Re: [PATCH liburing 0/3] __io_uring_get_cqe() fix/optimization

[Jens Axboe <axboe@kernel.dk>]

On 5/19/20 3:52 PM, Bijan Mottahedeh wrote:
> This patch set and a corresponding kernel patch set are fixes and
> optimizations resulting from running unit test 500f9fbadef8-test.
> 
> - Patch 1 is a fix to the test hanging when it runs on a non-mq queue.
> 
> The patch preserves the value of wait_nr if SETUP_IOPOLL is set
> since otherwise __sys_io_uring_enter() could never be called
> __io_uring_peek_cqe() could never find new completions.
> 
> With this patch applied, two problems were hit in the kernel as described
> in the kernel patch set, which caused 500f9fbadef8-test to fail and
> to hang.  With all three patches, 500f9fbadef8-test either passes
> successfully or skips the test gracefully with the following message:
> 
> Polling not supported in current dir, test skipped
> 
> - Patch 2 is an optimization for io_uring_enter() system calls.
> 
> If we want to wait for completions (wait_nr > 0), account for the
> completion we might fetch with __io_uring_peek_cqe().  For example,
> with wait_nr=1 and submit=0, there is no need to call io_uring_enter()
> if the peek call finds a completion.
> 
> Below are the perf results for 500f9fbadef8-test without/with the fix:
> 
> perf stat -e syscalls:sys_enter_io_uring_enter 500f9fbadef8-test
> 
> 12,289     syscalls:sys_enter_io_uring_enter
> 8,193      syscalls:sys_enter_io_uring_enter
> 
> - Patch 3 is a cleanup with no functional changes.
> 
> Since we always have
> 
> io_uring_wait_cqe_nr()
> -> __io_uring_get_cqe()
>    -> __io_uring_peek_cqe()
> 
> remove the direct call from io_uring_wait_cqe_nr() to __io_uring_peek_cqe().
> 
> After the removal, __io_uring_peek_cqe() is called only from
> __io_uring_get_cqe() so move the two routines together(). Without the
> move, compilation fails with a 'defined but not used' error.

LGTM, thanks! I've applied them.

-- 
Jens Axboe