net: sched: refine software bypass handling in tc_run

This patch addresses issues with filter counting in block (tcf_block),
particularly for software bypass scenarios, by introducing a more
accurate mechanism using useswcnt.

Previously, filtercnt and skipswcnt were introduced by:

Commit 2081fd3445fe ("net: sched: cls_api: add filter counter") and
Commit f631ef39d819 ("net: sched: cls_api: add skip_sw counter")

filtercnt tracked all tp (tcf_proto) objects added to a block, and
skipswcnt counted tp objects with the skipsw attribute set.

The problem is: a single tp can contain multiple filters, some with skipsw
and others without. The current implementation fails in the case:

When the first filter in a tp has skipsw, both skipswcnt and filtercnt
are incremented, then adding a second filter without skipsw to the same
tp does not modify these counters because tp->counted is already set.

This results in bypass software behavior based solely on skipswcnt
equaling filtercnt, even when the block includes filters without
skipsw. Consequently, filters without skipsw are inadvertently bypassed.

To address this, the patch introduces useswcnt in block to explicitly count
tp objects containing at least one filter without skipsw. Key changes
include:

Whenever a filter without skipsw is added, its tp is marked with usesw
and counted in useswcnt. tc_run() now uses useswcnt to determine software
bypass, eliminating reliance on filtercnt and skipswcnt.

This refined approach prevents software bypass for blocks containing
mixed filters, ensuring correct behavior in tc_run().

Additionally, as atomic operations on useswcnt ensure thread safety and
tp->lock guards access to tp->usesw and tp->counted, the broader lock
down_write(&block->cb_lock) is no longer required in tc_new_tfilter(),
and this resolves a performance regression caused by the filter counting
mechanism during parallel filter insertions.

The improvement can be demonstrated using the following script:

# cat insert_tc_rules.sh

tc qdisc add dev ens1f0np0 ingress
for i in $(seq 16); do
taskset -c $i tc -b rules_$i.txt &
done
wait

Each of rules_$i.txt files above includes 100000 tc filter rules to a
mlx5 driver NIC ens1f0np0.

Without this patch:

# time sh insert_tc_rules.sh

real 0m50.780s
user 0m23.556s
sys 4m13.032s

With this patch:

# time sh insert_tc_rules.sh

real 0m17.718s
user 0m7.807s
sys 3m45.050s

Fixes: 047f340b36fc ("net: sched: make skip_sw actually skip software")
Reported-by: Shuang Li <shuali@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Tested-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Signed-off-by: David S. Miller <davem@davemloft.net>