[v4,1/3] platform/chrome: cros_ec_spi: Move to real time priority for transfers
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Message ID 20190515164814.258898-2-dianders@chromium.org
State New
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Series
  • spi: A better solution for cros_ec_spi reliability
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Commit Message

Doug Anderson May 15, 2019, 4:48 p.m. UTC
In commit 37a186225a0c ("platform/chrome: cros_ec_spi: Transfer
messages at high priority") we moved transfers to a high priority
workqueue.  This helped make them much more reliable.

...but, we still saw failures.

We were actually finding ourselves competing for time with dm-crypt
which also scheduled work on HIGHPRI workqueues.  While we can
consider reverting the change that made dm-crypt run its work at
HIGHPRI, the argument in commit a1b89132dc4f ("dm crypt: use
WQ_HIGHPRI for the IO and crypt workqueues") is somewhat compelling.
It does make sense for IO to be scheduled at a priority that's higher
than the default user priority.  It also turns out that dm-crypt isn't
alone in using high priority like this.  loop_prepare_queue() does
something similar for loopback devices.

Looking in more detail, it can be seen that the high priority
workqueue isn't actually that high of a priority.  It runs at MIN_NICE
which is _fairly_ high priority but still below all real time
priority.

Should we move cros_ec_spi to real time priority to fix our problems,
or is this just escalating a priority war?  I'll argue here that
cros_ec_spi _does_ belong at real time priority.  Specifically
cros_ec_spi actually needs to run quickly for correctness.  As I
understand this is exactly what real time priority is for.

There currently doesn't appear to be any way to use the standard
workqueue APIs with a real time priority, so we'll switch over to
using using a kthread worker.  We'll match the priority that the SPI
core uses when it wants to do things on a realtime thread and just use
"MAX_RT_PRIO - 1".

This commit plus the patch ("platform/chrome: cros_ec_spi: Request the
SPI thread be realtime") are enough to get communications very close
to 100% reliable (the only known problem left is when serial console
is turned on, which isn't something that happens in shipping devices).
Specifically this test case now passes (tested on rk3288-veyron-jerry):

  dd if=/dev/zero of=/var/log/foo.txt bs=4M count=512&
  while true; do
    ectool version > /dev/null;
  done

It should be noted that "/var/log" is encrypted (and goes through
dm-crypt) and also passes through a loopback device.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v4:
- No needless init of err in cros_ec_spi_devm_high_pri_alloc (Guenter).
- Removed blank lines near #includes (Guenter).
- Switch to kthread_create_worker() and fix error handling (Guenter).
- Cleaner devm code (Guenter).

Changes in v3:
- cros_ec realtime patch replaces revert; now patch #1

Changes in v2: None

 drivers/platform/chrome/cros_ec_spi.c | 65 ++++++++++++++++++++++-----
 1 file changed, 53 insertions(+), 12 deletions(-)

Comments

Guenter Roeck May 15, 2019, 5:02 p.m. UTC | #1
On Wed, May 15, 2019 at 9:48 AM Douglas Anderson <dianders@chromium.org> wrote:
>
> In commit 37a186225a0c ("platform/chrome: cros_ec_spi: Transfer
> messages at high priority") we moved transfers to a high priority
> workqueue.  This helped make them much more reliable.
>
> ...but, we still saw failures.
>
> We were actually finding ourselves competing for time with dm-crypt
> which also scheduled work on HIGHPRI workqueues.  While we can
> consider reverting the change that made dm-crypt run its work at
> HIGHPRI, the argument in commit a1b89132dc4f ("dm crypt: use
> WQ_HIGHPRI for the IO and crypt workqueues") is somewhat compelling.
> It does make sense for IO to be scheduled at a priority that's higher
> than the default user priority.  It also turns out that dm-crypt isn't
> alone in using high priority like this.  loop_prepare_queue() does
> something similar for loopback devices.
>
> Looking in more detail, it can be seen that the high priority
> workqueue isn't actually that high of a priority.  It runs at MIN_NICE
> which is _fairly_ high priority but still below all real time
> priority.
>
> Should we move cros_ec_spi to real time priority to fix our problems,
> or is this just escalating a priority war?  I'll argue here that
> cros_ec_spi _does_ belong at real time priority.  Specifically
> cros_ec_spi actually needs to run quickly for correctness.  As I
> understand this is exactly what real time priority is for.
>
> There currently doesn't appear to be any way to use the standard
> workqueue APIs with a real time priority, so we'll switch over to
> using using a kthread worker.  We'll match the priority that the SPI
> core uses when it wants to do things on a realtime thread and just use
> "MAX_RT_PRIO - 1".
>
> This commit plus the patch ("platform/chrome: cros_ec_spi: Request the
> SPI thread be realtime") are enough to get communications very close
> to 100% reliable (the only known problem left is when serial console
> is turned on, which isn't something that happens in shipping devices).
> Specifically this test case now passes (tested on rk3288-veyron-jerry):
>
>   dd if=/dev/zero of=/var/log/foo.txt bs=4M count=512&
>   while true; do
>     ectool version > /dev/null;
>   done
>
> It should be noted that "/var/log" is encrypted (and goes through
> dm-crypt) and also passes through a loopback device.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Enric Balletbo i Serra May 21, 2019, 7:52 a.m. UTC | #2
Hi,

On 15/5/19 19:02, Guenter Roeck wrote:
> On Wed, May 15, 2019 at 9:48 AM Douglas Anderson <dianders@chromium.org> wrote:
>>
>> In commit 37a186225a0c ("platform/chrome: cros_ec_spi: Transfer
>> messages at high priority") we moved transfers to a high priority
>> workqueue.  This helped make them much more reliable.
>>
>> ...but, we still saw failures.
>>
>> We were actually finding ourselves competing for time with dm-crypt
>> which also scheduled work on HIGHPRI workqueues.  While we can
>> consider reverting the change that made dm-crypt run its work at
>> HIGHPRI, the argument in commit a1b89132dc4f ("dm crypt: use
>> WQ_HIGHPRI for the IO and crypt workqueues") is somewhat compelling.
>> It does make sense for IO to be scheduled at a priority that's higher
>> than the default user priority.  It also turns out that dm-crypt isn't
>> alone in using high priority like this.  loop_prepare_queue() does
>> something similar for loopback devices.
>>
>> Looking in more detail, it can be seen that the high priority
>> workqueue isn't actually that high of a priority.  It runs at MIN_NICE
>> which is _fairly_ high priority but still below all real time
>> priority.
>>
>> Should we move cros_ec_spi to real time priority to fix our problems,
>> or is this just escalating a priority war?  I'll argue here that
>> cros_ec_spi _does_ belong at real time priority.  Specifically
>> cros_ec_spi actually needs to run quickly for correctness.  As I
>> understand this is exactly what real time priority is for.
>>
>> There currently doesn't appear to be any way to use the standard
>> workqueue APIs with a real time priority, so we'll switch over to
>> using using a kthread worker.  We'll match the priority that the SPI
>> core uses when it wants to do things on a realtime thread and just use
>> "MAX_RT_PRIO - 1".
>>
>> This commit plus the patch ("platform/chrome: cros_ec_spi: Request the
>> SPI thread be realtime") are enough to get communications very close
>> to 100% reliable (the only known problem left is when serial console
>> is turned on, which isn't something that happens in shipping devices).
>> Specifically this test case now passes (tested on rk3288-veyron-jerry):
>>
>>   dd if=/dev/zero of=/var/log/foo.txt bs=4M count=512&
>>   while true; do
>>     ectool version > /dev/null;
>>   done
>>
>> It should be noted that "/var/log" is encrypted (and goes through
>> dm-crypt) and also passes through a loopback device.
>>
>> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> 
> Reviewed-by: Guenter Roeck <groeck@chromium.org>
> 

Added to the for-next branch for the autobuilders to play with, if all goes well
will be queued in chrome-platform-5.3

Thanks,
 Enric

Patch
diff mbox series

diff --git a/drivers/platform/chrome/cros_ec_spi.c b/drivers/platform/chrome/cros_ec_spi.c
index 8e9451720e73..1e38a885c539 100644
--- a/drivers/platform/chrome/cros_ec_spi.c
+++ b/drivers/platform/chrome/cros_ec_spi.c
@@ -12,7 +12,7 @@ 
 #include <linux/platform_device.h>
 #include <linux/slab.h>
 #include <linux/spi/spi.h>
-
+#include <uapi/linux/sched/types.h>
 
 /* The header byte, which follows the preamble */
 #define EC_MSG_HEADER			0xec
@@ -67,12 +67,14 @@ 
  *      is sent when we want to turn on CS at the start of a transaction.
  * @end_of_msg_delay: used to set the delay_usecs on the spi_transfer that
  *      is sent when we want to turn off CS at the end of a transaction.
+ * @high_pri_worker: Used to schedule high priority work.
  */
 struct cros_ec_spi {
 	struct spi_device *spi;
 	s64 last_transfer_ns;
 	unsigned int start_of_msg_delay;
 	unsigned int end_of_msg_delay;
+	struct kthread_worker *high_pri_worker;
 };
 
 typedef int (*cros_ec_xfer_fn_t) (struct cros_ec_device *ec_dev,
@@ -89,7 +91,7 @@  typedef int (*cros_ec_xfer_fn_t) (struct cros_ec_device *ec_dev,
  */
 
 struct cros_ec_xfer_work_params {
-	struct work_struct work;
+	struct kthread_work work;
 	cros_ec_xfer_fn_t fn;
 	struct cros_ec_device *ec_dev;
 	struct cros_ec_command *ec_msg;
@@ -632,7 +634,7 @@  static int do_cros_ec_cmd_xfer_spi(struct cros_ec_device *ec_dev,
 	return ret;
 }
 
-static void cros_ec_xfer_high_pri_work(struct work_struct *work)
+static void cros_ec_xfer_high_pri_work(struct kthread_work *work)
 {
 	struct cros_ec_xfer_work_params *params;
 
@@ -644,12 +646,14 @@  static int cros_ec_xfer_high_pri(struct cros_ec_device *ec_dev,
 				 struct cros_ec_command *ec_msg,
 				 cros_ec_xfer_fn_t fn)
 {
-	struct cros_ec_xfer_work_params params;
-
-	INIT_WORK_ONSTACK(&params.work, cros_ec_xfer_high_pri_work);
-	params.ec_dev = ec_dev;
-	params.ec_msg = ec_msg;
-	params.fn = fn;
+	struct cros_ec_spi *ec_spi = ec_dev->priv;
+	struct cros_ec_xfer_work_params params = {
+		.work = KTHREAD_WORK_INIT(params.work,
+					  cros_ec_xfer_high_pri_work),
+		.ec_dev = ec_dev,
+		.ec_msg = ec_msg,
+		.fn = fn,
+	};
 
 	/*
 	 * This looks a bit ridiculous.  Why do the work on a
@@ -660,9 +664,8 @@  static int cros_ec_xfer_high_pri(struct cros_ec_device *ec_dev,
 	 * context switched out for too long and the EC giving up on
 	 * the transfer.
 	 */
-	queue_work(system_highpri_wq, &params.work);
-	flush_work(&params.work);
-	destroy_work_on_stack(&params.work);
+	kthread_queue_work(ec_spi->high_pri_worker, &params.work);
+	kthread_flush_work(&params.work);
 
 	return params.ret;
 }
@@ -694,6 +697,40 @@  static void cros_ec_spi_dt_probe(struct cros_ec_spi *ec_spi, struct device *dev)
 		ec_spi->end_of_msg_delay = val;
 }
 
+static void cros_ec_spi_high_pri_release(void *worker)
+{
+	kthread_destroy_worker(worker);
+}
+
+static int cros_ec_spi_devm_high_pri_alloc(struct device *dev,
+					   struct cros_ec_spi *ec_spi)
+{
+	struct sched_param sched_priority = {
+		.sched_priority = MAX_RT_PRIO - 1,
+	};
+	int err;
+
+	ec_spi->high_pri_worker =
+		kthread_create_worker(0, "cros_ec_spi_high_pri");
+
+	if (IS_ERR(ec_spi->high_pri_worker)) {
+		err = PTR_ERR(ec_spi->high_pri_worker);
+		dev_err(dev, "Can't create cros_ec high pri worker: %d\n", err);
+		return err;
+	}
+
+	err = devm_add_action_or_reset(dev, cros_ec_spi_high_pri_release,
+				       ec_spi->high_pri_worker);
+	if (err)
+		return err;
+
+	err = sched_setscheduler_nocheck(ec_spi->high_pri_worker->task,
+					 SCHED_FIFO, &sched_priority);
+	if (err)
+		dev_err(dev, "Can't set cros_ec high pri priority: %d\n", err);
+	return err;
+}
+
 static int cros_ec_spi_probe(struct spi_device *spi)
 {
 	struct device *dev = &spi->dev;
@@ -732,6 +769,10 @@  static int cros_ec_spi_probe(struct spi_device *spi)
 
 	ec_spi->last_transfer_ns = ktime_get_ns();
 
+	err = cros_ec_spi_devm_high_pri_alloc(dev, ec_spi);
+	if (err)
+		return err;
+
 	err = cros_ec_register(ec_dev);
 	if (err) {
 		dev_err(dev, "cannot register EC\n");