Messages in this thread | | | Date | Thu, 29 Apr 2010 11:03:21 -0700 (PDT) | From | David Rientjes <> | Subject | Re: [PATCH 1/2] mm: fix bugs of mpol_rebind_nodemask() |
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On Thu, 29 Apr 2010, Miao Xie wrote:
> > That's been the behavior for at least three years so changing it from > > under the applications isn't acceptable, see > > Documentation/vm/numa_memory_policy.txt regarding mempolicy rebinds and > > the two flags that are defined that can be used to adjust the behavior. > > Is the flags what you said MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES? > But the codes that I changed isn't under MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES. > The documentation doesn't say what we should do if either of these two flags is not set. >
MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES allow you to adjust the behavior of the rebind: the former requires specific nodes to be assigned to the mempolicy and could suppress the rebind completely, if necessary; the latter ensures the mempolicy nodemask has a certain weight as nodes are assigned in a round-robin manner. The behavior that you're referring to is provided via MPOL_F_RELATIVE_NODES, which guarantees whatever weight is passed via set_mempolicy() will be preserved when mems are added to a cpuset.
Regardless of whether the behavior is documented when either flag is passed, we can't change the long-standing default behavior that people use when their cpuset mems are rebound: we can only extend the functionality and the behavior you're seeking is already available with a MPOL_F_RELATIVE_NODES flag modifier.
> Furthermore, in order to fix no node to alloc memory, when we want to update mempolicy > and mems_allowed, we expand the set of nodes first (set all the newly nodes) and > shrink the set of nodes lazily(clean disallowed nodes).
That's a cpuset implementation choice, not a mempolicy one; mempolicies have nothing to do with an empty current->mems_allowed.
> But remap() breaks the expanding, so if we don't remove remap(), the problem can't be > fixed. Otherwise, cpuset has to do the rebinding by itself and the code is ugly. > Like this: > > static void cpuset_change_task_nodemask(struct task_struct *tsk, nodemask_t *newmems) > { > nodemask_t tmp; > ... > /* expand the set of nodes */ > if (!mpol_store_user_nodemask(tsk->mempolicy)) { > nodes_remap(tmp, ...); > nodes_or(tsk->mempolicy->v.nodes, tsk->mempolicy->v.nodes, tmp); > } > ... > > /* shrink the set of nodes */ > if (!mpol_store_user_nodemask(tsk->mempolicy)) > tsk->mempolicy->v.nodes = tmp; > } >
I don't see why this is even necessary, the mempolicy code could simply return numa_node_id() when nodes_empty(current->mempolicy->v.nodes) to close the race.
[ Your pseudo-code is also lacking task_lock(tsk), which is required to safely dereference tsk->mempolicy, and this is only available so far in -mm since the oom killer rewrite. ]
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