[EXTERNAL] Re: [PATCH net-next v3 3/3] net: ti: icssg-prueth: Add XDP support

[Malladi, Meghana]

On 3/3/2025 6:01 PM, Dan Carpenter wrote:
> On Mon, Mar 03, 2025 at 05: 36: 41PM +0530, Malladi, Meghana wrote: > > 
>  > +static int emac_run_xdp(struct prueth_emac *emac, struct xdp_buff 
> *xdp, > > > + struct page *page) > > > +{ > > > + struct net_device
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> On Mon, Mar 03, 2025 at 05:36:41PM +0530, Malladi, Meghana wrote:
>> > > +static int emac_run_xdp(struct prueth_emac *emac, struct xdp_buff *xdp,
>> > > +			struct page *page)
>> > > +{
>> > > +	struct net_device *ndev = emac->ndev;
>> > > +	int err, result = ICSSG_XDP_PASS;
>> > > +	struct bpf_prog *xdp_prog;
>> > > +	struct xdp_frame *xdpf;
>> > > +	int q_idx;
>> > > +	u32 act;
>> > > +
>> > > +	xdp_prog = READ_ONCE(emac->xdp_prog);
>> > > +	act = bpf_prog_run_xdp(xdp_prog, xdp);
>> > > +	switch (act) {
>> > > +	case XDP_PASS:
>> > > +		break;
>> > > +	case XDP_TX:
>> > > +		/* Send packet to TX ring for immediate transmission */
>> > > +		xdpf = xdp_convert_buff_to_frame(xdp);
>> > > +		if (unlikely(!xdpf))
>> > 
>> > This is the second unlikely() macro which is added in this patchset.
>> > The rule with likely/unlikely() is that it should only be added if it
>> > likely makes a difference in benchmarking.  Quite often the compiler
>> > is able to predict that valid pointers are more likely than NULL
>> > pointers so often these types of annotations don't make any difference
>> > at all to the compiled code.  But it depends on the compiler and the -O2
>> > options.
>> > 
>> 
>> Do correct me if I am wrong, but from my understanding, XDP feature depends
>> alot of performance and benchmarking and having unlikely does make a
>> difference. Atleast in all the other drivers I see this being used for XDP.
>> 
> 
> Which compiler are you on when you say that "having unlikely does make a
> difference"?

I'm on gcc version 10.3.1.

> 
> I'm on gcc version 14.2.0 (Debian 14.2.0-16) and it doesn't make a
> difference to the compiled code.  This matches what one would expect from
> a compiler.  Valid pointers are fast path and NULL pointers are slow path.
> 

Can you tell me how did you verify this? I actually don't know what 
level of optimization to expect from a compiler. I said so, because I 
have checked with other drivers which implemented XDP and everywhere 
unlikely is used. But now I understand its not the driver but the 
compiler that plays the major role in defining the optimization.

> Adding an unlikely() is a micro optimization.  There are so many other
> things you can do to speed up the code.  I wouldn't start with that.
>

Ok, if you believe that unlikely is doing more harm than good, I am ok 
with dropping them off.

> regards,
> dan
>