hi,
In io_file_get() and io_put_file(), currently we use percpu_ref_get() and
percpu_ref_put() for registered files, but it's hard to say they're very
light-weight synchronization primitives, especially in arm platform. In one
our arm machine, I get below perf data(registered files enabled):
Samples: 98K of event 'cycles:ppp', Event count (approx.): 63789396810
Overhead Command Shared Object Symbol
...
0.78% io_uring-sq [kernel.vmlinux] [k] io_file_get
There is an obvious overhead that can not be ignored.
Currently I don't find any good and generic solution for this issue, but
in IOPOLL mode, given that we can always ensure get/put registered files
under uring_lock, we can use a simple and plain u64 counter to synchronize
with registered files update operations in __io_sqe_files_update().
With this patch, perf data show shows:
Samples: 104K of event 'cycles:ppp', Event count (approx.): 67478249890
Overhead Command Shared Object Symbol
...
0.27% io_uring-sq [kernel.vmlinux] [k] io_file_get
The above %0.78 => %0.27 improvements are observed in arm machine with 4.19 kernel.
In upstream mainline codes, since this patch "2b0d3d3e4fcf percpu_ref: reduce memory
footprint of percpu_ref in fast path", I believe the io_file_get's overhead would
be further smaller. I have same tests in same machine, in upstream codes with my patch,
now the io_file_get's overhead is %0.44.
This patch's idea is simple, and now seems it only gives minor performance improvement,
do you have any comments about this patch, should I continue re-send it?
Regards,
Xiaoguang Wang
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>
---
fs/io_uring.c | 58 ++++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 50 insertions(+), 8 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index ce69bd9b0838..186072861af9 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -195,6 +195,11 @@ struct fixed_file_table {
struct fixed_file_ref_node {
struct percpu_ref refs;
+ /*
+ * Track the number of reqs that reference this node, currently it's
+ * only used in IOPOLL mode.
+ */
+ u64 count;
struct list_head node;
struct list_head file_list;
struct fixed_file_data *file_data;
@@ -651,7 +656,10 @@ struct io_kiocb {
*/
struct list_head inflight_entry;
- struct percpu_ref *fixed_file_refs;
+ union {
+ struct percpu_ref *fixed_file_refs;
+ struct fixed_file_ref_node *fixed_file_ref_node;
+ };
struct callback_head task_work;
/* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
struct hlist_node hash_node;
@@ -1544,9 +1552,20 @@ static struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx,
static inline void io_put_file(struct io_kiocb *req, struct file *file,
bool fixed)
{
- if (fixed)
- percpu_ref_put(req->fixed_file_refs);
- else
+ struct io_ring_ctx *ctx = req->ctx;
+
+ if (fixed) {
+ /* See same comments in io_sqe_files_unregister(). */
+ if (ctx->flags & IORING_SETUP_IOPOLL) {
+ struct fixed_file_ref_node *ref_node = req->fixed_file_ref_node;
+ struct percpu_ref *refs = &ref_node->refs;
+
+ ref_node->count--;
+ if ((ctx->file_data->cur_refs != refs) && !ref_node->count)
+ percpu_ref_kill(refs);
+ } else
+ percpu_ref_put(req->fixed_file_refs);
+ } else
fput(file);
}
@@ -5967,8 +5986,21 @@ static int io_file_get(struct io_submit_state *state, struct io_kiocb *req,
fd = array_index_nospec(fd, ctx->nr_user_files);
file = io_file_from_index(ctx, fd);
if (file) {
- req->fixed_file_refs = ctx->file_data->cur_refs;
- percpu_ref_get(req->fixed_file_refs);
+ struct percpu_ref *refs = ctx->file_data->cur_refs;
+
+ /*
+ * In IOPOLL mode, we can always ensure get/put registered files under
+ * uring_lock, so we can use a simple and plain u64 counter to synchronize
+ * with registered files update operations in __io_sqe_files_update.
+ */
+ if (ctx->flags & IORING_SETUP_IOPOLL) {
+ req->fixed_file_ref_node = container_of(refs,
+ struct fixed_file_ref_node, refs);
+ req->fixed_file_ref_node->count++;
+ } else {
+ req->fixed_file_refs = refs;
+ percpu_ref_get(refs);
+ }
}
} else {
trace_io_uring_file_get(ctx, fd);
@@ -6781,7 +6813,12 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
ref_node = list_first_entry(&data->ref_list,
struct fixed_file_ref_node, node);
spin_unlock(&data->lock);
- if (ref_node)
+ /*
+ * If count is not zero, that means we're in IOPOLL mode, and there are
+ * still reqs that reference this ref_node, let the final req do the
+ * percpu_ref_kill job.
+ */
+ if (ref_node && !ref_node->count)
percpu_ref_kill(&ref_node->refs);
percpu_ref_kill(&data->refs);
@@ -7363,7 +7400,12 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
}
if (needs_switch) {
- percpu_ref_kill(data->cur_refs);
+ struct fixed_file_ref_node *old_ref_node = container_of(data->cur_refs,
+ struct fixed_file_ref_node, refs);
+
+ /* See same comments in io_sqe_files_unregister(). */
+ if (!old_ref_node->count)
+ percpu_ref_kill(data->cur_refs);
spin_lock(&data->lock);
list_add(&ref_node->node, &data->ref_list);
data->cur_refs = &ref_node->refs;
