aio: allocate kiocbs in batches

In testing aio on a fast storage device, I found that the context lock
takes up a fair amount of cpu time in the I/O submission path.  The reason
is that we take it for every I/O submitted (see __aio_get_req).  Since we
know how many I/Os are passed to io_submit, we can preallocate the kiocbs
in batches, reducing the number of times we take and release the lock.

In my testing, I was able to reduce the amount of time spent in
_raw_spin_lock_irq by .56% (average of 3 runs).  The command I used to
test this was:

   aio-stress -O -o 2 -o 3 -r 8 -d 128 -b 32 -i 32 -s 16384 <dev>

I also tested the patch with various numbers of events passed to
io_submit, and I ran the xfstests aio group of tests to ensure I didn't
break anything.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Daniel Ehrenberg <dehrenberg@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 108 insertions, 29 deletions

diff --git a/fs/aio.c b/fs/aio.c
index 632b235f4fbe..78c514cfd212 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -440,8 +440,6 @@ void exit_aio(struct mm_struct *mm)
 static struct kiocb *__aio_get_req(struct kioctx *ctx)
 {
         struct kiocb *req = NULL;
-        struct aio_ring *ring;
-        int okay = 0;
 
         req = kmem_cache_alloc(kiocb_cachep, GFP_KERNEL);
         if (unlikely(!req))
@@ -459,39 +457,114 @@ static struct kiocb *__aio_get_req(struct kioctx *ctx)
         INIT_LIST_HEAD(&req->ki_run_list);
         req->ki_eventfd = NULL;
 
-        /* Check if the completion queue has enough free space to
-         * accept an event from this io.
-         */
+        return req;
+}
+
+/*
+ * struct kiocb's are allocated in batches to reduce the number of
+ * times the ctx lock is acquired and released.
+ */
+#define KIOCB_BATCH_SIZE        32L
+struct kiocb_batch {
+        struct list_head head;
+        long count; /* number of requests left to allocate */
+};
+
+static void kiocb_batch_init(struct kiocb_batch *batch, long total)
+{
+        INIT_LIST_HEAD(&batch->head);
+        batch->count = total;
+}
+
+static void kiocb_batch_free(struct kiocb_batch *batch)
+{
+        struct kiocb *req, *n;
+
+        list_for_each_entry_safe(req, n, &batch->head, ki_batch) {
+                list_del(&req->ki_batch);
+                kmem_cache_free(kiocb_cachep, req);
+        }
+}
+
+/*
+ * Allocate a batch of kiocbs.  This avoids taking and dropping the
+ * context lock a lot during setup.
+ */
+static int kiocb_batch_refill(struct kioctx *ctx, struct kiocb_batch *batch)
+{
+        unsigned short allocated, to_alloc;
+        long avail;
+        bool called_fput = false;
+        struct kiocb *req, *n;
+        struct aio_ring *ring;
+
+        to_alloc = min(batch->count, KIOCB_BATCH_SIZE);
+        for (allocated = 0; allocated < to_alloc; allocated++) {
+                req = __aio_get_req(ctx);
+                if (!req)
+                        /* allocation failed, go with what we've got */
+                        break;
+                list_add(&req->ki_batch, &batch->head);
+        }
+
+        if (allocated == 0)
+                goto out;
+
+retry:
         spin_lock_irq(&ctx->ctx_lock);
-        ring = kmap_atomic(ctx->ring_info.ring_pages[0], KM_USER0);
-        if (ctx->reqs_active < aio_ring_avail(&ctx->ring_info, ring)) {
+        ring = kmap_atomic(ctx->ring_info.ring_pages[0]);
+
+        avail = aio_ring_avail(&ctx->ring_info, ring) - ctx->reqs_active;
+        BUG_ON(avail < 0);
+        if (avail == 0 && !called_fput) {
+                /*
+                 * Handle a potential starvation case.  It is possible that
+                 * we hold the last reference on a struct file, causing us
+                 * to delay the final fput to non-irq context.  In this case,
+                 * ctx->reqs_active is artificially high.  Calling the fput
+                 * routine here may free up a slot in the event completion
+                 * ring, allowing this allocation to succeed.
+                 */
+                kunmap_atomic(ring);
+                spin_unlock_irq(&ctx->ctx_lock);
+                aio_fput_routine(NULL);
+                called_fput = true;
+                goto retry;
+        }
+
+        if (avail < allocated) {
+                /* Trim back the number of requests. */
+                list_for_each_entry_safe(req, n, &batch->head, ki_batch) {
+                        list_del(&req->ki_batch);
+                        kmem_cache_free(kiocb_cachep, req);
+                        if (--allocated <= avail)
+                                break;
+                }
+        }
+
+        batch->count -= allocated;
+        list_for_each_entry(req, &batch->head, ki_batch) {
                 list_add(&req->ki_list, &ctx->active_reqs);
                 ctx->reqs_active++;
-                okay = 1;
         }
-        kunmap_atomic(ring, KM_USER0);
-        spin_unlock_irq(&ctx->ctx_lock);
 
-        if (!okay) {
-                kmem_cache_free(kiocb_cachep, req);
-                req = NULL;
-        }
+        kunmap_atomic(ring);
+        spin_unlock_irq(&ctx->ctx_lock);
 
-        return req;
+out:
+        return allocated;
 }
 
-static inline struct kiocb *aio_get_req(struct kioctx *ctx)
+static inline struct kiocb *aio_get_req(struct kioctx *ctx,
+                                        struct kiocb_batch *batch)
 {
         struct kiocb *req;
-        /* Handle a potential starvation case -- should be exceedingly rare as 
-         * requests will be stuck on fput_head only if the aio_fput_routine is 
-         * delayed and the requests were the last user of the struct file.
-         */
-        req = __aio_get_req(ctx);
-        if (unlikely(NULL == req)) {
-                aio_fput_routine(NULL);
-                req = __aio_get_req(ctx);
-        }
+
+        if (list_empty(&batch->head))
+                if (kiocb_batch_refill(ctx, batch) == 0)
+                        return NULL;
+        req = list_first_entry(&batch->head, struct kiocb, ki_batch);
+        list_del(&req->ki_batch);
         return req;
 }
 
@@ -1515,7 +1588,8 @@ static ssize_t aio_setup_iocb(struct kiocb *kiocb, bool compat)
 }
 
 static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
-                         struct iocb *iocb, bool compat)
+                         struct iocb *iocb, struct kiocb_batch *batch,
+                         bool compat)
 {
         struct kiocb *req;
         struct file *file;
@@ -1541,7 +1615,7 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
         if (unlikely(!file))
                 return -EBADF;
 
-        req = aio_get_req(ctx);         /* returns with 2 references to req */
+        req = aio_get_req(ctx, batch);  /* returns with 2 references to req */
         if (unlikely(!req)) {
                 fput(file);
                 return -EAGAIN;
@@ -1621,8 +1695,9 @@ long do_io_submit(aio_context_t ctx_id, long nr,
 {
         struct kioctx *ctx;
         long ret = 0;
-        int i;
+        int i = 0;
         struct blk_plug plug;
+        struct kiocb_batch batch;
 
         if (unlikely(nr < 0))
                 return -EINVAL;
@@ -1639,6 +1714,8 @@ long do_io_submit(aio_context_t ctx_id, long nr,
                 return -EINVAL;
         }
 
+        kiocb_batch_init(&batch, nr);
+
         blk_start_plug(&plug);
 
         /*
@@ -1659,12 +1736,13 @@ long do_io_submit(aio_context_t ctx_id, long nr,
                         break;
                 }
 
-                ret = io_submit_one(ctx, user_iocb, &tmp, compat);
+                ret = io_submit_one(ctx, user_iocb, &tmp, &batch, compat);
                 if (ret)
                         break;
         }
         blk_finish_plug(&plug);
 
+        kiocb_batch_free(&batch);
         put_ioctx(ctx);
         return i ? i : ret;
 }
diff --git a/include/linux/aio.h b/include/linux/aio.h
index 2dcb72bff4b6..2314ad8b3c9c 100644
--- a/include/linux/aio.h
+++ b/include/linux/aio.h
@@ -117,6 +117,7 @@ struct kiocb {
 
         struct list_head        ki_list;        /* the aio core uses this
                                                  * for cancellation */
+        struct list_head        ki_batch;       /* batch allocation */
 
         /*
          * If the aio_resfd field of the userspace iocb is not zero,