[Devel] Re: [RFC] cpuset update_cgroup_cpus_allowed
Paul Menage
menage at google.com
Mon Oct 15 17:20:15 PDT 2007
Paul Jackson wrote:
> Paul M wrote:
>> Here's an alternative for consideration, below.
>
> I don't see the alternative -- I just see my patch, with the added
> blurbage:
>
> #12 - /usr/local/google/home/menage/kernel9/linux/kernel/cpuset.c ====
> # action=edit type=text
>
> Should I be increasing my caffeine intake?
>
Bah. Trying again:
Here's an alternative for consideration, below. The main differences are:
- currently against an older kernel with pre-cgroup cpusets, so it uses tasklist_lock and do_each_thread(); a cgroup version would use cgroup iterators as yours does
- solves the race between sched_setaffinity() and update_cpumask() by having sched_setaffinity() check for changes to cpuset_cpus_allowed() after doing set_cpus_allowed()
- guarantees to only act on each process once (so guarantees forward progress, in the absence of fork bombs. (And could be adapted to handle fork bombs too)
- uses a priority heap to pick the processes to act on, based on start time
- uses lock_cpu_hotplug() to avoid races with CPU hotplug; sadly I think this is gone in more recent kernels, so some other synchronization would be needed
Cause writes to cpuset "cpus" file to update cpus_allowed for
member tasks:
- collect batches of tasks under tasklist_lock and then call
set_cpus_allowed() on them outside the lock (since this can
sleep).
- add a simple generic priority heap type to allow efficient
collection of batches of tasks to be processed without
duplicating or missing any tasks in subsequent batches.
- avoid races with hotplug events via lock_cpu_hotplug()
- make "cpus" file update a no-op if the mask hasn't changed
- fix race between update_cpumask() and sched_setaffinity() by
making sched_setaffinity() to post-check that it's not
running on any cpus outside cpuset_cpus_allowed().
include/linux/prio_heap.h | 56 +++++++++++++++++++++++++
kernel/cpuset.c | 103 ++++++++++++++++++++++++++++++++++++++++++++--
kernel/sched.c | 13 +++++
lib/Makefile | 2
lib/prio_heap.c | 68 ++++++++++++++++++++++++++++++
5 files changed, 238 insertions(+), 4 deletions(-)
--- /dev/null 1969-12-31 16:00:00.000000000 -0800
+++ linux/include/linux/prio_heap.h 2007-10-12 16:43:27.000000000 -0700
@@ -0,0 +1,56 @@
+#ifndef _LINUX_PRIO_HEAP_H
+#define _LINUX_PRIO_HEAP_H
+
+/*
+ * Simple insertion-only static-sized priority heap containing
+ * pointers, based on CLR, chapter 7
+ */
+
+#include <linux/gfp.h>
+
+/**
+ * struct ptr_heap - simple static-sized priority heap
+ * @ptrs - pointer to data area
+ * @max - max number of elements that can be stored in @ptrs
+ * @size - current number of valid elements in @ptrs (in the range 0.. at size-1
+ */
+struct ptr_heap {
+ void **ptrs;
+ int max;
+ int size;
+};
+
+/**
+ * heap_init - initialize an empty heap with a given memory size
+ * @heap: the heap structure to be initialized
+ * @size: amount of memory to use in bytes
+ * @gfp_mask: mask to pass to kmalloc()
+ */
+extern int heap_init(struct ptr_heap *heap, size_t size, gfp_t gfp_mask);
+
+/**
+ * heap_free - release a heap's storage
+ * @heap: the heap structure whose data should be released
+ */
+void heap_free(struct ptr_heap *heap);
+
+/**
+ * heap_insert - insert a value into the heap and return any overflowed value
+ * @heap: the heap to be operated on
+ * @p: the pointer to be inserted
+ * @gt: comparison operator, which should implement "greater than"
+ *
+ * Attempts to insert the given value into the priority heap. If the
+ * heap is full prior to the insertion, then the resulting heap will
+ * consist of the smallest @max elements of the original heap and the
+ * new element; the greatest element will be removed from the heap and
+ * returned. Note that the returned element will be the new element
+ * (i.e. no change to the heap) if the new element is greater than all
+ * elements currently in the heap.
+ */
+extern void *heap_insert(struct ptr_heap *heap, void *p,
+ int (*gt)(void *, void *));
+
+
+
+#endif /* _LINUX_PRIO_HEAP_H */
==== linux/kernel/cpuset.c
--- linux/kernel/cpuset.c 2007-10-05 17:46:09.000000000 -0700
+++ linux/kernel/cpuset.c 2007-10-12 16:24:49.000000000 -0700
@@ -37,6 +37,7 @@
#include <linux/mount.h>
#include <linux/namei.h>
#include <linux/pagemap.h>
+#include <linux/prio_heap.h>
#include <linux/proc_fs.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
@@ -839,6 +840,36 @@
unlock_cpu_hotplug();
}
+static int inline started_after_time(struct task_struct *t1,
+ struct timespec *time,
+ struct task_struct *t2)
+{
+ int start_diff = timespec_compare(&t1->start_time, time);
+ if (start_diff > 0) {
+ return 1;
+ } else if (start_diff < 0) {
+ return 0;
+ } else {
+ /*
+ * Arbitrarily, if two processes started at the same
+ * time, we'll say that the lower pointer value
+ * started first. Note that t2 may have exited by now
+ * so this may not be a valid pointer any longer, but
+ * that's fine - it still serves to distinguish
+ * between two tasks started (effectively)
+ * simultaneously.
+ */
+ return t1 > t2;
+ }
+}
+
+static int inline started_after(void *p1, void *p2)
+{
+ struct task_struct *t1 = p1;
+ struct task_struct *t2 = p2;
+ return started_after_time(t1, &t2->start_time, t2);
+}
+
/*
* Call with manage_mutex held. May take callback_mutex during call.
*/
@@ -846,7 +877,12 @@
static int update_cpumask(struct cpuset *cs, char *buf)
{
struct cpuset trialcs;
- int retval, cpus_unchanged;
+ int retval, i;
+ struct task_struct *g, *p, *dropped;
+ /* Never dereference latest_task, since it's not refcounted */
+ struct task_struct *latest_task = NULL;
+ struct ptr_heap heap;
+ struct timespec latest_time = { 0, 0 };
/* top_cpuset.cpus_allowed tracks cpu_online_map; it's read-only */
if (cs == &top_cpuset)
@@ -862,11 +898,72 @@
retval = validate_change(cs, &trialcs);
if (retval < 0)
return retval;
- cpus_unchanged = cpus_equal(cs->cpus_allowed, trialcs.cpus_allowed);
+ if (cpus_equal(cs->cpus_allowed, trialcs.cpus_allowed))
+ return 0;
+ retval = heap_init(&heap, PAGE_SIZE, GFP_KERNEL);
+ if (retval)
+ return retval;
+
mutex_lock(&callback_mutex);
cs->cpus_allowed = trialcs.cpus_allowed;
mutex_unlock(&callback_mutex);
- if (is_cpu_exclusive(cs) && !cpus_unchanged)
+ again:
+ read_lock(&tasklist_lock);
+ /*
+ * Scan tasks in the cpuset, and update the cpumasks of any
+ * that need an update. Since we can't call set_cpus_allowed()
+ * while holding tasklist_lock, gather tasks to be processed
+ * in a heap structure. If the statically-sized heap fills up,
+ * overflow tasks that started later, and in future iterations
+ * only consider tasks that started after the latest task in
+ * the previous pass. This guarantees forward progress and
+ * that we don't miss any tasks
+ */
+ heap.size = 0;
+ do_each_thread(g, p) {
+ /* Only affect tasks from this cpuset */
+ if (p->cpuset != cs)
+ continue;
+ /* Only affect tasks that don't have the right cpus_allowed */
+ if (cpus_equal(p->cpus_allowed, cs->cpus_allowed))
+ continue;
+ /*
+ * Only process tasks that started after the last task
+ * we processed
+ */
+ if (!started_after_time(p, &latest_time, latest_task))
+ continue;
+ dropped = heap_insert(&heap, p, &started_after);
+ if (dropped == NULL) {
+ get_task_struct(p);
+ } else if (dropped != p) {
+ get_task_struct(p);
+ put_task_struct(dropped);
+ }
+ } while_each_thread(g, p);
+ read_unlock(&tasklist_lock);
+ if (heap.size) {
+ for (i = 0; i < heap.size; i++) {
+ struct task_struct *p = heap.ptrs[i];
+ if (i == 0) {
+ latest_time = p->start_time;
+ latest_task = p;
+ }
+ set_cpus_allowed(p, cs->cpus_allowed);
+ put_task_struct(p);
+ }
+ /*
+ * If we had to process any tasks at all, scan again
+ * in case some of them were in the middle of forking
+ * children that didn't notice the new cpumask
+ * restriction. Not the most efficient way to do it,
+ * but it avoids having to take callback_mutex in the
+ * fork path
+ */
+ goto again;
+ }
+ heap_free(&heap);
+ if (is_cpu_exclusive(cs))
update_cpu_domains(cs);
return 0;
}
==== linux/kernel/sched.c
--- linux/kernel/sched.c 2007-10-11 20:07:17.000000000 -0700
+++ linux/kernel/sched.c 2007-10-11 22:04:45.000000000 -0700
@@ -4411,8 +4411,21 @@
cpus_allowed = cpuset_cpus_allowed(p);
cpus_and(new_mask, new_mask, cpus_allowed);
+ again:
retval = set_cpus_allowed(p, new_mask);
+ if (!retval) {
+ cpus_allowed = cpuset_cpus_allowed(p);
+ if (!cpus_subset(new_mask, cpus_allowed)) {
+ /*
+ * We must have raced with a concurrent cpuset
+ * update. Just reset the cpus_allowed to the
+ * cpuset's cpus_allowed
+ */
+ new_mask = cpus_allowed;
+ goto again;
+ }
+ }
out_unlock:
put_task_struct(p);
unlock_cpu_hotplug();
==== linux/lib/Makefile
--- linux/lib/Makefile 2007-10-15 14:09:45.000000000 -0700
+++ linux/lib/Makefile 2007-10-12 16:29:22.000000000 -0700
@@ -5,7 +5,7 @@
lib-y := errno.o ctype.o string.o vsprintf.o cmdline.o \
bust_spinlocks.o rbtree.o radix-tree.o dump_stack.o \
idr.o div64.o int_sqrt.o bitmap.o extable.o prio_tree.o \
- sha1.o
+ sha1.o prio_heap.o
lib-$(CONFIG_SMP) += cpumask.o
==== linux/lib/prio_heap.c
--- /dev/null 1969-12-31 16:00:00.000000000 -0800
+++ linux/lib/prio_heap.c 2007-10-12 16:30:27.000000000 -0700
@@ -0,0 +1,68 @@
+/*
+ * Simple insertion-only static-sized priority heap containing
+ * pointers, based on CLR, chapter 7
+ */
+
+#include <linux/slab.h>
+#include <linux/prio_heap.h>
+
+int heap_init(struct ptr_heap *heap, size_t size, gfp_t gfp_mask)
+{
+ heap->ptrs = kmalloc(size, gfp_mask);
+ if (!heap->ptrs)
+ return -ENOMEM;
+ heap->size = 0;
+ heap->max = size / sizeof(void *);
+ return 0;
+}
+
+void heap_free(struct ptr_heap *heap)
+{
+ kfree(heap->ptrs);
+}
+
+void *heap_insert(struct ptr_heap *heap, void *p, int (*gt)(void *, void *))
+{
+ void *res;
+ void **ptrs = heap->ptrs;
+ int pos;
+
+ if (heap->size < heap->max) {
+ /* Heap insertion */
+ int pos = heap->size++;
+ while (pos > 0 && gt(p, ptrs[(pos-1)/2])) {
+ ptrs[pos] = ptrs[(pos-1)/2];
+ pos = (pos-1)/2;
+ }
+ ptrs[pos] = p;
+ return NULL;
+ }
+
+ /* The heap is full, so something will have to be dropped */
+
+ /* If the new pointer is greater than the current max, drop it */
+ if (gt(p, ptrs[0]))
+ return p;
+
+ /* Replace the current max and heapify */
+ res = ptrs[0];
+ ptrs[0] = p;
+ pos = 0;
+
+ while (1) {
+ int left = 2 * pos + 1;
+ int right = 2 * pos + 2;
+ int largest = pos;
+ if (left < heap->size && gt(ptrs[left], p))
+ largest = left;
+ if (right < heap->size && gt(ptrs[right], ptrs[largest]))
+ largest = right;
+ if (largest == pos)
+ break;
+ /* Push p down the heap one level and bump one up */
+ ptrs[pos] = ptrs[largest];
+ ptrs[largest] = p;
+ pos = largest;
+ }
+ return res;
+}
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