netdev - Re: [PATCH] net/core: Export dev_core_stats_rx_dropped

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Date: Tue, 12 Sep 2023 20:03:54 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Yajun Deng <yajun.deng@...ux.dev>, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, horms@...nel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net/core: Export dev_core_stats_rx_dropped_inc sets

On Tue, Sep 12, 2023 at 7:44 PM Alexander Lobakin
<aleksander.lobakin@...el.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Tue, 12 Sep 2023 19:28:50 +0200
>
> > On Tue, Sep 12, 2023 at 7:16 PM Alexander Lobakin
> > <aleksander.lobakin@...el.com> wrote:
> >>
> >> From: Eric Dumazet <edumazet@...gle.com>
> >> Date: Tue, 12 Sep 2023 18:04:44 +0200
> >>
> >>> On Tue, Sep 12, 2023 at 5:58 PM Alexander Lobakin
> >>> <aleksander.lobakin@...el.com> wrote:
> >>>>
> >>>> From: Eric Dumazet <edumazet@...gle.com>
> >>>> Date: Tue, 12 Sep 2023 06:23:24 +0200
> >>>>
> >>>>> On Mon, Sep 11, 2023 at 10:20 AM Yajun Deng <yajun.deng@...ux.dev> wrote:
> >>>>>>
> >>>>>> Although there is a kfree_skb_reason() helper function that can be used
> >>>>>> to find the reason for dropped packets, but most callers didn't increase
> >>>>>> one of rx_dropped, tx_dropped, rx_nohandler and rx_otherhost_dropped.
> >>>>
> >>>> [...]
> >>>>
> >>>>>>  EXPORT_SYMBOL(netdev_stats_to_stats64);
> >>>>>>
> >>>>>> -struct net_device_core_stats __percpu *netdev_core_stats_alloc(struct net_device *dev)
> >>>>>> +static struct net_device_core_stats __percpu *netdev_core_stats_alloc(struct net_device *dev)
> >>>>>>  {
> >>>>>>         struct net_device_core_stats __percpu *p;
> >>>>>>
> >>>>>> @@ -10488,7 +10488,33 @@ struct net_device_core_stats __percpu *netdev_core_stats_alloc(struct net_device
> >>>>>>         /* This READ_ONCE() pairs with the cmpxchg() above */
> >>>>>>         return READ_ONCE(dev->core_stats);
> >>>>>>  }
> >>>>>> -EXPORT_SYMBOL(netdev_core_stats_alloc);
> >>>>>> +
> >>>>>> +static inline struct net_device_core_stats __percpu *dev_core_stats(struct net_device *dev)
> >>>>>
> >>>>> Please remove this inline attritbute. Consider using __cold instead.
> >>>>
> >>>> __cold? O_o I thought the author's inlining it as it's a couple
> >>>> locs/intstructions, while the compilers would most likely keep it
> >>>> non-inlined as it's referenced 4 times. __cold will for sure keep it
> >>>> standalone and place it in .text.cold, i.e. far away from the call sites.
> >>>> I realize dev_core_stats_*() aren't called frequently, but why making
> >>>> only one small helper cold rather than all of them then?
> >>>>
> >>>
> >>> This helper is used at least one time per netdevice lifetime.
> >>> This is definitely cold.
> >>
> >> But then each dev_stats_*_inc() (not cold) has to call it from a
> >> completely different piece of .text far from their. I either don't
> >> understand the idea or dunno. Why not make them cold as well then?
> >>
> >
> > The __cold attribute is only applied to the helper _allocating_ the
> > memory, once.
>
> Then it should be applied to netdev_core_stats_alloc(), not
> dev_core_stats(). The latter only dereferences the already existing
> pointer or calls the former, which actually does the allocation.
> That's why I don't get why make one if/else non-inline or even cold.

Sure, this was what was suggested (perhaps not _very_ precisely, but
the general idea was pretty clear).
v2 seems ok, right ?

It seems we are all on the same page.

+static __cold struct net_device_core_stats __percpu
*dev_core_stats(struct net_device *dev)
+{
+       /* This READ_ONCE() pairs with the write in netdev_core_stats_alloc() */
+       struct net_device_core_stats __percpu *p = READ_ONCE(dev->core_stats);
+
+       if (likely(p))
+               return p;
+
+       return netdev_core_stats_alloc(dev);
+}
+
+#define DEV_CORE_STATS_INC(FIELD)                              \
+void dev_core_stats_##FIELD##_inc(struct net_device *dev)      \
+{                                                              \
+       struct net_device_core_stats __percpu *p;               \
+                                                               \
+       p = dev_core_stats(dev);                                \
+       if (p)                                                  \
+               this_cpu_inc(p->FIELD);                         \
+}                                                              \
+EXPORT_SYMBOL(dev_core_stats_##FIELD##_inc)