time, signal: Protect resource use statistics with seqlock

Both times() and clock_gettime(CLOCK_PROCESS_CPUTIME_ID) have scalability
issues on large systems, due to both functions being serialized with a
lock.

The lock protects against reporting a wrong value, due to a thread in the
task group exiting, its statistics reporting up to the signal struct, and
that exited task's statistics being counted twice (or not at all).

Protecting that with a lock results in times() and clock_gettime() being
completely serialized on large systems.

This can be fixed by using a seqlock around the events that gather and
propagate statistics. As an additional benefit, the protection code can
be moved into thread_group_cputime(), slightly simplifying the calling
functions.

In the case of posix_cpu_clock_get_task() things can be simplified a
lot, because the calling function already ensures that the task sticks
around, and the rest is now taken care of in thread_group_cputime().

This way the statistics reporting code can run lockless.

Signed-off-by: Rik van Riel <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Alex Thorlton <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Daeseok Youn <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Dongsheng Yang <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Guillaume Morin <[email protected]>
Cc: Ionut Alexa <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michal Schmidt <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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