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>

diff --git a/fs/aio.c b/fs/aio.c
index 632b235f4fbe02c237de17fbde271c8bfe6f1240..78c514cfd212d66b8e6311d4a25435be6493c26d 100644 (file)
--- 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 2dcb72bff4b614c44999d4c9d3bf607b3f1b19d0..2314ad8b3c9cced6a4679441d7c6b25afe500348 100644 (file)
--- 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,