aio: percpu reqs_available

See the previous patch ("aio: reqs_active -> reqs_available") for why we
want to do this - this basically implements a per cpu allocator for
reqs_available that doesn't actually allocate anything.

Note that we need to increase the size of the ringbuffer we allocate,
since a single thread won't necessarily be able to use all the
reqs_available slots - some (up to about half) might be on other per cpu
lists, unavailable for the current thread.

We size the ringbuffer based on the nr_events userspace passed to
io_setup(), so this is a slight behaviour change - but nr_events wasn't
being used as a hard limit before, it was being rounded up to the next
page before so this doesn't change the actual semantics.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
Cc: Zach Brown <zab@redhat.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Asai Thambi S P <asamymuthupa@micron.com>
Cc: Selvan Mani <smani@micron.com>
Cc: Sam Bradshaw <sbradshaw@micron.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Reviewed-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

1 files changed, 99 insertions, 7 deletions

diff --git a/fs/aio.c b/fs/aio.c
index 0e23dfa77b0e..bb1a6c433110 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -26,6 +26,7 @@
 #include <linux/mm.h>
 #include <linux/mman.h>
 #include <linux/mmu_context.h>
+#include <linux/percpu.h>
 #include <linux/slab.h>
 #include <linux/timer.h>
 #include <linux/aio.h>
@@ -64,6 +65,10 @@ struct aio_ring {
 
 #define AIO_RING_PAGES  8
 
+struct kioctx_cpu {
+        unsigned                reqs_available;
+};
+
 struct kioctx {
         atomic_t                users;
         atomic_t                dead;
@@ -72,6 +77,13 @@ struct kioctx {
         unsigned long           user_id;
         struct hlist_node       list;
 
+        struct __percpu kioctx_cpu *cpu;
+
+        /*
+         * For percpu reqs_available, number of slots we move to/from global
+         * counter at a time:
+         */
+        unsigned                req_batch;
         /*
          * This is what userspace passed to io_setup(), it's not used for
          * anything but counting against the global max_reqs quota.
@@ -99,6 +111,8 @@ struct kioctx {
                  * so we avoid overflowing it: it's decremented (if positive)
                  * when allocating a kiocb and incremented when the resulting
                  * io_event is pulled off the ringbuffer.
+                 *
+                 * We batch accesses to it with a percpu version.
                  */
                 atomic_t        reqs_available;
         } ____cacheline_aligned_in_smp;
@@ -379,6 +393,8 @@ static int kiocb_cancel(struct kioctx *ctx, struct kiocb *kiocb,
 static void free_ioctx_rcu(struct rcu_head *head)
 {
         struct kioctx *ctx = container_of(head, struct kioctx, rcu_head);
+
+        free_percpu(ctx->cpu);
         kmem_cache_free(kioctx_cachep, ctx);
 }
 
@@ -392,7 +408,7 @@ static void free_ioctx(struct kioctx *ctx)
         struct aio_ring *ring;
         struct io_event res;
         struct kiocb *req;
-        unsigned head, avail;
+        unsigned cpu, head, avail;
 
         spin_lock_irq(&ctx->ctx_lock);
 
@@ -406,6 +422,13 @@ static void free_ioctx(struct kioctx *ctx)
 
         spin_unlock_irq(&ctx->ctx_lock);
 
+        for_each_possible_cpu(cpu) {
+                struct kioctx_cpu *kcpu = per_cpu_ptr(ctx->cpu, cpu);
+
+                atomic_add(kcpu->reqs_available, &ctx->reqs_available);
+                kcpu->reqs_available = 0;
+        }
+
         ring = kmap_atomic(ctx->ring_pages[0]);
         head = ring->head;
         kunmap_atomic(ring);
@@ -454,6 +477,18 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
         struct kioctx *ctx;
         int err = -ENOMEM;
 
+        /*
+         * We keep track of the number of available ringbuffer slots, to prevent
+         * overflow (reqs_available), and we also use percpu counters for this.
+         *
+         * So since up to half the slots might be on other cpu's percpu counters
+         * and unavailable, double nr_events so userspace sees what they
+         * expected: additionally, we move req_batch slots to/from percpu
+         * counters at a time, so make sure that isn't 0:
+         */
+        nr_events = max(nr_events, num_possible_cpus() * 4);
+        nr_events *= 2;
+
         /* Prevent overflows */
         if ((nr_events > (0x10000000U / sizeof(struct io_event))) ||
             (nr_events > (0x10000000U / sizeof(struct kiocb)))) {
@@ -479,10 +514,16 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
 
         INIT_LIST_HEAD(&ctx->active_reqs);
 
-        if (aio_setup_ring(ctx) < 0)
+        ctx->cpu = alloc_percpu(struct kioctx_cpu);
+        if (!ctx->cpu)
                 goto out_freectx;
 
+        if (aio_setup_ring(ctx) < 0)
+                goto out_freepcpu;
+
         atomic_set(&ctx->reqs_available, ctx->nr_events - 1);
+        ctx->req_batch = (ctx->nr_events - 1) / (num_possible_cpus() * 4);
+        BUG_ON(!ctx->req_batch);
 
         /* limit the number of system wide aios */
         spin_lock(&aio_nr_lock);
@@ -506,6 +547,8 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
 out_cleanup:
         err = -EAGAIN;
         aio_free_ring(ctx);
+out_freepcpu:
+        free_percpu(ctx->cpu);
 out_freectx:
         if (ctx->aio_ring_file)
                 fput(ctx->aio_ring_file);
@@ -610,6 +653,52 @@ void exit_aio(struct mm_struct *mm)
         }
 }
 
+static void put_reqs_available(struct kioctx *ctx, unsigned nr)
+{
+        struct kioctx_cpu *kcpu;
+
+        preempt_disable();
+        kcpu = this_cpu_ptr(ctx->cpu);
+
+        kcpu->reqs_available += nr;
+        while (kcpu->reqs_available >= ctx->req_batch * 2) {
+                kcpu->reqs_available -= ctx->req_batch;
+                atomic_add(ctx->req_batch, &ctx->reqs_available);
+        }
+
+        preempt_enable();
+}
+
+static bool get_reqs_available(struct kioctx *ctx)
+{
+        struct kioctx_cpu *kcpu;
+        bool ret = false;
+
+        preempt_disable();
+        kcpu = this_cpu_ptr(ctx->cpu);
+
+        if (!kcpu->reqs_available) {
+                int old, avail = atomic_read(&ctx->reqs_available);
+
+                do {
+                        if (avail < ctx->req_batch)
+                                goto out;
+
+                        old = avail;
+                        avail = atomic_cmpxchg(&ctx->reqs_available,
+                                               avail, avail - ctx->req_batch);
+                } while (avail != old);
+
+                kcpu->reqs_available += ctx->req_batch;
+        }
+
+        ret = true;
+        kcpu->reqs_available--;
+out:
+        preempt_enable();
+        return ret;
+}
+
 /* aio_get_req
  *      Allocate a slot for an aio request.  Increments the ki_users count
  * of the kioctx so that the kioctx stays around until all requests are
@@ -624,7 +713,7 @@ static inline struct kiocb *aio_get_req(struct kioctx *ctx)
 {
         struct kiocb *req;
 
-        if (atomic_dec_if_positive(&ctx->reqs_available) <= 0)
+        if (!get_reqs_available(ctx))
                 return NULL;
 
         req = kmem_cache_alloc(kiocb_cachep, GFP_KERNEL|__GFP_ZERO);
@@ -633,10 +722,9 @@ static inline struct kiocb *aio_get_req(struct kioctx *ctx)
 
         atomic_set(&req->ki_users, 2);
         req->ki_ctx = ctx;
-
         return req;
 out_put:
-        atomic_inc(&ctx->reqs_available);
+        put_reqs_available(ctx, 1);
         return NULL;
 }
 
@@ -725,6 +813,10 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
          */
         if (unlikely(xchg(&iocb->ki_cancel,
                           KIOCB_CANCELLED) == KIOCB_CANCELLED)) {
+                /*
+                 * Can't use the percpu reqs_available here - could race with
+                 * free_ioctx()
+                 */
                 atomic_inc(&ctx->reqs_available);
                 /* Still need the wake_up in case free_ioctx is waiting */
                 goto put_rq;
@@ -863,7 +955,7 @@ static long aio_read_events_ring(struct kioctx *ctx,
 
         pr_debug("%li  h%u t%u\n", ret, head, ctx->tail);
 
-        atomic_add(ret, &ctx->reqs_available);
+        put_reqs_available(ctx, ret);
 out:
         mutex_unlock(&ctx->ring_lock);
 
@@ -1247,7 +1339,7 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
         aio_put_req(req);       /* drop extra ref to req */
         return 0;
 out_put_req:
-        atomic_inc(&ctx->reqs_available);
+        put_reqs_available(ctx, 1);
         aio_put_req(req);       /* drop extra ref to req */
         aio_put_req(req);       /* drop i/o ref to req */
         return ret;