summary

There is a list xfrm_km_list in kernel.
Each node of the list is struct xfrm_mgr,
which has several methods to notify usersapce by netlink message.
Different methods has corresponding method,
and it broadcast the netlink message with different xfrm groups.

struct xfrm_mgr has many methods, for example:
1. notify: notify the sa change, ex: add, delete, expire ..
2. acquire: notify when sp is match, while no SA is got.
3. compile_policy:
4. new_mapping:
5: notify_policy: notify sp change. add, delete, expire.
6. report:
7. mirgrate:

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In order to understand kernel route LC-Trie,
summary the basic functions here.

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kprobe is a useful tool when debug kernel.

It can directly hook the related function of kernel,
before or after it run.

It also can be used to patch kernel in some special case.

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usage:

In version management, we often meet this case:

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When a line/file is deleted for git repo?
or 
who did it ?

In this case, we need git bisect.

1. git bisect start
2. git bisect good commit_id1
3. git bisect bad commit_id2
4. git bisect bad/good

repeat to step 4, until finish.

more auto method for step4 is run with git bisect run command.

by the way git blame is used to check the added line/file.

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Q: Does kernel IPv6 route support route cache ?

yes. but it is very different with IPv4.
It should be ‘clone’ strictly speaking.

For simple let us only focus on ip6_pol_route, and variable struct rt6_info *nrt.

1. When a route is matched, a nrt will be created by rt6_alloc_cow or rt6_alloc_clone.

2. the nrt will be set with flag RTF_CACHE;

3. ip6_ins_rt insert the new nrt to fib tree.

4. kernel starts fib6 gc for nrt.

so there will be a new cache route, and after a period it will disappear.

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##summary
IP4 route register a call back notify in IPv4 address.
When a address is added, fib_inetaddr_event will be called.

and then 4 route entry maybe added in fib_add_ifaddr

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##summary
There is a struct in_device __rcu *ip_ptr under every net_device.
which stores all the IPv4 related information for this device.

Each IPv4 address is stored a struct in_ifaddr,
all the IPv4 address of a net device are stored into a list(netdev->ip_ptr->ifa_list).

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unregister_netdev is used to delete a net device. In fact, it equals:

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rtnl_lock();
rollback_registered_many( a temp list with a single net device);
list_add_tail(&dev->todo_list, &net_todo_list);
rtnl_unlock();

a temporary list stores a single net device, which is to be deleted.
net_todo_list stores all the net devices are being deleted.

The core function is rollback_registered_many, which efficiently deletes many devices in a list.
But here, in this case, one a single netdevice in the list.

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##summary

There is a struct rt_rq in struct rq. struct rt_rq is used to store all
the realtime task in current struct rq. which has a struct rt_prio_array active.

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summary

There are four sched_class,
stop_sched_class --> rt_sched_class --> fair_sched_class --> idle_sched_class

They are linked one by one staticly by struct sched_class->next by their defination.

Each sched_class has method pick_next_task, which is used to select
a perfect process to run from each sched_class‘s runqueue.

When we need schedule, the four pick_next_task will be called one by one.

As a optimization, most time there is no rt task in running state,
in this case we can directly call fair_sched_class.

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