tracing: Cache comms only after an event occurred

Whenever an event is registered, the comm of tasks are saved at
every task switch instead of saving them at every event. But if
an event isn't executed much, the comm cache will be filled up
by tasks that did not record the event and you lose out on the comms
that did.

Here's an example, if you enable the following events:

echo 1 > /debug/tracing/events/kvm/kvm_cr/enable
echo 1 > /debug/tracing/events/net/net_dev_xmit/enable

Note, there's no kvm running on this machine so the first event will
never be triggered, but because it is enabled, the storing of comms
will continue. If we now disable the network event:

echo 0 > /debug/tracing/events/net/net_dev_xmit/enable

and look at the trace:

cat /debug/tracing/trace
            sshd-2672  [001] ..s2   375.731616: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=242 rc=0
            sshd-2672  [001] ..s1   375.731617: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=242 rc=0
            sshd-2672  [001] ..s2   375.859356: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=242 rc=0
            sshd-2672  [001] ..s1   375.859357: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=242 rc=0
            sshd-2672  [001] ..s2   375.947351: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=242 rc=0
            sshd-2672  [001] ..s1   375.947352: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=242 rc=0
            sshd-2672  [001] ..s2   376.035383: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=242 rc=0
            sshd-2672  [001] ..s1   376.035383: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=242 rc=0
            sshd-2672  [001] ..s2   377.563806: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=226 rc=0
            sshd-2672  [001] ..s1   377.563807: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=226 rc=0
            sshd-2672  [001] ..s2   377.563834: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6be0 len=114 rc=0
            sshd-2672  [001] ..s1   377.563842: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6be0 len=114 rc=0

We see that process 2672 which triggered the events has the comm "sshd".
But if we run hackbench for a bit and look again:

cat /debug/tracing/trace
           <...>-2672  [001] ..s2   375.731616: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=242 rc=0
           <...>-2672  [001] ..s1   375.731617: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=242 rc=0
           <...>-2672  [001] ..s2   375.859356: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=242 rc=0
           <...>-2672  [001] ..s1   375.859357: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=242 rc=0
           <...>-2672  [001] ..s2   375.947351: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=242 rc=0
           <...>-2672  [001] ..s1   375.947352: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=242 rc=0
           <...>-2672  [001] ..s2   376.035383: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=242 rc=0
           <...>-2672  [001] ..s1   376.035383: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=242 rc=0
           <...>-2672  [001] ..s2   377.563806: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6de0 len=226 rc=0
           <...>-2672  [001] ..s1   377.563807: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6de0 len=226 rc=0
           <...>-2672  [001] ..s2   377.563834: net_dev_xmit: dev=eth0 skbaddr=ffff88005cbb6be0 len=114 rc=0
           <...>-2672  [001] ..s1   377.563842: net_dev_xmit: dev=br0 skbaddr=ffff88005cbb6be0 len=114 rc=0

The stored "sshd" comm has been flushed out and we get a useless "<...>".

But by only storing comms after a trace event occurred, we can run
hackbench all day and still get the same output.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>