Re: [PATCH] net: add support for threaded NAPI polling

[Eric Dumazet <eric.dumazet@gmail.com>]

On 7/30/20 10:21 AM, Sebastian Gottschall wrote:
> 
> Am 30.07.2020 um 18:08 schrieb Eric Dumazet:
>>
>> On 7/30/20 7:30 AM, Sebastian Gottschall wrote:
>>> Am 29.07.2020 um 19:44 schrieb Eric Dumazet:
>>>> On 7/29/20 9:50 AM, Felix Fietkau wrote:
>>>>> For some drivers (especially 802.11 drivers), doing a lot of work in the NAPI
>>>>> poll function does not perform well. Since NAPI poll is bound to the CPU it
>>>>> was scheduled from, we can easily end up with a few very busy CPUs spending
>>>>> most of their time in softirq/ksoftirqd and some idle ones.
>>>>>
>>>>> Introduce threaded NAPI for such drivers based on a workqueue. The API is the
>>>>> same except for using netif_threaded_napi_add instead of netif_napi_add.
>>>>>
>>>>> In my tests with mt76 on MT7621 using threaded NAPI + a thread for tx scheduling
>>>>> improves LAN->WLAN bridging throughput by 10-50%. Throughput without threaded
>>>>> NAPI is wildly inconsistent, depending on the CPU that runs the tx scheduling
>>>>> thread.
>>>>>
>>>>> With threaded NAPI, throughput seems stable and consistent (and higher than
>>>>> the best results I got without it).
>>>>>
>>>>> Based on a patch by Hillf Danton
>>>>>
>>>>> Cc: Hillf Danton <hdanton@sina.com>
>>>>> Signed-off-by: Felix Fietkau <nbd@nbd.name>
>>>>> ---
>>>>> Changes since RFC v2:
>>>>> - fix unused but set variable reported by kbuild test robot
>>>>>
>>>>> Changes since RFC:
>>>>> - disable softirq around threaded poll functions
>>>>> - reuse most parts of napi_poll()
>>>>> - fix re-schedule condition
>>>>>
>>>>>    include/linux/netdevice.h |  23 ++++++
>>>>>    net/core/dev.c            | 162 ++++++++++++++++++++++++++------------
>>>>>    2 files changed, 133 insertions(+), 52 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>>>>> index ac2cd3f49aba..3a39211c7598 100644
>>>>> --- a/include/linux/netdevice.h
>>>>> +++ b/include/linux/netdevice.h
>>>>> @@ -347,6 +347,7 @@ struct napi_struct {
>>>>>        struct list_head    dev_list;
>>>>>        struct hlist_node    napi_hash_node;
>>>>>        unsigned int        napi_id;
>>>>> +    struct work_struct    work;
>>>>>    };
>>>>>      enum {
>>>>> @@ -357,6 +358,7 @@ enum {
>>>>>        NAPI_STATE_HASHED,    /* In NAPI hash (busy polling possible) */
>>>>>        NAPI_STATE_NO_BUSY_POLL,/* Do not add in napi_hash, no busy polling */
>>>>>        NAPI_STATE_IN_BUSY_POLL,/* sk_busy_loop() owns this NAPI */
>>>>> +    NAPI_STATE_THREADED,    /* Use threaded NAPI */
>>>>>    };
>>>>>      enum {
>>>>> @@ -367,6 +369,7 @@ enum {
>>>>>        NAPIF_STATE_HASHED     = BIT(NAPI_STATE_HASHED),
>>>>>        NAPIF_STATE_NO_BUSY_POLL = BIT(NAPI_STATE_NO_BUSY_POLL),
>>>>>        NAPIF_STATE_IN_BUSY_POLL = BIT(NAPI_STATE_IN_BUSY_POLL),
>>>>> +    NAPIF_STATE_THREADED     = BIT(NAPI_STATE_THREADED),
>>>>>    };
>>>>>      enum gro_result {
>>>>> @@ -2315,6 +2318,26 @@ static inline void *netdev_priv(const struct net_device *dev)
>>>>>    void netif_napi_add(struct net_device *dev, struct napi_struct *napi,
>>>>>                int (*poll)(struct napi_struct *, int), int weight);
>>>>>    +/**
>>>>> + *    netif_threaded_napi_add - initialize a NAPI context
>>>>> + *    @dev:  network device
>>>>> + *    @napi: NAPI context
>>>>> + *    @poll: polling function
>>>>> + *    @weight: default weight
>>>>> + *
>>>>> + * This variant of netif_napi_add() should be used from drivers using NAPI
>>>>> + * with CPU intensive poll functions.
>>>>> + * This will schedule polling from a high priority workqueue that
>>>>> + */
>>>>> +static inline void netif_threaded_napi_add(struct net_device *dev,
>>>>> +                       struct napi_struct *napi,
>>>>> +                       int (*poll)(struct napi_struct *, int),
>>>>> +                       int weight)
>>>>> +{
>>>>> +    set_bit(NAPI_STATE_THREADED, &napi->state);
>>>>> +    netif_napi_add(dev, napi, poll, weight);
>>>>> +}
>>>>> +
>>>>>    /**
>>>>>     *    netif_tx_napi_add - initialize a NAPI context
>>>>>     *    @dev:  network device
>>>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>>>> index 19f1abc26fcd..11b027f3a2b9 100644
>>>>> --- a/net/core/dev.c
>>>>> +++ b/net/core/dev.c
>>>>> @@ -158,6 +158,7 @@ static DEFINE_SPINLOCK(offload_lock);
>>>>>    struct list_head ptype_base[PTYPE_HASH_SIZE] __read_mostly;
>>>>>    struct list_head ptype_all __read_mostly;    /* Taps */
>>>>>    static struct list_head offload_base __read_mostly;
>>>>> +static struct workqueue_struct *napi_workq __read_mostly;
>>>>>      static int netif_rx_internal(struct sk_buff *skb);
>>>>>    static int call_netdevice_notifiers_info(unsigned long val,
>>>>> @@ -6286,6 +6287,11 @@ void __napi_schedule(struct napi_struct *n)
>>>>>    {
>>>>>        unsigned long flags;
>>>>>    +    if (test_bit(NAPI_STATE_THREADED, &n->state)) {
>>>>> +        queue_work(napi_workq, &n->work);
>>>>> +        return;
>>>>> +    }
>>>>> +
>>>> Where is the corresponding cancel_work_sync() or flush_work() at device dismantle ?
>>>>
>>>> Just hoping the thread will eventually run seems optimistic to me.
>>>>
>>>>
>>>> Quite frankly, I do believe this STATE_THREADED status should be a generic NAPI attribute
>>>> that can be changed dynamically, at admin request, instead of having to change/recompile
>>>> a driver.
>>> thats not that easy. wifi devices do use dummy netdev devices. they are not visible to sysfs and other administrative options.
>>> so changing it would just be possible if a special mac80211 based control would be implemented for these drivers.
>>> for standard netdev devices it isnt a big thing to implement a administrative control by sysfs (if you are talking about such a feature)
>> We do not want to add code in fast path only for one device. We need something truly generic.
>>
>> I am not saying only the admin can chose, it is fine if a driver does not give the choice
>> and will simply call netif_threaded_napi_add()
> what could make sense if the feature can be disabled / enabled, but it will only affect drivers using the netif_threaded_napi_add call, but it should not affect drivers
> using the old api in any way since not all drivers will work with this feature.


If we provide something in core NAPI stack, we want to make sure we can test/use it with other drivers.

ethtool, or a /sys/class/net/ethXXX entry could be used.

The argument about not affecting other drivers is misleading, since the patch adds another conditional test in
standard NAPI layer.

Lets keep NAPI generic please.

Lets make sure syzbot will find bugs without having to attach a specific mac80211 hardware.

Another concern I have with this patch is that we no longer can contain NIC processing is done
on a selected set of cpus (as commanded in /proc/irq/XXX/smp_affinity).
Or can we ?