Re: [PATCH v10 13/35] vmscan: per-node deferred work

Andrew Morton
2013-06-05T23:08:15Z

On Mon, 3 Jun 2013 23:29:42 +0400 Glauber Costa <glommer@openvz.org> wrote:

> We already keep per-node LRU lists for objects being shrunk, but the
> work that is deferred from one run to another is kept global. This
> creates an impedance problem, where upon node pressure, work deferred
> will accumulate and end up being flushed in other nodes.

This changelog would be more useful if it had more specificity. Where
do we keep these per-node LRU lists (names of variables?). Where do we
keep the global data? In what function does this other-node flushing
happen?

Generally so that readers can go and look at the data structures and
functions which you're talking about.

> In large machines, many nodes can accumulate at the same time, all
> adding to the global counter.

What global counter?

> As we accumulate more and more, we start
> to ask for the caches to flush even bigger numbers.

Where does this happen?

> The result is that
> the caches are depleted and do not stabilize. To achieve stable steady
> state behavior, we need to tackle it differently.
>
> In this patch we keep the deferred count per-node, and will never
> accumulate that to other nodes.
>
> ...
>
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -19,6 +19,8 @@ struct shrink_control {
>
> /* shrink from these nodes */
> nodemask_t nodes_to_scan;
> + /* current node being shrunk (for NUMA aware shrinkers) */
> + int nid;
> };
>
> /*
> @@ -42,6 +44,8 @@ struct shrink_control {
> * objects freed during the scan, or -1 if progress cannot be made due to
> * potential deadlocks. If -1 is returned, then no further attempts to call the
> * @scan_objects will be made from the current reclaim context.
> + *
> + * @flags determine the shrinker abilities, like numa awareness
> */
> struct shrinker {
> int (*shrink)(struct shrinker *, struct shrink_control *sc);
> @@ -50,12 +54,34 @@ struct shrinker {
>
> int seeks; /* seeks to recreate an obj */
> long batch; /* reclaim batch size, 0 = default */
> + unsigned long flags;
>
> /* These are for internal use */
> struct list_head list;
> - atomic_long_t nr_in_batch; /* objs pending delete */
> + /*
> + * We would like to avoid allocating memory when registering a new
> + * shrinker.

That's quite surprising. What are the reasons for this?

> All shrinkers will need to keep track of deferred objects,

What is a deferred object and why does this deferral happen?

> + * and we need a counter for this. If the shrinkers are not NUMA aware,
> + * this is a small and bounded space that fits into an atomic_long_t.
> + * This is because that the deferring decisions are global, and we will

s/that//

> + * not allocate in this case.
> + *
> + * When the shrinker is NUMA aware, we will need this to be a per-node
> + * array. Numerically speaking, the minority of shrinkers are NUMA
> + * aware, so this saves quite a bit.
> + */

I don't really understand what's going on here :(

> + union {
> + /* objs pending delete */
> + atomic_long_t nr_deferred;
> + /* objs pending delete, per node */
> + atomic_long_t *nr_deferred_node;
> + };
> };
> #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
> -extern void register_shrinker(struct shrinker *);
> +
> +/* Flags */
> +#define SHRINKER_NUMA_AWARE (1 << 0)
> +
> +extern int register_shrinker(struct shrinker *);
> extern void unregister_shrinker(struct shrinker *);
> #endif
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 53e647f..08eec9d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -155,14 +155,36 @@ static unsigned long get_lru_size(struct lruvec *lruvec, enum lru_list lru)
> }
>
> /*
> - * Add a shrinker callback to be called from the vm
> + * Add a shrinker callback to be called from the vm.
> + *
> + * It cannot fail, unless the flag SHRINKER_NUMA_AWARE is specified.
> + * With this flag set, this function will allocate memory and may fail.
> */

Again, I don't see what the big deal is with memory allocation.
register_shrinker() is pretty rare, is likely to happen when the system
is under little stress and GFP_KERNEL is quite strong. Why all the
concern?

> -void register_shrinker(struct shrinker *shrinker)
> +int register_shrinker(struct shrinker *shrinker)
> {
> - atomic_long_set(&shrinker->nr_in_batch, 0);
> + /*
> + * If we only have one possible node in the system anyway, save
> + * ourselves the trouble and disable NUMA aware behavior. This way we
> + * will allocate nothing and save memory and some small loop time
> + * later.
> + */
> + if (nr_node_ids == 1)
> + shrinker->flags &= ~SHRINKER_NUMA_AWARE;
> +
> + if (shrinker->flags & SHRINKER_NUMA_AWARE) {
> + size_t size;
> +
> + size = sizeof(*shrinker->nr_deferred_node) * nr_node_ids;
> + shrinker->nr_deferred_node = kzalloc(size, GFP_KERNEL);
> + if (!shrinker->nr_deferred_node)
> + return -ENOMEM;
> + } else
> + atomic_long_set(&shrinker->nr_deferred, 0);
> +
> down_write(&shrinker_rwsem);
> list_add_tail(&shrinker->list, &shrinker_list);
> up_write(&shrinker_rwsem);
> + return 0;
> }
> EXPORT_SYMBOL(register_shrinker);

What would be the cost if we were to do away with SHRINKER_NUMA_AWARE
and treat all shrinkers the same way? The need to allocate extra
memory per shrinker? That sounds pretty cheap?

> @@ -186,6 +208,116 @@ static inline int do_shrinker_shrink(struct shrinker *shrinker,
> }
>
> #define SHRINK_BATCH 128
> +
> +static unsigned long
> +shrink_slab_node(struct shrink_control *shrinkctl, struct shrinker *shrinker,
> + unsigned long nr_pages_scanned, unsigned long lru_pages,
> + atomic_long_t *deferred)
> +{
> + unsigned long freed = 0;
> + unsigned long long delta;
> + long total_scan;
> + long max_pass;
> + long nr;
> + long new_nr;
> + long batch_size = shrinker->batch ? shrinker->batch
> + : SHRINK_BATCH;
> +
> + if (shrinker->scan_objects) {
> + max_pass = shrinker->count_objects(shrinker, shrinkctl);
> + WARN_ON(max_pass < 0);
> + } else
> + max_pass = do_shrinker_shrink(shrinker, shrinkctl, 0);
> + if (max_pass <= 0)
> + return 0;
> +
> + /*
> + * copy the current shrinker scan count into a local variable
> + * and zero it so that other concurrent shrinker invocations
> + * don't also do this scanning work.
> + */
> + nr = atomic_long_xchg(deferred, 0);

This comment seems wrong. It implies that "deferred" refers to "the
current shrinker scan count". But how are these two the same thing? A
"scan count" would refer to the number of objects to be scanned (or
which were scanned - it's unclear). Whereas "deferred" would refer to
the number of those to-be-scanned objects which we didn't process and
is hence less than or equal to the "scan count".

It's all very foggy :( This whole concept of deferral needs more
explanation, please.

> + total_scan = nr;
> + delta = (4 * nr_pages_scanned) / shrinker->seeks;
> + delta *= max_pass;
> + do_div(delta, lru_pages + 1);
> + total_scan += delta;
> + if (total_scan < 0) {
> + printk(KERN_ERR
> + "shrink_slab: %pF negative objects to delete nr=%ld\n",
> + shrinker->shrink, total_scan);
> + total_scan = max_pass;
> + }
> +
> + /*
> + * We need to avoid excessive windup on filesystem shrinkers
> + * due to large numbers of GFP_NOFS allocations causing the
> + * shrinkers to return -1 all the time. This results in a large
> + * nr being built up so when a shrink that can do some work
> + * comes along it empties the entire cache due to nr >>>
> + * max_pass. This is bad for sustaining a working set in
> + * memory.
> + *
> + * Hence only allow the shrinker to scan the entire cache when
> + * a large delta change is calculated directly.
> + */

That was an important comment. So the whole problem we're tackling
here is fs shrinkers baling out in GFP_NOFS allocations?

> + if (delta < max_pass / 4)
> + total_scan = min(total_scan, max_pass / 2);
> +
> + /*
> + * Avoid risking looping forever due to too large nr value:
> + * never try to free more than twice the estimate number of

"estimated"

> + * freeable entries.
> + */
> + if (total_scan > max_pass * 2)
> + total_scan = max_pass * 2;
> +
> + trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
> + nr_pages_scanned, lru_pages,
> + max_pass, delta, total_scan);
> +
> + while (total_scan >= batch_size) {
> + long ret;
> +
> + if (shrinker->scan_objects) {
> + shrinkctl->nr_to_scan = batch_size;
> + ret = shrinker->scan_objects(shrinker, shrinkctl);
> +
> + if (ret == -1)
> + break;
> + freed += ret;
> + } else {
> + int nr_before;
> + nr_before = do_shrinker_shrink(shrinker, shrinkctl, 0);
> + ret = do_shrinker_shrink(shrinker, shrinkctl,
> + batch_size);
> + if (ret == -1)
> + break;
> + if (ret < nr_before)
> + freed += nr_before - ret;
> + }
> +
> + count_vm_events(SLABS_SCANNED, batch_size);
> + total_scan -= batch_size;
> +
> + cond_resched();
> + }
> +
> + /*
> + * move the unused scan count back into the shrinker in a
> + * manner that handles concurrent updates. If we exhausted the
> + * scan, there is no need to do an update.
> + */
> + if (total_scan > 0)
> + new_nr = atomic_long_add_return(total_scan, deferred);
> + else
> + new_nr = atomic_long_read(deferred);
> +
> + trace_mm_shrink_slab_end(shrinker, freed, nr, new_nr);
> + return freed;
> +}
>
> ...
>

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