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Message-ID: <5eb18175-b5b9-6e4e-2541-7533e21ccb21@gmail.com>
Date: Thu, 30 Jul 2020 09:08:55 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: Sebastian Gottschall <s.gottschall@...wrt.com>,
Eric Dumazet <eric.dumazet@...il.com>,
Felix Fietkau <nbd@....name>, netdev@...r.kernel.org
Cc: Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH] net: add support for threaded NAPI polling
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@...a.com>
>>> Signed-off-by: Felix Fietkau <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()
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