Re: [PATCH v1] io_uring: reserve word at cqring tail+4 for the user

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On 17/09/2019 17.54, Jens Axboe wrote:
On 9/17/19 3:13 AM, Avi Kivity wrote:
In some applications, a thread waits for I/O events generated by
the kernel, and also events generated by other threads in the same
application. Typically events from other threads are passed using
in-memory queues that are not known to the kernel. As long as the
threads is active, it polls for both kernel completions and
inter-thread completions; when it is idle, it tells the other threads
to use an I/O event to wait it up (e.g. an eventfd or a pipe) and
then enters the kernel, waiting for such an event or an ordinary
I/O completion.

When such a thread goes idle, it typically spins for a while to
avoid the kernel entry/exit cost in case an event is forthcoming
shortly. While it spins it polls both I/O completions and
inter-thread queues.

The x86 instruction pair UMONITOR/UMWAIT allows waiting for a cache
line to be written to. This can be used with io_uring to wait for a
wakeup without spinning (and wasting power and slowing down the other
hyperthread). Other threads can also wake up the waiter by doing a
safe write to the tail word (which triggers the wakeup), but safe
writes are slow as they require an atomic instruction. To speed up
those wakeups, reserve a word after the tail for user writes.

A thread consuming an io_uring completion queue can then use the
following sequences:

    - while busy:
      - pick up work from the completion queue and from other threads,
        and process it

    - while idle:
      - use UMONITOR/UMWAIT to wait on completions and notifications
        from other threads for a short period
      - if no work is picked up, let other threads know you will need
        a kernel wakeup, and use io_uring_enter to wait indefinitely
This is cool, I like it. A few comments:

diff --git a/fs/io_uring.c b/fs/io_uring.c
index cfb48bd088e1..4bd7905cee1d 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -77,12 +77,13 @@
#define IORING_MAX_ENTRIES 4096
   #define IORING_MAX_FIXED_FILES	1024
struct io_uring {
-	u32 head ____cacheline_aligned_in_smp;
-	u32 tail ____cacheline_aligned_in_smp;
+	u32 head ____cacheline_aligned;
+	u32 tail ____cacheline_aligned;
+	u32 reserved_for_user; // for cq ring and UMONITOR/UMWAIT (or similar) wakeups
   };
Since we have that full cacheline, maybe name this one a bit more
appropriately as we can add others if we need it. Not a big deal.


You mean, name it for its intended purpose of serving as a write target for umonitor/umwait wakes?


Note that the user won't see the name, and that it's only accurate for an io_uring that's used for completions.


But definitely use /* */ style comments :-)


Sorry, in C++-land for a while. You're lucky I didn't turn the whole thing into a virtual template something.



diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 1e1652f25cc1..1a6a826a66f3 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -103,10 +103,14 @@ struct io_sqring_offsets {
    */
   #define IORING_SQ_NEED_WAKEUP	(1U << 0) /* needs io_uring_enter wakeup */
struct io_cqring_offsets {
   	__u32 head;
+	// tail is guaranteed to be aligned on a cache line, and to have the
+	// following __u32 free for user use. This allows using e.g.
+	// UMONITOR/UMWAIT to wait on both writes to head and writes from
+	// other threads to the following word.
   	__u32 tail;
   	__u32 ring_mask;
   	__u32 ring_entries;
   	__u32 overflow;
   	__u32 cqes;
Ditto on the comments here.


Sure.


Would be ideal if we could pair this with an example for liburing, a basic
test case would be fine. Something that shows how to use it, and verifies
that it works.


I'll have to look for a machine with waitpkg for that.


Also, this patch is against master, it should be against for-5.4/io_iuring as
it won't apply there right now.


Sure, will rebase.





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