Hi,
When looking at perf data for my aio-stress test runs against some
relatively fast storage, I noticed that __aio_get_req was taking up more
CPU than I expected. The command line for aio-stress was:
aio-stress -O -r 8 -d 256 -b 32 -l -s 16384 /dev/sdk
which means we're sending 32 iocbs to io_submit at a time. Here's the
perf report -g output:
Events: 41K cycles
- 7.33% aio-stress [kernel.kallsyms] [k] _raw_spin_lock_irq
- _raw_spin_lock_irq
+ 75.38% scsi_request_fn
- 10.67% __aio_get_req
io_submit_one
The spinlock is the ctx_lock, taken when allocating the request. This
seemed like pretty low-hanging fruit to me, so I cooked up a patch to
allocate 32 kiocbs at a time. This is the result:
Events: 50K cycles
- 5.54% aio-stress [kernel.kallsyms] [k] _raw_spin_lock_irq
- _raw_spin_lock_irq
+ 83.23% scsi_request_fn
- 5.14% io_submit_one
So, aio-stress now takes up ~2% less CPU. This seems like a worth-while
thing to do, but I don't have a system handy that would show any real
performance gains from it. The higher the number of iocbs passed to
io_submit, the more of a win this will be.
FWIW, I tested this with multiple different batch sizes (1, 32, 33, 64,
256) and it didn't blow up.
Comments?
Cheers,
Jeff
Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
diff --git a/fs/aio.c b/fs/aio.c
index e29ec48..ae6066f 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))
@@ -459,40 +457,107 @@ 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;
+}
+
+#define KIOCB_STASH_SIZE 32
+struct kiocb_stash {
+ unsigned head;
+ unsigned tail;
+ unsigned total;
+ unsigned used;
+ struct kiocb *kiocbs[KIOCB_STASH_SIZE];
+};
+static void kiocb_stash_init(struct kiocb_stash *stash, int nr)
+{
+ stash->head = stash->tail = 0;
+ stash->total = nr;
+ stash->used = 0;
+}
+static void kiocb_stash_free(struct kiocb_stash *stash, int used)
+{
+ int i;
+
+ for (i = used; i < stash->total; i++)
+ kmem_cache_free(kiocb_cachep, stash->kiocbs[i]);
+}
+static inline unsigned kiocb_stash_avail(struct kiocb_stash *stash)
+{
+ return stash->tail - stash->head;
+}
+
+/*
+ * Allocate nr kiocbs. This aovids taking and dropping the lock a
+ * lot during setup.
+ */
+static int kiocb_stash_refill(struct kioctx *ctx, struct kiocb_stash *stash)
+{
+ int i, nr = KIOCB_STASH_SIZE;
+ int avail;
+ int called_fput = 0;
+ struct aio_ring *ring;
+
+ /*
+ * Allocate the requests up-front. Later, we'll trim the list
+ * if there isn't enough room in the completion ring.
*/
+ stash->head = 0;
+ if ((stash->total - stash->used) % KIOCB_STASH_SIZE)
+ /* don't need to allocate a full batch */
+ nr = stash->total - stash->used;
+
+ for (i = 0; i < nr; i++) {
+ stash->kiocbs[i] = __aio_get_req(ctx);
+ if (!stash->kiocbs[i] && !called_fput) {
+ /* 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.
+ */
+ aio_fput_routine(NULL);
+ called_fput = 1;
+ i--;
+ continue;
+ }
+ }
+ stash->tail = i;
+
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)) {
- list_add(&req->ki_list, &ctx->active_reqs);
+
+ avail = aio_ring_avail(&ctx->ring_info, ring) + ctx->reqs_active;
+ if (avail < stash->tail) {
+ /* Trim back the number of requests. */
+ for (i = avail; i < stash->tail; i++)
+ kmem_cache_free(kiocb_cachep, stash->kiocbs[i]);
+ stash->tail = avail;
+ }
+ for (i = 0; i < stash->tail; i++) {
+ list_add(&stash->kiocbs[i]->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;
- }
-
- return req;
+ if (stash->tail == 0)
+ return -1;
+ return 0;
}
-static inline struct kiocb *aio_get_req(struct kioctx *ctx)
+static inline struct kiocb *aio_get_req(struct kioctx *ctx,
+ struct kiocb_stash *stash)
{
- 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);
+ int ret;
+
+ if (kiocb_stash_avail(stash) == 0) {
+ ret = kiocb_stash_refill(ctx, stash);
+ if (ret)
+ return NULL;
}
- return req;
+ stash->used++;
+ return stash->kiocbs[stash->head++];
}
static inline void really_put_req(struct kioctx *ctx, struct kiocb *req)
@@ -1515,7 +1580,8 @@ 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_stash *stash,
+ bool compat)
{
struct kiocb *req;
struct file *file;
@@ -1541,7 +1607,7 @@ 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, stash); /* returns with 2 references to req */
if (unlikely(!req)) {
fput(file);
return -EAGAIN;
@@ -1621,8 +1687,9 @@ 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_stash kiocbs;
if (unlikely(nr < 0))
return -EINVAL;
@@ -1639,6 +1706,8 @@ long do_io_submit(aio_context_t ctx_id, long nr,
return -EINVAL;
}
+ kiocb_stash_init(&kiocbs, nr);
+
blk_start_plug(&plug);
/*
@@ -1659,12 +1728,13 @@ 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, &kiocbs, compat);
if (ret)
break;
}
blk_finish_plug(&plug);
+ kiocb_stash_free(&kiocbs, i);
put_ioctx(ctx);
return i ? i : ret;
}
--
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