Re: [PATCH net-next v11 3/3] net: add sysfs attribute to control napi threaded mode

This patch adds a new sysfs attribute to the network device class.
Said attribute provides a per-device control to enable/disable the
threaded mode for all the napi instances of the given network device,
without the need for a device up/down.
User sets it to 1 or 0 to enable or disable threaded mode.
Note: when switching between threaded and the current softirq based mode
for a napi instance, it will not immediately take effect if the napi is
currently being polled. The mode switch will happen for the next time
napi_schedule() is called.

Co-developed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Co-developed-by: Hannes Frederic Sowa <redacted>
Signed-off-by: Hannes Frederic Sowa <redacted>
Co-developed-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Wei Wang <redacted>
---
 Documentation/ABI/testing/sysfs-class-net | 15 +++++++
 include/linux/netdevice.h                 |  2 +
 net/core/dev.c                            | 48 ++++++++++++++++++++++-
 net/core/net-sysfs.c                      | 40 +++++++++++++++++++
 4 files changed, 103 insertions(+), 2 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-class-net b/Documentation/ABI/testing/sysfs-class-net
index 1f2002df5ba2..1419103d11f9 100644
--- a/Documentation/ABI/testing/sysfs-class-net
+++ b/Documentation/ABI/testing/sysfs-class-net

@@ -337,3 +337,18 @@ Contact:   netdev@vger.kernel.org
 Description:
                32-bit unsigned integer counting the number of times the link has
                been down
+
+What:          /sys/class/net/<iface>/threaded
+Date:          Jan 2021
+KernelVersion: 5.12
+Contact:       netdev@vger.kernel.org
+Description:
+               Boolean value to control the threaded mode per device. User could
+               set this value to enable/disable threaded mode for all napi
+               belonging to this device, without the need to do device up/down.
+
+               Possible values:
+               == ==================================
+               0  threaded mode disabled for this dev
+               1  threaded mode enabled for this dev
+               == ==================================

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 99fb4ec9573e..1340327f7abf 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h

@@ -497,6 +497,8 @@ static inline bool napi_complete(struct napi_struct *n)
        return napi_complete_done(n, 0);
 }

+int dev_set_threaded(struct net_device *dev, bool threaded);
+
 /**
  *     napi_disable - prevent NAPI from scheduling
  *     @n: NAPI context

diff --git a/net/core/dev.c b/net/core/dev.c
index 1e35f4f44f3b..7647278e46f0 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -4291,8 +4291,9 @@ static inline void ____napi_schedule(struct softnet_data *sd,

        if (test_bit(NAPI_STATE_THREADED, &napi->state)) {
                /* Paired with smp_mb__before_atomic() in
-                * napi_enable(). Use READ_ONCE() to guarantee
-                * a complete read on napi->thread. Only call
+                * napi_enable()/dev_set_threaded().
+                * Use READ_ONCE() to guarantee a complete
+                * read on napi->thread. Only call
                 * wake_up_process() when it's not NULL.
                 */
                thread = READ_ONCE(napi->thread);

@@ -6738,6 +6739,49 @@ static void init_gro_hash(struct napi_struct *napi)
        napi->gro_bitmask = 0;
 }

+int dev_set_threaded(struct net_device *dev, bool threaded)
+{
+       struct napi_struct *napi;
+       int err = 0;
+
+       if (dev->threaded == threaded)
+               return 0;
+
+       if (threaded) {
+               list_for_each_entry(napi, &dev->napi_list, dev_list) {
+                       if (!napi->thread) {
+                               err = napi_kthread_create(napi);
+                               if (err) {
+                                       threaded = false;
+                                       break;
+                               }
+                       }
+               }
+       }
+

+       dev->threaded = threaded;
+
+       /* Make sure kthread is created before THREADED bit
+        * is set.
+        */
+       smp_mb__before_atomic();
+
+       /* Setting/unsetting threaded mode on a napi might not immediately
+        * take effect, if the current napi instance is actively being
+        * polled. In this case, the switch between threaded mode and
+        * softirq mode will happen in the next round of napi_schedule().
+        * This should not cause hiccups/stalls to the live traffic.
+        */
+       list_for_each_entry(napi, &dev->napi_list, dev_list) {
+               if (threaded)
+                       set_bit(NAPI_STATE_THREADED, &napi->state);
+               else
+                       clear_bit(NAPI_STATE_THREADED, &napi->state);
+       }
+
+       return err;
+}
+
 void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
                    int (*poll)(struct napi_struct *, int), int weight)
 {

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index daf502c13d6d..e72d474c2623 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c

@@ -538,6 +538,45 @@ static ssize_t phys_switch_id_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(phys_switch_id);

+static ssize_t threaded_show(struct device *dev,
+                            struct device_attribute *attr, char *buf)
+{
+       struct net_device *netdev = to_net_dev(dev);
+       ssize_t ret = -EINVAL;
+
+       if (!rtnl_trylock())
+               return restart_syscall();
+
+       if (dev_isalive(netdev))
+               ret = sprintf(buf, fmt_dec, netdev->threaded);
+
+       rtnl_unlock();
+       return ret;
+}
+
+static int modify_napi_threaded(struct net_device *dev, unsigned long val)
+{
+       int ret;
+
+       if (list_empty(&dev->napi_list))
+               return -EOPNOTSUPP;
+
+       if (val != 0 && val != 1)
+               return -EOPNOTSUPP;
+
+       ret = dev_set_threaded(dev, val);
+
+       return ret;
+}
+
+static ssize_t threaded_store(struct device *dev,
+                             struct device_attribute *attr,
+                             const char *buf, size_t len)
+{
+       return netdev_store(dev, attr, buf, len, modify_napi_threaded);
+}
+static DEVICE_ATTR_RW(threaded);
+
 static struct attribute *net_class_attrs[] __ro_after_init = {
        &dev_attr_netdev_group.attr,
        &dev_attr_type.attr,

@@ -570,6 +609,7 @@ static struct attribute *net_class_attrs[] __ro_after_init = {
        &dev_attr_proto_down.attr,
        &dev_attr_carrier_up_count.attr,
        &dev_attr_carrier_down_count.attr,
+       &dev_attr_threaded.attr,
        NULL,
 };
 ATTRIBUTE_GROUPS(net_class);