[PATCH v2 08/10] engines/libblkio: Add option libblkio_force_enable_completion_eventfd

[Alberto Faria <afaria@redhat.com>]

When set, the queue's completion fd is enabled even when it isn't used,
i.e., even if option libblkio_wait_mode is _not_ set to "eventfd".

Depending on the libblkio driver, this can have an impact on
performance. This option allows evaluating that overhead.

Signed-off-by: Alberto Faria <afaria@redhat.com>
---
 HOWTO.rst          |  9 ++++++++-
 engines/libblkio.c | 21 +++++++++++++++++++--
 fio.1              | 10 +++++++++-
 3 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/HOWTO.rst b/HOWTO.rst
index e9602f59..1cea16c4 100644
--- a/HOWTO.rst
+++ b/HOWTO.rst
@@ -2338,7 +2338,8 @@ with the caveat that when used on the command line, they must come after the
    [libblkio]
 
 	Use poll queues. This is incompatible with
-	:option:`libblkio_wait_mode=eventfd <libblkio_wait_mode>`.
+	:option:`libblkio_wait_mode=eventfd <libblkio_wait_mode>` and
+	:option:`libblkio_force_enable_completion_eventfd`.
 
    [pvsync2]
 
@@ -2907,6 +2908,12 @@ with the caveat that when used on the command line, they must come after the
 	**loop**
 		Use a busy loop with a non-blocking call to ``blkioq_do_io()``.
 
+.. option:: libblkio_force_enable_completion_eventfd : [libblkio]
+
+	Enable the queue's completion eventfd even when unused. This may impact
+	performance. The default is to enable it only if
+	:option:`libblkio_wait_mode=eventfd <libblkio_wait_mode>`.
+
 I/O depth
 ~~~~~~~~~
 
diff --git a/engines/libblkio.c b/engines/libblkio.c
index 8a4828c3..7f5dcf36 100644
--- a/engines/libblkio.c
+++ b/engines/libblkio.c
@@ -24,7 +24,7 @@
 struct fio_blkio_data {
 	struct blkio *b;
 	struct blkioq *q;
-	int completion_fd; /* -1 if not FIO_BLKIO_WAIT_MODE_EVENTFD */
+	int completion_fd; /* may be -1 if not FIO_BLKIO_WAIT_MODE_EVENTFD */
 
 	bool has_mem_region; /* whether mem_region is valid */
 	struct blkio_mem_region mem_region; /* only if allocated by libblkio */
@@ -50,6 +50,7 @@ struct fio_blkio_options {
 	unsigned int vectored;
 	unsigned int write_zeroes_on_trim;
 	enum fio_blkio_wait_mode wait_mode;
+	unsigned int force_enable_completion_eventfd;
 };
 
 static struct fio_option options[] = {
@@ -132,6 +133,16 @@ static struct fio_option options[] = {
 		.category = FIO_OPT_C_ENGINE,
 		.group	= FIO_OPT_G_LIBBLKIO,
 	},
+	{
+		.name	= "libblkio_force_enable_completion_eventfd",
+		.lname	= "Force enable the completion eventfd, even if unused",
+		.type	= FIO_OPT_STR_SET,
+		.off1	= offsetof(struct fio_blkio_options,
+				   force_enable_completion_eventfd),
+		.help	= "This can impact performance",
+		.category = FIO_OPT_C_ENGINE,
+		.group	= FIO_OPT_G_LIBBLKIO,
+	},
 	{
 		.name = NULL,
 	},
@@ -276,6 +287,11 @@ static int fio_blkio_setup(struct thread_data *td)
 		return 1;
 	}
 
+	if (options->hipri && options->force_enable_completion_eventfd) {
+		log_err("fio: option hipri is incompatible with option libblkio_force_enable_completion_eventfd\n");
+		return 1;
+	}
+
 	if (fio_blkio_create_and_connect(td, &b) != 0)
 		return 1;
 
@@ -342,7 +358,8 @@ static int fio_blkio_init(struct thread_data *td)
 	else
 		data->q = blkio_get_queue(data->b, 0);
 
-	if (options->wait_mode == FIO_BLKIO_WAIT_MODE_EVENTFD) {
+	if (options->wait_mode == FIO_BLKIO_WAIT_MODE_EVENTFD ||
+		options->force_enable_completion_eventfd) {
 		/* enable completion fd and make it blocking */
 		blkioq_set_completion_fd_enabled(data->q, true);
 		data->completion_fd = blkioq_get_completion_fd(data->q);
diff --git a/fio.1 b/fio.1
index 1a810214..da548303 100644
--- a/fio.1
+++ b/fio.1
@@ -2633,7 +2633,8 @@ properties depend on the libblkio version in use and are listed at
 \fIhttps://libblkio.gitlab.io/libblkio/blkio.html#properties\fR
 .TP
 .BI (libblkio)hipri
-Use poll queues. This is incompatible with \fBlibblkio_wait_mode=eventfd\fR.
+Use poll queues. This is incompatible with \fBlibblkio_wait_mode=eventfd\fR and
+\fBlibblkio_force_enable_completion_eventfd\fR.
 .TP
 .BI (libblkio)libblkio_vectored
 Submit vectored read and write requests.
@@ -2654,6 +2655,13 @@ Use a blocking call to \fBread()\fR on the completion eventfd.
 .TP
 .B loop
 Use a busy loop with a non-blocking call to \fBblkioq_do_io()\fR.
+.RE
+.RE
+.TP
+.BI (libblkio)libblkio_force_enable_completion_eventfd
+Enable the queue's completion eventfd even when unused. This may impact
+performance. The default is to enable it only if
+\fBlibblkio_wait_mode=eventfd\fR.
 .SS "I/O depth"
 .TP
 .BI iodepth \fR=\fPint
-- 
2.38.1