Re: [net-next] tcp: allow drivers to tweak TSQ logic

[Eric Dumazet <eric.dumazet@gmail.com>]

On Sat, 2017-11-11 at 15:27 +0100, Johannes Berg wrote:
> Thanks Eric!
> 
> > We expect wifi drivers to set this field to smaller values (tests have
> > been done with values from 6 to 9)
> 
> I suppose we should test each driver or so.
> 
> > They would have to use following template :
> > 
> > if (skb->sk && skb->sk->sk_pacing_shift != MY_PACING_SHIFT)
> >      skb->sk->sk_pacing_shift = MY_PACING_SHIFT;
> 
> Hm. I wish we wouldn't have to do this on every skb, but perhaps it
> doesn't matter that much.

Yes, it does not matter, even at 40Gbit ;)

> 
> 
> >  	u16			sk_gso_max_segs;
> > +	u8			sk_pacing_shift;
> 
> I guess you tried to fill a hole, but weren't we saying that it would
> be better in the same cacheline? Then again, perhaps both cachelines
> are resident anyway, haven't looked at this now.

Same cache line already ;)

	u32                        sk_pacing_rate;       /* 0x1c0   0x4 */
	u32                        sk_max_pacing_rate;   /* 0x1c4   0x4 */
	struct page_frag           sk_frag;              /* 0x1c8  0x10 */
	netdev_features_t          sk_route_caps;        /* 0x1d8   0x8 */
	netdev_features_t          sk_route_nocaps;      /* 0x1e0   0x8 */
	int                        sk_gso_type;          /* 0x1e8   0x4 */
	unsigned int               sk_gso_max_size;      /* 0x1ec   0x4 */
	gfp_t                      sk_allocation;        /* 0x1f0   0x4 */
	__u32                      sk_txhash;            /* 0x1f4   0x4 */
	unsigned int               __sk_flags_offset[0]; /* 0x1f8     0 */
	unsigned int               sk_padding:1;         /* 0x1f8:0x1f 0x4 */
	unsigned int               sk_kern_sock:1;       /* 0x1f8:0x1e 0x4 */
	unsigned int               sk_no_check_tx:1;     /* 0x1f8:0x1d 0x4 */
	unsigned int               sk_no_check_rx:1;     /* 0x1f8:0x1c 0x4 */
	unsigned int               sk_userlocks:4;       /* 0x1f8:0x18 0x4 */
	unsigned int               sk_protocol:8;        /* 0x1f8:0x10 0x4 */
	unsigned int               sk_type:16;           /* 0x1f8: 0 0x4 */
	u16                        sk_gso_max_segs;      /* 0x1fc   0x2 */
	u8			   sk_pacing_shift;	 /* 0x1fe   0x1 */






> 
> Unrelated to that, I think this is missing a documentation update since
> the struct has kernel-doc comments.

Yeah, I believe these kernel-doc on gigantic struct sock are useless and
we should remove them, they have zero useful info.