Re: [patch 3/7 -mm] oom: select task from tasklist for mempolicy ooms

[KOSAKI Motohiro]

> The oom killer presently kills current whenever there is no more memory
> free or reclaimable on its mempolicy's nodes.  There is no guarantee that
> current is a memory-hogging task or that killing it will free any
> substantial amount of memory, however.
> 
> In such situations, it is better to scan the tasklist for nodes that are
> allowed to allocate on current's set of nodes and kill the task with the
> highest badness() score.  This ensures that the most memory-hogging task,
> or the one configured by the user with /proc/pid/oom_adj, is always
> selected in such scenarios.
> 
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>  include/linux/mempolicy.h |   13 +++++++-
>  mm/mempolicy.c            |   39 +++++++++++++++++++++++
>  mm/oom_kill.c             |   77 +++++++++++++++++++++++++++-----------------
>  3 files changed, 98 insertions(+), 31 deletions(-)
> 
> diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
> --- a/include/linux/mempolicy.h
> +++ b/include/linux/mempolicy.h
> @@ -202,6 +202,8 @@ extern struct zonelist *huge_zonelist(struct vm_area_struct *vma,
>  				unsigned long addr, gfp_t gfp_flags,
>  				struct mempolicy **mpol, nodemask_t **nodemask);
>  extern bool init_nodemask_of_mempolicy(nodemask_t *mask);
> +extern bool mempolicy_nodemask_intersects(struct task_struct *tsk,
> +				const nodemask_t *mask);
>  extern unsigned slab_node(struct mempolicy *policy);
>  
>  extern enum zone_type policy_zone;
> @@ -329,7 +331,16 @@ static inline struct zonelist *huge_zonelist(struct vm_area_struct *vma,
>  	return node_zonelist(0, gfp_flags);
>  }
>  
> -static inline bool init_nodemask_of_mempolicy(nodemask_t *m) { return false; }
> +static inline bool init_nodemask_of_mempolicy(nodemask_t *m)
> +{
> +	return false;
> +}
> +
> +static inline bool mempolicy_nodemask_intersects(struct task_struct *tsk,
> +			const nodemask_t *mask)
> +{
> +	return false;
> +}
>  
>  static inline int do_migrate_pages(struct mm_struct *mm,
>  			const nodemask_t *from_nodes,
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1638,6 +1638,45 @@ bool init_nodemask_of_mempolicy(nodemask_t *mask)
>  }
>  #endif
>  
> +/*
> + * mempolicy_nodemask_intersects
> + *
> + * If tsk's mempolicy is "default" [NULL], return 'true' to indicate default
> + * policy.  Otherwise, check for intersection between mask and the policy
> + * nodemask for 'bind' or 'interleave' policy, or mask to contain the single
> + * node for 'preferred' or 'local' policy.
> + */
> +bool mempolicy_nodemask_intersects(struct task_struct *tsk,
> +					const nodemask_t *mask)
> +{
> +	struct mempolicy *mempolicy;
> +	bool ret = true;
> +
> +	mempolicy = tsk->mempolicy;
> +	mpol_get(mempolicy);

Why is this refcount increment necessary? mempolicy is grabbed by tsk,
IOW it never be freed in this function.


> +	if (!mask || !mempolicy)
> +		goto out;
> +
> +	switch (mempolicy->mode) {
> +	case MPOL_PREFERRED:
> +		if (mempolicy->flags & MPOL_F_LOCAL)
> +			ret = node_isset(numa_node_id(), *mask);

Um? Is this good heuristic?
The task can migrate various cpus, then "node_isset(numa_node_id(), *mask) == 0"
doesn't mean the task doesn't consume *mask's memory.


> +		else
> +			ret = node_isset(mempolicy->v.preferred_node,
> +					 *mask);
> +		break;
> +	case MPOL_BIND:
> +	case MPOL_INTERLEAVE:
> +		ret = nodes_intersects(mempolicy->v.nodes, *mask);
> +		break;
> +	default:
> +		BUG();
> +	}
> +out:
> +	mpol_put(mempolicy);
> +	return ret;
> +}
> +
>  /* Allocate a page in interleaved policy.
>     Own path because it needs to do special accounting. */
>  static struct page *alloc_page_interleave(gfp_t gfp, unsigned order,
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -26,6 +26,7 @@
>  #include <linux/module.h>
>  #include <linux/notifier.h>
>  #include <linux/memcontrol.h>
> +#include <linux/mempolicy.h>
>  #include <linux/security.h>
>  
>  int sysctl_panic_on_oom;
> @@ -36,19 +37,35 @@ static DEFINE_SPINLOCK(zone_scan_lock);
>  
>  /*
>   * Do all threads of the target process overlap our allowed nodes?
> + * @tsk: task struct of which task to consider
> + * @mask: nodemask passed to page allocator for mempolicy ooms
>   */
> -static int has_intersects_mems_allowed(struct task_struct *tsk)
> +static bool has_intersects_mems_allowed(struct task_struct *tsk,
> +						const nodemask_t *mask)
>  {
> -	struct task_struct *t;
> +	struct task_struct *start = tsk;
>  
> -	t = tsk;
>  	do {
> -		if (cpuset_mems_allowed_intersects(current, t))
> -			return 1;
> -		t = next_thread(t);
> -	} while (t != tsk);
> -
> -	return 0;
> +		if (mask) {
> +			/*
> +			 * If this is a mempolicy constrained oom, tsk's
> +			 * cpuset is irrelevant.  Only return true if its
> +			 * mempolicy intersects current, otherwise it may be
> +			 * needlessly killed.
> +			 */
> +			if (mempolicy_nodemask_intersects(tsk, mask))
> +				return true;
> +		} else {
> +			/*
> +			 * This is not a mempolicy constrained oom, so only
> +			 * check the mems of tsk's cpuset.
> +			 */
> +			if (cpuset_mems_allowed_intersects(current, tsk))
> +				return true;
> +		}
> +		tsk = next_thread(tsk);
> +	} while (tsk != start);
> +	return false;
>  }
>  
>  /**
> @@ -236,7 +253,8 @@ static enum oom_constraint constrained_alloc(struct zonelist *zonelist,
>   * (not docbooked, we don't want this one cluttering up the manual)
>   */
>  static struct task_struct *select_bad_process(unsigned long *ppoints,
> -						struct mem_cgroup *mem)
> +		struct mem_cgroup *mem, enum oom_constraint constraint,
> +		const nodemask_t *mask)
>  {
>  	struct task_struct *p;
>  	struct task_struct *chosen = NULL;
> @@ -258,7 +276,9 @@ static struct task_struct *select_bad_process(unsigned long *ppoints,
>  			continue;
>  		if (mem && !task_in_mem_cgroup(p, mem))
>  			continue;
> -		if (!has_intersects_mems_allowed(p))
> +		if (!has_intersects_mems_allowed(p,
> +				constraint == CONSTRAINT_MEMORY_POLICY ? mask :
> +									 NULL))
>  			continue;
>  
>  		/*
> @@ -478,7 +498,7 @@ void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask)
>  
>  	read_lock(&tasklist_lock);
>  retry:
> -	p = select_bad_process(&points, mem);
> +	p = select_bad_process(&points, mem, CONSTRAINT_NONE, NULL);
>  	if (PTR_ERR(p) == -1UL)
>  		goto out;
>  
> @@ -560,7 +580,8 @@ void clear_zonelist_oom(struct zonelist *zonelist, gfp_t gfp_mask)
>  /*
>   * Must be called with tasklist_lock held for read.
>   */
> -static void __out_of_memory(gfp_t gfp_mask, int order)
> +static void __out_of_memory(gfp_t gfp_mask, int order,
> +			enum oom_constraint constraint, const nodemask_t *mask)
>  {
>  	struct task_struct *p;
>  	unsigned long points;
> @@ -574,7 +595,7 @@ retry:
>  	 * Rambo mode: Shoot down a process and hope it solves whatever
>  	 * issues we may have.
>  	 */
> -	p = select_bad_process(&points, NULL);
> +	p = select_bad_process(&points, NULL, constraint, mask);
>  
>  	if (PTR_ERR(p) == -1UL)
>  		return;
> @@ -615,7 +636,8 @@ void pagefault_out_of_memory(void)
>  		panic("out of memory from page fault. panic_on_oom is selected.\n");
>  
>  	read_lock(&tasklist_lock);
> -	__out_of_memory(0, 0); /* unknown gfp_mask and order */
> +	/* unknown gfp_mask and order */
> +	__out_of_memory(0, 0, CONSTRAINT_NONE, NULL);
>  	read_unlock(&tasklist_lock);
>  
>  	/*
> @@ -632,6 +654,7 @@ rest_and_return:
>   * @zonelist: zonelist pointer
>   * @gfp_mask: memory allocation flags
>   * @order: amount of memory being requested as a power of 2
> + * @nodemask: nodemask passed to page allocator
>   *
>   * If we run out of memory, we have the choice between either
>   * killing a random task (bad), letting the system crash (worse)
> @@ -660,24 +683,18 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
>  	 */
>  	constraint = constrained_alloc(zonelist, gfp_mask, nodemask);
>  	read_lock(&tasklist_lock);
> -
> -	switch (constraint) {
> -	case CONSTRAINT_MEMORY_POLICY:
> -		oom_kill_process(current, gfp_mask, order, 0, NULL,
> -				"No available memory (MPOL_BIND)");
> -		break;
> -
> -	case CONSTRAINT_NONE:
> -		if (sysctl_panic_on_oom) {
> +	if (unlikely(sysctl_panic_on_oom)) {
> +		/*
> +		 * panic_on_oom only affects CONSTRAINT_NONE, the kernel
> +		 * should not panic for cpuset or mempolicy induced memory
> +		 * failures.
> +		 */
> +		if (constraint == CONSTRAINT_NONE) {
>  			dump_header(NULL, gfp_mask, order, NULL);
> -			panic("out of memory. panic_on_oom is selected\n");
> +			panic("Out of memory: panic_on_oom is enabled\n");

enabled? Its feature is enabled at boot time. triggered? or fired?


>  		}
> -		/* Fall-through */
> -	case CONSTRAINT_CPUSET:
> -		__out_of_memory(gfp_mask, order);
> -		break;
>  	}
> -
> +	__out_of_memory(gfp_mask, order, constraint, nodemask);
>  	read_unlock(&tasklist_lock);
>  
>  	/*
> 
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