Re: [PATCH -next 3/4] sched: remove qdisc_rehape_fail

Eric Dumazet [off-list ref] wrote:

On Wed, 2016-06-08 at 17:35 +0200, Florian Westphal wrote:

quoted

After the removal of TCA_CBQ_POLICE in cbq scheduler qdisc->reshape_fail
is always NULL, i.e. qdisc_rehape_fail is now the same as qdisc_drop.

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 include/net/sch_generic.h | 19 -------------------
 net/sched/sch_fifo.c      |  4 ++--
 net/sched/sch_netem.c     |  4 ++--
 net/sched/sch_plug.c      |  2 +-
 net/sched/sch_tbf.c       |  4 ++--
 5 files changed, 7 insertions(+), 26 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index c069ac1..a9aec63 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h

@@ -63,9 +63,6 @@ struct Qdisc {
 	struct list_head	list;
 	u32			handle;
 	u32			parent;
-	int			(*reshape_fail)(struct sk_buff *skb,
-					struct Qdisc *q);
-
 	void			*u32_node;
 

You removed 2 pointers from Qdisc, so now next_sched & gso_skb are in a
different cache line than ->state

Some performance penalty is expected, unless you move a read_mostly
field there to compensate.

Would you mind an annotation rather than covering the hole?

--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h

@@ -71,11 +71,11 @@ struct Qdisc {
        struct gnet_stats_basic_cpu __percpu *cpu_bstats;
        struct gnet_stats_queue __percpu *cpu_qstats;
 
-       struct Qdisc            *next_sched;
-       struct sk_buff          *gso_skb;
        /*
         * For performance sake on SMP, we put highly modified fields at the end
         */
+       struct Qdisc            *next_sched ____cacheline_aligned_in_smp;
+       struct sk_buff          *gso_skb;

... it creates 16 byte hole after cpu_qstats and keeps the rest as-is
 (i.e. next_sched is at beginning of 2nd cacheline, as before the removal).

I could also cover the hole by moving rcu_head there but it seems fragile
and doesn't reduce total struct size anyway (we get larger hole at end).

If you have no objection I'd resubmit the series as-is but with this patch.

Let me know, thanks Eric!