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SubjectRe: [PATCH 0/18] Basic scheduler support for automatic NUMA balancing V5

Subject: sched, numa: Break stuff..
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue Jul 23 14:58:41 CEST 2013

This patch is mostly a comment in code. I don't believe the current
scan period adjustment scheme can work properly nor do I think it a
good idea to ratelimit the numa faults as a whole based on migration.

Reasons are in the modified comments...

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
kernel/sched/fair.c | 41 ++++++++++++++++++++++++++++++++---------
1 file changed, 32 insertions(+), 9 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1108,7 +1108,6 @@ static void task_numa_placement(struct t

/* Preferred node as the node with the most faults */
if (max_faults && max_nid != p->numa_preferred_nid) {
- int old_migrate_seq = p->numa_migrate_seq;

/* Queue task on preferred node if possible */
p->numa_preferred_nid = max_nid;
@@ -1116,14 +1115,19 @@ static void task_numa_placement(struct t
numa_migrate_preferred(p);

/*
+ int old_migrate_seq = p->numa_migrate_seq;
+ *
* If preferred nodes changes frequently then the scan rate
* will be continually high. Mitigate this by increasing the
* scan rate only if the task was settled.
- */
+ *
+ * APZ: disabled because we don't lower it again :/
+ *
if (old_migrate_seq >= sysctl_numa_balancing_settle_count) {
p->numa_scan_period = max(p->numa_scan_period >> 1,
task_scan_min(p));
}
+ */
}
}

@@ -1167,10 +1171,20 @@ void task_numa_fault(int last_nidpid, in
/*
* If pages are properly placed (did not migrate) then scan slower.
* This is reset periodically in case of phase changes
- */
- if (!migrated)
+ *
+ * APZ: it seems to me that one can get a ton of !migrated faults;
+ * consider the scenario where two threads fight over a shared memory
+ * segment. We'll win half the faults, half of that will be local, half
+ * of that will be remote. This means we'll see 1/4-th of the total
+ * memory being !migrated. Using a fixed increment will completely
+ * flatten the scan speed for a sufficiently large workload. Another
+ * scenario is due to that migration rate limit.
+ *
+ if (!migrated) {
p->numa_scan_period = min(p->numa_scan_period_max,
p->numa_scan_period + jiffies_to_msecs(10));
+ }
+ */

task_numa_placement(p);

@@ -1216,12 +1230,15 @@ void task_numa_work(struct callback_head
if (p->flags & PF_EXITING)
return;

+#if 0
/*
* We do not care about task placement until a task runs on a node
* other than the first one used by the address space. This is
* largely because migrations are driven by what CPU the task
* is running on. If it's never scheduled on another node, it'll
* not migrate so why bother trapping the fault.
+ *
+ * APZ: seems like a bad idea for pure shared memory workloads.
*/
if (mm->first_nid == NUMA_PTE_SCAN_INIT)
mm->first_nid = numa_node_id();
@@ -1233,6 +1250,7 @@ void task_numa_work(struct callback_head

mm->first_nid = NUMA_PTE_SCAN_ACTIVE;
}
+#endif

/*
* Enforce maximal scan/migration frequency..
@@ -1254,9 +1272,14 @@ void task_numa_work(struct callback_head
* Do not set pte_numa if the current running node is rate-limited.
* This loses statistics on the fault but if we are unwilling to
* migrate to this node, it is less likely we can do useful work
- */
+ *
+ * APZ: seems like a bad idea; even if this node can't migrate anymore
+ * other nodes might and we want up-to-date information to do balance
+ * decisions.
+ *
if (migrate_ratelimited(numa_node_id()))
return;
+ */

start = mm->numa_scan_offset;
pages = sysctl_numa_balancing_scan_size;
@@ -1297,10 +1320,10 @@ void task_numa_work(struct callback_head

out:
/*
- * It is possible to reach the end of the VMA list but the last few VMAs are
- * not guaranteed to the vma_migratable. If they are not, we would find the
- * !migratable VMA on the next scan but not reset the scanner to the start
- * so check it now.
+ * It is possible to reach the end of the VMA list but the last few
+ * VMAs are not guaranteed to the vma_migratable. If they are not, we
+ * would find the !migratable VMA on the next scan but not reset the
+ * scanner to the start so check it now.
*/
if (vma)
mm->numa_scan_offset = start;

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