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Date:   Sat, 25 Jul 2020 16:08:41 +0200
From:   Sebastian Gottschall <s.gottschall@...wrt.com>
To:     Hillf Danton <hdanton@...a.com>,
        David Laight <David.Laight@...LAB.COM>,
        Andrew Lunn <andrew@...n.ch>,
        Rakesh Pillai <pillair@...eaurora.org>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ath10k@...ts.infradead.org" <ath10k@...ts.infradead.org>,
        "dianders@...omium.org" <dianders@...omium.org>,
        Markus Elfring <Markus.Elfring@....de>,
        "evgreen@...omium.org" <evgreen@...omium.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "johannes@...solutions.net" <johannes@...solutions.net>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kvalo@...eaurora.org" <kvalo@...eaurora.org>
Subject: Re: [RFC 0/7] Add support to process rx packets in thread


Am 25.07.2020 um 14:25 schrieb Hillf Danton:
> On Sat, 25 Jul 2020 12:38:00 +0200 Sebastian Gottschall wrote:
>> you may consider this
>>
>> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1142611.html 
>>
>> <https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1142611.html> 
>>
> Thanks very much for your link.
>
>> years ago someone already wanted to bring this feature upstream, but it
>> was denied. i already tested this patch the last 2 days and it worked so
>> far (with some little modifications)
>> so such a solution existed already and may be considered here
> I don't see outstanding difference in principle from Paolo's work in
> 2016 except for the use of kthread_create() and friends because kworker
> made use of them even before 2016. This is a simpler one as shown by
> the diff stat in his cover letter.
i agree. i just can say that i tested this patch recently due this 
discussion here. and it can be changed by sysfs. but it doesnt work for
wifi drivers which are mainly using dummy netdev devices. for this i 
made a small patch to get them working using napi_set_threaded manually 
hardcoded in the drivers. (see patch bellow)
i also tested various networking drivers. one thing i notice doesnt 
work. some napi code is used for tx polling. so from my experience this 
concept just works good for rx with the most drivers.
so far i tested mt76, ath10k and some soc ethernet chipsets with good 
success. on ath10k i had about 10 - 20% performance gain on multicore 
systems. using standard iperf3 with 4 parallel streams.

-5439,7 +5441,7 @@ int napi_set_threaded(struct napi_struct *n, bool
                 clear_bit(NAPI_STATE_THREADED, &n->state);

         /* if the device is initializing, nothing todo */
-       if (test_bit(__LINK_STATE_START, &n->dev->state))
+       if (test_bit(__LINK_STATE_START, &n->dev->state) && 
n->dev->reg_state != NETREG_DUMMY)
                 return 0;

         napi_thread_stop(n)
;


>
> Paolo, feel free to correct me if I misread anything.
>
> Finally I don't see the need to add sysfs attr, given 
> CONFIG_THREADED_NAPI
> in this work.
>
> BTW let us know if anyone has plans to pick up the 2016 RFC.
>
> Hillf
>
> Paolo Abeni (2):
>    net: implement threaded-able napi poll loop support
>    net: add sysfs attribute to control napi threaded mode
>
>   include/linux/netdevice.h |   4 ++
>   net/core/dev.c            | 113 
> ++++++++++++++++++++++++++++++++++++++++++++++
>   net/core/net-sysfs.c      | 102 
> +++++++++++++++++++++++++++++++++++++++++
>   3 files changed, 219 insertions(+)
>> Sebastian
>>
>>
>> someone
>>
>> Am 25.07.2020 um 10:16 schrieb Hillf Danton:
>>> On Wed, 22 Jul 2020 09:12:42 +0000 David Laight wrote:
>>>>> On 21 July 2020 18:25 Andrew Lunn wrote:
>>>>>
>>>>> On Tue, Jul 21, 2020 at 10:44:19PM +0530, Rakesh Pillai wrote:
>>>>>> NAPI gets scheduled on the CPU core which got the
>>>>>> interrupt. The linux scheduler cannot move it to a
>>>>>> different core, even if the CPU on which NAPI is running
>>>>>> is heavily loaded. This can lead to degraded wifi
>>>>>> performance when running traffic at peak data rates.
>>>>>>
>>>>>> A thread on the other hand can be moved to different
>>>>>> CPU cores, if the one on which its running is heavily
>>>>>> loaded. During high incoming data traffic, this gives
>>>>>> better performance, since the thread can be moved to a
>>>>>> less loaded or sometimes even a more powerful CPU core
>>>>>> to account for the required CPU performance in order
>>>>>> to process the incoming packets.
>>>>>>
>>>>>> This patch series adds the support to use a high priority
>>>>>> thread to process the incoming packets, as opposed to
>>>>>> everything being done in NAPI context.
>>>>> I don't see why this problem is limited to the ath10k driver. I 
>>>>> expect
>>>>> it applies to all drivers using NAPI. So shouldn't you be solving 
>>>>> this
>>>>> in the NAPI core? Allow a driver to request the NAPI core uses a
>>>>> thread?
>>>> It's not just NAPI the problem is with the softint processing.
>>>> I suspect a lot of systems would work better if it ran as
>>>> a (highish priority) kernel thread.
>>> Hi folks
>>>
>>> Below is a minimunm poc implementation I can imagine on top of 
>>> workqueue
>>> to make napi threaded. Thoughts are appreciated.
>>>
>>>> I've had to remove the main locks from a multi-threaded application
>>>> and replace them with atomic counters.
>>>> Consider what happens when the threads remove items from a shared
>>>> work list.
>>>> The code looks like:
>>>>     mutex_enter();
>>>>     remove_item_from_list();
>>>>     mutex_exit().
>>>> The mutex is only held for a few instructions, so while you'd expect
>>>> the cache line to be 'hot' you wouldn't get real contention.
>>>> However the following scenarios happen:
>>>> 1) An ethernet interrupt happens while the mutex is held.
>>>>      This stops the other threads until all the softint processing
>>>>      has finished.
>>>> 2) An ethernet interrupt (and softint) runs on a thread that is
>>>>      waiting for the mutex.
>>>>      (Or on the cpu that the thread's processor affinity ties it to.)
>>>>      In this case the 'fair' (ticket) mutex code won't let any other
>>>>      thread acquire the mutex.
>>>>      So again everything stops until the softints all complete.
>>>>
>>>> The second one is also a problem when trying to wake up all
>>>> the threads (eg after adding a lot of items to the list).
>>>> The ticket locks force them to wake in order, but
>>>> sometimes the 'thundering herd' would work better.
>>>>
>>>> IIRC this is actually worse for processes running under the RT
>>>> scheduler (without CONFIG_PREEMPT) because the they are almost
>>>> always scheduled on the same cpu they ran on last.
>>>> If it is busy, but cannot be pre-empted, they are not moved
>>>> to an idle cpu.
>>>>      To confound things there is a very broken workaround for broken
>>>> hardware in the driver for the e1000 interface on (at least)
>>>> Ivy Bridge cpu that can cause the driver to spin for a very
>>>> long time (IIRC milliseconds) whenever it has to write to a
>>>> MAC register (ie on every transmit setup).
>>>>
>>>>     David
>>>>
>>>> -
>>>> Registered Address Lakeside, Bramley Road, Mount Farm, Milton 
>>>> Keynes, MK1 1PT, UK
>>>> Registration No: 1397386 (Wales)
>>>>
>>>>
>>> To make napi threaded, if either irq or softirq thread is entirely 
>>> ruled
>>> out, add napi::work that will be queued on a highpri workqueue. It is
>>> actually a unbound one to facilitate scheduler to catter napi loads 
>>> on to
>>> idle CPU cores. What users need to do with the threaded napi
>>> is s/netif_napi_add/netif_threaded_napi_add/ and no more.
>>>
>>> --- a/include/linux/netdevice.h
>>> +++ b/include/linux/netdevice.h
>>> @@ -338,6 +338,9 @@ struct napi_struct {
>>>        struct list_head    dev_list;
>>>        struct hlist_node    napi_hash_node;
>>>        unsigned int        napi_id;
>>> +#ifdef CONFIG_THREADED_NAPI
>>> +    struct work_struct    work;
>>> +#endif
>>>    };
>>>       enum {
>>> @@ -2234,6 +2237,19 @@ static inline void *netdev_priv(const st
>>>    void netif_napi_add(struct net_device *dev, struct napi_struct 
>>> *napi,
>>>                int (*poll)(struct napi_struct *, int), int weight);
>>>    +#ifdef CONFIG_THREADED_NAPI
>>> +void netif_threaded_napi_add(struct net_device *dev, struct 
>>> napi_struct *napi,
>>> +            int (*poll)(struct napi_struct *, int), int weight);
>>> +#else
>>> +static inline void netif_threaded_napi_add(struct net_device *dev,
>>> +                    struct napi_struct *napi,
>>> +                    int (*poll)(struct napi_struct *, int),
>>> +                    int weight)
>>> +{
>>> +    netif_napi_add(dev, napi, poll, weight);
>>> +}
>>> +#endif
>>> +
>>>    /**
>>>     *    netif_tx_napi_add - initialize a NAPI context
>>>     *    @dev:  network device
>>> --- a/net/core/dev.c
>>> +++ b/net/core/dev.c
>>> @@ -6277,6 +6277,61 @@ static int process_backlog(struct napi_s
>>>        return work;
>>>    }
>>>    +#ifdef CONFIG_THREADED_NAPI
>>> +/* unbound highpri workqueue for threaded napi */
>>> +static struct workqueue_struct *napi_workq;
>>> +
>>> +static void napi_workfn(struct work_struct *work)
>>> +{
>>> +    struct napi_struct *n = container_of(work, struct napi_struct, 
>>> work);
>>> +
>>> +    for (;;) {
>>> +        if (!test_bit(NAPI_STATE_SCHED, &n->state))
>>> +            return;
>>> +
>>> +        if (n->poll(n, n->weight) < n->weight)
>>> +            return;
>>> +
>>> +        if (need_resched()) {
>>> +            /*
>>> +             * have to pay for the latency of task switch even if
>>> +             * napi is scheduled
>>> +             */
>>> +            if (test_bit(NAPI_STATE_SCHED, &n->state))
>>> +                queue_work(napi_workq, work);
>>> +            return;
>>> +        }
>>> +    }
>>> +}
>>> +
>>> +void netif_threaded_napi_add(struct net_device *dev,
>>> +                struct napi_struct *napi,
>>> +                int (*poll)(struct napi_struct *, int),
>>> +                int weight)
>>> +{
>>> +    netif_napi_add(dev, napi, poll, weight);
>>> +    INIT_WORK(&napi->work, napi_workfn);
>>> +}
>>> +
>>> +static inline bool is_threaded_napi(struct napi_struct *n)
>>> +{
>>> +    return n->work.func == napi_workfn;
>>> +}
>>> +
>>> +static inline void threaded_napi_sched(struct napi_struct *n)
>>> +{
>>> +    if (is_threaded_napi(n))
>>> +        queue_work(napi_workq, &n->work);
>>> +    else
>>> +        ____napi_schedule(this_cpu_ptr(&softnet_data), n);
>>> +}
>>> +#else
>>> +static inline void threaded_napi_sched(struct napi_struct *n)
>>> +{
>>> +    ____napi_schedule(this_cpu_ptr(&softnet_data), n);
>>> +}
>>> +#endif
>>> +
>>>    /**
>>>     * __napi_schedule - schedule for receive
>>>     * @n: entry to schedule
>>> @@ -6289,7 +6344,7 @@ void __napi_schedule(struct napi_struct
>>>        unsigned long flags;
>>>           local_irq_save(flags);
>>> -    ____napi_schedule(this_cpu_ptr(&softnet_data), n);
>>> +    threaded_napi_sched(n);
>>>        local_irq_restore(flags);
>>>    }
>>>    EXPORT_SYMBOL(__napi_schedule);
>>> @@ -6335,7 +6390,7 @@ EXPORT_SYMBOL(napi_schedule_prep);
>>>     */
>>>    void __napi_schedule_irqoff(struct napi_struct *n)
>>>    {
>>> -    ____napi_schedule(this_cpu_ptr(&softnet_data), n);
>>> +    threaded_napi_sched(n);
>>>    }
>>>    EXPORT_SYMBOL(__napi_schedule_irqoff);
>>>    @@ -10685,6 +10740,10 @@ static int __init net_dev_init(void)
>>>            sd->backlog.weight = weight_p;
>>>        }
>>>    +#ifdef CONFIG_THREADED_NAPI
>>> +    napi_workq = alloc_workqueue("napi_workq", WQ_UNBOUND | 
>>> WQ_HIGHPRI,
>>> +                        WQ_UNBOUND_MAX_ACTIVE);
>>> +#endif
>>>        dev_boot_phase = 0;
>>>           /* The loopback device is special if any other network 
>>> devices
>>>
>>>
>>> _______________________________________________
>>> ath10k mailing list
>>> ath10k@...ts.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/ath10k
>

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