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Date: Fri, 28 Jun 2013 13:41:20 +0530
From: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To: Mel Gorman <mgorman@...e.de>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Ingo Molnar <mingo@...nel.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8] sched: Favour moving tasks towards the preferred node
* Mel Gorman <mgorman@...e.de> [2013-06-26 15:38:04]:
> This patch favours moving tasks towards the preferred NUMA node when
> it has just been selected. Ideally this is self-reinforcing as the
> longer the the task runs on that node, the more faults it should incur
> causing task_numa_placement to keep the task running on that node. In
> reality a big weakness is that the nodes CPUs can be overloaded and it
> would be more effficient to queue tasks on an idle node and migrate to
> the new node. This would require additional smarts in the balancer so
> for now the balancer will simply prefer to place the task on the
> preferred node for a tunable number of PTE scans.
>
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> ---
> Documentation/sysctl/kernel.txt | 8 +++++++-
> include/linux/sched.h | 1 +
> kernel/sched/core.c | 4 +++-
> kernel/sched/fair.c | 40 ++++++++++++++++++++++++++++++++++++++--
> kernel/sysctl.c | 7 +++++++
> 5 files changed, 56 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index 0fe678c..246b128 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -374,7 +374,8 @@ feature should be disabled. Otherwise, if the system overhead from the
> feature is too high then the rate the kernel samples for NUMA hinting
> faults may be controlled by the numa_balancing_scan_period_min_ms,
> numa_balancing_scan_delay_ms, numa_balancing_scan_period_reset,
> -numa_balancing_scan_period_max_ms and numa_balancing_scan_size_mb sysctls.
> +numa_balancing_scan_period_max_ms, numa_balancing_scan_size_mb and
> +numa_balancing_settle_count sysctls.
>
> ==============================================================
>
> @@ -418,6 +419,11 @@ scanned for a given scan.
> numa_balancing_scan_period_reset is a blunt instrument that controls how
> often a tasks scan delay is reset to detect sudden changes in task behaviour.
>
> +numa_balancing_settle_count is how many scan periods must complete before
> +the schedule balancer stops pushing the task towards a preferred node. This
> +gives the scheduler a chance to place the task on an alternative node if the
> +preferred node is overloaded.
> +
> ==============================================================
>
> osrelease, ostype & version:
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 42f9818..82a6136 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -815,6 +815,7 @@ enum cpu_idle_type {
> #define SD_ASYM_PACKING 0x0800 /* Place busy groups earlier in the domain */
> #define SD_PREFER_SIBLING 0x1000 /* Prefer to place tasks in a sibling domain */
> #define SD_OVERLAP 0x2000 /* sched_domains of this level overlap */
> +#define SD_NUMA 0x4000 /* cross-node balancing */
>
> extern int __weak arch_sd_sibiling_asym_packing(void);
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b00b81a..ba9470e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1591,7 +1591,7 @@ static void __sched_fork(struct task_struct *p)
>
> p->node_stamp = 0ULL;
> p->numa_scan_seq = p->mm ? p->mm->numa_scan_seq : 0;
> - p->numa_migrate_seq = p->mm ? p->mm->numa_scan_seq - 1 : 0;
> + p->numa_migrate_seq = 0;
> p->numa_scan_period = sysctl_numa_balancing_scan_delay;
> p->numa_preferred_nid = -1;
> p->numa_work.next = &p->numa_work;
> @@ -5721,6 +5721,7 @@ struct sched_domain_topology_level;
> void sched_setnuma(struct task_struct *p, int nid)
> {
> p->numa_preferred_nid = nid;
> + p->numa_migrate_seq = 0;
> }
> #endif /* CONFIG_NUMA_BALANCING */
>
> @@ -6150,6 +6151,7 @@ sd_numa_init(struct sched_domain_topology_level *tl, int cpu)
> | 0*SD_SHARE_PKG_RESOURCES
> | 1*SD_SERIALIZE
> | 0*SD_PREFER_SIBLING
> + | 1*SD_NUMA
> | sd_local_flags(level)
> ,
> .last_balance = jiffies,
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5893399..5e7f728 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -791,6 +791,15 @@ unsigned int sysctl_numa_balancing_scan_size = 256;
> /* Scan @scan_size MB every @scan_period after an initial @scan_delay in ms */
> unsigned int sysctl_numa_balancing_scan_delay = 1000;
>
> +/*
> + * Once a preferred node is selected the scheduler balancer will prefer moving
> + * a task to that node for sysctl_numa_balancing_settle_count number of PTE
> + * scans. This will give the process the chance to accumulate more faults on
> + * the preferred node but still allow the scheduler to move the task again if
> + * the nodes CPUs are overloaded.
> + */
> +unsigned int sysctl_numa_balancing_settle_count __read_mostly = 3;
> +
> static void task_numa_placement(struct task_struct *p)
> {
> int seq, nid, max_nid = 0;
> @@ -802,6 +811,7 @@ static void task_numa_placement(struct task_struct *p)
> if (p->numa_scan_seq == seq)
> return;
> p->numa_scan_seq = seq;
> + p->numa_migrate_seq++;
>
> /* Find the node with the highest number of faults */
> for (nid = 0; nid < nr_node_ids; nid++) {
> @@ -3897,6 +3907,28 @@ task_hot(struct task_struct *p, u64 now, struct sched_domain *sd)
> return delta < (s64)sysctl_sched_migration_cost;
> }
>
> +/* Returns true if the destination node has incurred more faults */
> +static bool migrate_improves_locality(struct task_struct *p, struct lb_env *env)
> +{
> + int src_nid, dst_nid;
> +
> + if (!p->numa_faults || !(env->sd->flags & SD_NUMA))
> + return false;
> +
> + src_nid = cpu_to_node(env->src_cpu);
> + dst_nid = cpu_to_node(env->dst_cpu);
> +
> + if (src_nid == dst_nid)
> + return false;
> +
> + if (p->numa_migrate_seq < sysctl_numa_balancing_settle_count &&
Lets say even if the numa_migrate_seq is greater than settle_count but running
on a wrong node, then shouldnt this be taken as a good opportunity to
move the task?
> + p->numa_preferred_nid == dst_nid)
> + return true;
> +
> + return false;
> +}
> +
> +
> /*
> * can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
> */
> @@ -3945,10 +3977,14 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>
> /*
> * Aggressive migration if:
> - * 1) task is cache cold, or
> - * 2) too many balance attempts have failed.
> + * 1) destination numa is preferred
> + * 2) task is cache cold, or
> + * 3) too many balance attempts have failed.
> */
>
> + if (migrate_improves_locality(p, env))
> + return 1;
Shouldnt this be under tsk_cache_hot check?
If the task is cache hot, then we would have to update the corresponding schedstat
metrics.
> +
> tsk_cache_hot = task_hot(p, env->src_rq->clock_task, env->sd);
> if (!tsk_cache_hot ||
> env->sd->nr_balance_failed > env->sd->cache_nice_tries) {
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index afc1dc6..263486f 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -393,6 +393,13 @@ static struct ctl_table kern_table[] = {
> .mode = 0644,
> .proc_handler = proc_dointvec,
> },
> + {
> + .procname = "numa_balancing_settle_count",
> + .data = &sysctl_numa_balancing_settle_count,
> + .maxlen = sizeof(unsigned int),
> + .mode = 0644,
> + .proc_handler = proc_dointvec,
> + },
> #endif /* CONFIG_NUMA_BALANCING */
> #endif /* CONFIG_SCHED_DEBUG */
> {
> --
> 1.8.1.4
>
> --
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--
Thanks and Regards
Srikar Dronamraju
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