* Mel Gorman <mgorman@xxxxxxx> [2013-06-26 15:38:03]:
> NUMA hinting faults counts and placement decisions are both recorded in the
> same array which distorts the samples in an unpredictable fashion. The values
> linearly accumulate during the scan and then decay creating a sawtooth-like
> pattern in the per-node counts. It also means that placement decisions are
> time sensitive. At best it means that it is very difficult to state that
> the buffer holds a decaying average of past faulting behaviour. At worst,
> it can confuse the load balancer if it sees one node with an artifically high
> count due to very recent faulting activity and may create a bouncing effect.
>
> This patch adds a second array. numa_faults stores the historical data
> which is used for placement decisions. numa_faults_buffer holds the
> fault activity during the current scan window. When the scan completes,
> numa_faults decays and the values from numa_faults_buffer are copied
> across.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> ---
> include/linux/sched.h | 13 +++++++++++++
> kernel/sched/core.c | 1 +
> kernel/sched/fair.c | 16 +++++++++++++---
> 3 files changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index ba46a64..42f9818 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1506,7 +1506,20 @@ struct task_struct {
> u64 node_stamp; /* migration stamp */
> struct callback_head numa_work;
>
> + /*
> + * Exponential decaying average of faults on a per-node basis.
> + * Scheduling placement decisions are made based on the these counts.
> + * The values remain static for the duration of a PTE scan
> + */
> unsigned long *numa_faults;
> +
> + /*
> + * numa_faults_buffer records faults per node during the current
> + * scan window. When the scan completes, the counts in numa_faults
> + * decay and these values are copied.
> + */
> + unsigned long *numa_faults_buffer;
> +
> int numa_preferred_nid;
> #endif /* CONFIG_NUMA_BALANCING */
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 019baae..b00b81a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1596,6 +1596,7 @@ static void __sched_fork(struct task_struct *p)
> p->numa_preferred_nid = -1;
> p->numa_work.next = &p->numa_work;
> p->numa_faults = NULL;
> + p->numa_faults_buffer = NULL;
> #endif /* CONFIG_NUMA_BALANCING */
> }
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f8c3f61..5893399 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -805,8 +805,14 @@ static void task_numa_placement(struct task_struct *p)
>
> /* Find the node with the highest number of faults */
> for (nid = 0; nid < nr_node_ids; nid++) {
> - unsigned long faults = p->numa_faults[nid];
> + unsigned long faults;
> +
> + /* Decay existing window and copy faults since last scan */
> p->numa_faults[nid] >>= 1;
> + p->numa_faults[nid] += p->numa_faults_buffer[nid];
> + p->numa_faults_buffer[nid] = 0;
> +
> + faults = p->numa_faults[nid];
> if (faults > max_faults) {
> max_faults = faults;
> max_nid = nid;
> @@ -831,9 +837,13 @@ void task_numa_fault(int node, int pages, bool migrated)
> if (unlikely(!p->numa_faults)) {
> int size = sizeof(*p->numa_faults) * nr_node_ids;
>
> - p->numa_faults = kzalloc(size, GFP_KERNEL);
> + /* numa_faults and numa_faults_buffer share the allocation */
> + p->numa_faults = kzalloc(size * 2, GFP_KERNEL);
Instead of allocating buffer to hold the current faults, cant we pass
the nr of pages and node information (and probably migrate) to
task_numa_placement()?.
Why should task_struct be passed as an argument to task_numa_placement().
It seems it always will be current.
> if (!p->numa_faults)
> return;
> +
> + BUG_ON(p->numa_faults_buffer);
> + p->numa_faults_buffer = p->numa_faults + nr_node_ids;
> }
>
> /*
> @@ -847,7 +857,7 @@ void task_numa_fault(int node, int pages, bool migrated)
> task_numa_placement(p);
>
> /* Record the fault, double the weight if pages were migrated */
> - p->numa_faults[node] += pages << migrated;
> + p->numa_faults_buffer[node] += pages << migrated;
> }
>
> static void reset_ptenuma_scan(struct task_struct *p)
> --
> 1.8.1.4
>
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--
Thanks and Regards
Srikar Dronamraju
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