On Wed, Mar 07, 2012 at 05:15:28PM +0800, Miao Xie wrote:
> On Tue, 6 Mar 2012 13:27:35 +0000, Mel Gorman wrote:
> [skip]
> > @@ -964,7 +964,6 @@ static void cpuset_change_task_nodemask(struct task_struct *tsk,
> > {
> > bool need_loop;
> >
> > -repeat:
> > /*
> > * Allow tasks that have access to memory reserves because they have
> > * been OOM killed to get memory anywhere.
> > @@ -983,45 +982,19 @@ repeat:
> > */
> > need_loop = task_has_mempolicy(tsk) ||
> > !nodes_intersects(*newmems, tsk->mems_allowed);
> > - nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems);
> > - mpol_rebind_task(tsk, newmems, MPOL_REBIND_STEP1);
> >
> > - /*
> > - * ensure checking ->mems_allowed_change_disable after setting all new
> > - * allowed nodes.
> > - *
> > - * the read-side task can see an nodemask with new allowed nodes and
> > - * old allowed nodes. and if it allocates page when cpuset clears newly
> > - * disallowed ones continuous, it can see the new allowed bits.
> > - *
> > - * And if setting all new allowed nodes is after the checking, setting
> > - * all new allowed nodes and clearing newly disallowed ones will be done
> > - * continuous, and the read-side task may find no node to alloc page.
> > - */
> > - smp_mb();
> > + if (need_loop)
> > + write_seqcount_begin(&tsk->mems_allowed_seq);
> >
> > - /*
> > - * Allocation of memory is very fast, we needn't sleep when waiting
> > - * for the read-side.
> > - */
> > - while (need_loop && ACCESS_ONCE(tsk->mems_allowed_change_disable)) {
> > - task_unlock(tsk);
> > - if (!task_curr(tsk))
> > - yield();
> > - goto repeat;
> > - }
> > -
> > - /*
> > - * ensure checking ->mems_allowed_change_disable before clearing all new
> > - * disallowed nodes.
> > - *
> > - * if clearing newly disallowed bits before the checking, the read-side
> > - * task may find no node to alloc page.
> > - */
> > - smp_mb();
> > + nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems);
> > + mpol_rebind_task(tsk, newmems, MPOL_REBIND_STEP1);
> >
> > mpol_rebind_task(tsk, newmems, MPOL_REBIND_STEP2);
> > tsk->mems_allowed = *newmems;
> > +
> > + if (need_loop)
> > + write_seqcount_end(&tsk->mems_allowed_seq);
> > +
> > task_unlock(tsk);
> > }
>
Thanks for reviewing this.
> With this patch, we needn't break the nodemask update into two steps.
>
Good point. At a glance it's sufficiently complex to warrent its own patch.
> Beside that, we need deal with fork() carefully, or it is possible that the child
> task will be set to a wrong nodemask.
>
Can you clarify this statement please? It's not clear what the old code
did that protected against problems in fork() versus this patch. fork is
not calling get_mems_allowed() for example or doing anything special
around mems_allowed.
Maybe you are talking about an existing problem whereby during fork
there should be get_mems_allowed/put_mems_allowed and the mems_allowed
mask gets copied explicitly?
--
Mel Gorman
SUSE Labs
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