Messages in this thread | | | Date | Wed, 05 May 2010 07:00:24 +0200 | From | Tejun Heo <> | Subject | Re: [RFC PATCHSET] sched,perf: unify tracers in sched and move perf on top of TP |
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Hello,
On 05/04/2010 07:29 PM, Peter Zijlstra wrote: >> * 0001-0007: Unify the three tracers (tracepoints, perf_events and >> preempt/sched notifiers) in scheduler. > > Right, so I really don't like the SCHED_EVENT() thing much, esp the 3 > different function postfixes.
Yeap, it's not the prettiest thing. I thought about renaming all of them so that they share the same postfix but then again I need a way to tell the script which to enable which is the easiest with specifying postfixes as macro argument. Any better ideas?
>> * 0008-0012: Move perf hooks in sched on top of tracepoints if TPs are >> enabled. > > This seems to add a terrible amount of overhead for no particular > reason.
Hmm... What overhead?
What matters more here is avoiding the overhead when perf or whatever tracing mechanism is disabled. When both perf and TPs are compiled in but not enabled, moving perf hooks on top of TPs means that the sched core code doesn't have to call into extra functions for perf and TPs can be nooped. ie. less overhead. Also, even when perf is enabled, there isn't any inherent extra runtime overhead other than during enabling/disabling which again is a much colder path.
The only place where noticeable overhead is added is the extra pair of irq enable/disable that I added for sched_out hook. After glancing through what perf does during context switch, I thought that overhead wouldn't be anything noticeable but if it is, they can definitely be separated. The only purpose of that change was colocating sched notifiers and perf hooks.
Also, if a few more perf hooks are converted to use the same mechanism, it would be possible to put perf properly on top of TPs other than init/exit code paths which will be cleaner for the rest of the kernel and there won't be any extra runtime overhead.
The code will be much cleaner if perf depends on TPs. With perf hooks completely removed, there won't be much point in SCHED_EVENT and the messy ifdefs in perf can also go away. Would this be a possibility?
Thanks.
-- tejun
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