Re: [RFC 0/7] Add support to process rx packets in thread

[Hillf Danton <hdanton@sina.com>]

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.

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@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/ath10k


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