -557,8 +559,17 @@
min = 1UL << order;
for (i = 0; zones[i] != NULL; i++) {
struct zone *z = zones[i];
+ unsigned long local_low;
+
+ /*
+ * This is the fabled 'incremental min'. We let real-time tasks
+ * dip their real-time paws a little deeper into reserves.
+ */
+ local_low = z->pages_low;
+ if (rt_task(p))
+ local_low >>= 1;
+ min += local_low;

- min += z->pages_low;
if (z->free_pages >= min ||
(!wait && z->free_pages >= z->pages_high)) {
page = buffered_rmqueue(z, order, cold);
@@ -594,7 +605,7 @@
/* here we're in the low on memory slow path */

rebalance:
- if ((current->flags & (PF_MEMALLOC | PF_MEMDIE)) && !in_interrupt()) {
+ if ((p->flags & (PF_MEMALLOC | PF_MEMDIE)) && !in_interrupt()) {
/* go through the zonelist yet again, ignoring mins */
for (i = 0; zones[i] != NULL; i++) {
struct zone *z = zones[i];
@@ -610,14 +621,14 @@
if (!wait)
goto nopage;

- current->flags |= PF_MEMALLOC;
+ p->flags |= PF_MEMALLOC;
reclaim_state.reclaimed_slab = 0;
- current->reclaim_state = &reclaim_state;
+ p->reclaim_state = &reclaim_state;

try_to_free_pages(classzone, gfp_mask, order);

- current->reclaim_state = NULL;
- current->flags &= ~PF_MEMALLOC;
+ p->reclaim_state = NULL;
+ p->flags &= ~PF_MEMALLOC;

/* go through the zonelist yet one more time */
min = 1UL << order;
@@ -657,7 +668,7 @@
if (!(gfp_mask & __GFP_NOWARN)) {
printk("%s: page allocation failure."
" order:%d, mode:0x%x\n",
- current->comm, order, gfp_mask);
+ p->comm, order, gfp_mask);
}
return NULL;
got_pg:
diff -urN linux-2.6.0-test2-mm4/mm/page-writeback.c linux/mm/page-writeback.c
--- linux-2.6.0-test2-mm4/mm/page-writeback.c 2003-08-05 14:53:47.000000000 -0700
+++ linux/mm/page-writeback.c 2003-08-05 13:48:43.000000000 -0700
@@ -227,7 +227,11 @@
if (dirty_exceeded)
ratelimit = 8;

- if (get_cpu_var(ratelimits)++ >= ratelimit) {
+ /*
+ * Check the rate limiting. Also, we do not want to throttle real-time
+ * tasks in balance_dirty_pages(). Period.
+ */
+ if (get_cpu_var(ratelimits)++ >= ratelimit && !rt_task(current)) {
__get_cpu_var(ratelimits) = 0;
put_cpu_var(ratelimits);
return balance_dirty_pages(mapping);


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