[PATCH 0/4] trace: add ability to collect call chains of non-current task.

[Andrew Vagin]

Without these patches, an event may only save call chain of a current
task, but some events may require call chains of non-current task.

E.g. You may want to know where and how long a task is sleeping. A time
slice is known when the task wakes up.  We send event in this moment,
but a target task isn't "current".

Usually when the system begin stuttering, it often waits a lock or an
operation for a long time. For investigation such situation you probaly
send sysrq-t and analyze backtraces, but perf may do it more effectively.

A macto __perf_task() is set task for wich a call chain will be collected.

Now this functionality is suitable for sched_stat_sleep and sched_stat_iowait.
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