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SubjectRe: [PATCH] cpuset: mm: Reduce large amounts of memory barrier related damage v2
On Tue, Mar 06, 2012 at 12:26:57PM -0800, Andrew Morton wrote:
> > <SNIP>
> > --- a/include/linux/cpuset.h
> > +++ b/include/linux/cpuset.h
> > @@ -92,33 +92,19 @@ extern void cpuset_print_task_mems_allowed(struct task_struct *p);
> > * reading current mems_allowed and mempolicy in the fastpath must protected
> > * by get_mems_allowed()
> > */
> > -static inline void get_mems_allowed(void)
> > +static inline unsigned int get_mems_allowed(void)
> > {
> > - current->mems_allowed_change_disable++;
> > -
> > - /*
> > - * ensure that reading mems_allowed and mempolicy happens after the
> > - * update of ->mems_allowed_change_disable.
> > - *
> > - * the write-side task finds ->mems_allowed_change_disable is not 0,
> > - * and knows the read-side task is reading mems_allowed or mempolicy,
> > - * so it will clear old bits lazily.
> > - */
> > - smp_mb();
> > + return read_seqcount_begin(&current->mems_allowed_seq);
> > }
>
> Perhaps we could tickle up the interface documentation? The current
> "documentation" is a grammatical mess and has a typo.
>

There is no guarantee that I will do a better job :) . How about this?

/*
* get_mems_allowed is required when making decisions involving mems_allowed
* such as during page allocation. mems_allowed can be updated in parallel
* and depending on the new value an operation can fail potentially causing
* process failure. A retry loop with get_mems_allowed and put_mems_allowed
* prevents these artificial failures.
*/
static inline unsigned int get_mems_allowed(void)
{
return read_seqcount_begin(&current->mems_allowed_seq);
}
/*
* If this returns false, the operation that took place after get_mems_allowed
* may have failed. It is up to the caller to retry the operation if
* appropriate.
*/
static inline bool put_mems_allowed(unsigned int seq)
{
return !read_seqcount_retry(&current->mems_allowed_seq, seq);
}
?

> > -static inline void put_mems_allowed(void)
> > +/*
> > + * If this returns false, the operation that took place after get_mems_allowed
> > + * may have failed. It is up to the caller to retry the operation if
> > + * appropriate
> > + */
> > +static inline bool put_mems_allowed(unsigned int seq)
> > {
> > - /*
> > - * ensure that reading mems_allowed and mempolicy before reducing
> > - * mems_allowed_change_disable.
> > - *
> > - * the write-side task will know that the read-side task is still
> > - * reading mems_allowed or mempolicy, don't clears old bits in the
> > - * nodemask.
> > - */
> > - smp_mb();
> > - --ACCESS_ONCE(current->mems_allowed_change_disable);
> > + return !read_seqcount_retry(&current->mems_allowed_seq, seq);
> > }
> >
> > static inline void set_mems_allowed(nodemask_t nodemask)
>
> How come set_mems_allowed() still uses task_lock()?
>

Consistency.

The task_lock is taken by kernel/cpuset.c when updating
mems_allowed so it is taken here. That said, it is unnecessary to take
as the two places where set_mems_allowed is used are not going to be
racing. In the unlikely event that set_mems_allowed() gets another user,
there is no harm is leaving the task_lock as it is. It's not in a hot
path of any description.

>
> > @@ -234,12 +220,14 @@ static inline void set_mems_allowed(nodemask_t nodemask)
> > {
> > }
> >
> > -static inline void get_mems_allowed(void)
> > +static inline unsigned int get_mems_allowed(void)
> > {
> > + return 0;
> > }
> >
> > -static inline void put_mems_allowed(void)
> > +static inline bool put_mems_allowed(unsigned int seq)
> > {
> > + return true;
> > }
> >
> > #endif /* !CONFIG_CPUSETS */
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 7d379a6..a0bb87a 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1498,7 +1498,7 @@ struct task_struct {
> > #endif
> > #ifdef CONFIG_CPUSETS
> > nodemask_t mems_allowed; /* Protected by alloc_lock */
> > - int mems_allowed_change_disable;
> > + seqcount_t mems_allowed_seq; /* Seqence no to catch updates */
>
> mems_allowed_seq never gets initialised. That happens to be OK as
> we're never using its spinlock.

Yes.

> But that's sloppy, and adding an
> initialisation to INIT_TASK() is free. But will copying a spinlock by
> value upset lockdep? To be fully anal we should run seqlock_init()
> against each new task_struct.
>

I did not check if lockdep throws a hissy fit but your point that
leaving it uninitialised is sloppy and fixing that is trivial.

> > int cpuset_mem_spread_rotor;
> > int cpuset_slab_spread_rotor;
> > #endif
> >
> > ...
> >
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -498,12 +498,15 @@ struct page *__page_cache_alloc(gfp_t gfp)
> > {
> > int n;
> > struct page *page;
> > + unsigned int cpuset_mems_cookie;
> >
> > if (cpuset_do_page_mem_spread()) {
> > - get_mems_allowed();
> > - n = cpuset_mem_spread_node();
> > - page = alloc_pages_exact_node(n, gfp, 0);
> > - put_mems_allowed();
> > + do {
> > + cpuset_mems_cookie = get_mems_allowed();
> > + n = cpuset_mem_spread_node();
> > + page = alloc_pages_exact_node(n, gfp, 0);
> > + } while (!put_mems_allowed(cpuset_mems_cookie) && !page);
>
> It would be a little tidier to move cpuset_mems_cookie's scope inwards.
>

True.

> > return page;
> > }
> > return alloc_pages(gfp, 0);
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 5f34bd8..5f1e959 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -460,8 +460,10 @@ static struct page *dequeue_huge_page_vma(struct hstate *h,
> > struct zonelist *zonelist;
> > struct zone *zone;
> > struct zoneref *z;
> > + unsigned int cpuset_mems_cookie;
> >
> > - get_mems_allowed();
> > +retry_cpuset:
> > + cpuset_mems_cookie = get_mems_allowed();
> > zonelist = huge_zonelist(vma, address,
> > htlb_alloc_mask, &mpol, &nodemask);
> > /*
> > @@ -490,7 +492,8 @@ static struct page *dequeue_huge_page_vma(struct hstate *h,
> > }
> > err:
> > mpol_cond_put(mpol);
> > - put_mems_allowed();
> > + if (unlikely(!put_mems_allowed(cpuset_mems_cookie) && !page))
> > + goto retry_cpuset;
> > return page;
> > }
>
> We didn't really want to retry the allocation if dequeue_huge_page_vma() has
> made one of its "goto err" decisions.
>

Very good point, thanks. Fixed.

> >
> > ...
> >
> > @@ -2416,9 +2417,19 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
> > page = __alloc_pages_slowpath(gfp_mask, order,
> > zonelist, high_zoneidx, nodemask,
> > preferred_zone, migratetype);
> > - put_mems_allowed();
> >
> > trace_mm_page_alloc(page, order, gfp_mask, migratetype);
> > +
> > +out:
> > + /*
> > + * When updating a tasks mems_allowed, it is possible to race with
>
> "task's"
>

Fixed

> > + * parallel threads in such a way that an allocation can fail while
> > + * the mask is being updated. If a page allocation is about to fail,
> > + * check if the cpuset changed during allocation and if so, retry.
> > + */
> > + if (unlikely(!put_mems_allowed(cpuset_mems_cookie) && !page))
> > + goto retry_cpuset;
> > +
> > return page;
> > }
> > EXPORT_SYMBOL(__alloc_pages_nodemask);
> >
> > ...
> >
> > @@ -3312,11 +3310,14 @@ static void *fallback_alloc(struct kmem_cache *cache, gfp_t flags)
> > enum zone_type high_zoneidx = gfp_zone(flags);
> > void *obj = NULL;
> > int nid;
> > + unsigned int cpuset_mems_cookie;
> >
> > if (flags & __GFP_THISNODE)
> > return NULL;
> >
> > - get_mems_allowed();
> > +retry_cpuset:
> > + cpuset_mems_cookie = get_mems_allowed();
> > +
> > zonelist = node_zonelist(slab_node(current->mempolicy), flags);
> > local_flags = flags & (GFP_CONSTRAINT_MASK|GFP_RECLAIM_MASK);
> >
> > @@ -3372,7 +3373,9 @@ retry:
> > }
> > }
> > }
> > - put_mems_allowed();
> > +
> > + if (unlikely(!put_mems_allowed(cpuset_mems_cookie) && !obj))
> > + goto retry_cpuset;
>
> We recalculate `zonelist' and `local_flags' each time around the loop.
> The former is probably unnecessary and the latter is surely so. I'd
> expect gcc to fix the `local_flags' one.
>

It's not at all obvious but zonelist needs to be recalculated. In
slab_node, we access nodemask information that can be changed if the
cpuset nodemask is altered and the retry loop needs the new information.
I moved the local_flags one outside the retry loop anyway.

> > return obj;
> > }
> >
> > ...
> >
> > @@ -1604,23 +1605,24 @@ static struct page *get_any_partial(struct kmem_cache *s, gfp_t flags,
> > get_cycles() % 1024 > s->remote_node_defrag_ratio)
> > return NULL;
> >
> > - get_mems_allowed();
> > - zonelist = node_zonelist(slab_node(current->mempolicy), flags);
> > - for_each_zone_zonelist(zone, z, zonelist, high_zoneidx) {
> > - struct kmem_cache_node *n;
> > -
> > - n = get_node(s, zone_to_nid(zone));
> > -
> > - if (n && cpuset_zone_allowed_hardwall(zone, flags) &&
> > - n->nr_partial > s->min_partial) {
> > - object = get_partial_node(s, n, c);
> > - if (object) {
> > - put_mems_allowed();
> > - return object;
> > + do {
> > + cpuset_mems_cookie = get_mems_allowed();
> > + zonelist = node_zonelist(slab_node(current->mempolicy), flags);
> > + for_each_zone_zonelist(zone, z, zonelist, high_zoneidx) {
> > + struct kmem_cache_node *n;
> > +
> > + n = get_node(s, zone_to_nid(zone));
> > +
> > + if (n && cpuset_zone_allowed_hardwall(zone, flags) &&
> > + n->nr_partial > s->min_partial) {
> > + object = get_partial_node(s, n, c);
> > + if (object) {
> > + put_mems_allowed(cpuset_mems_cookie);
> > + return object;
>
> Confused. If put_mems_allowed() returned false, doesn't that mean the
> result is unstable and we should retry? Needs a comment explaining
> what's going on?
>

There is a race between the allocator and the cpuset being updated. If
the cpuset is being updated but the allocation succeeded, I decided to
return the object as if the collision had never occurred. The
alternative was to free the object again and retry which seemed
completely unnecessary.

I added a comment.

> > + }
> > }
> > }
> > - }
> > - put_mems_allowed();
> > + } while (!put_mems_allowed(cpuset_mems_cookie));
> > #endif
> > return NULL;
> > }
> >
> > ...
> >
>

--
Mel Gorman
SUSE Labs


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