Messages in this thread | | | Date | Fri, 22 May 2009 21:18:13 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC PATCH] perf_counter: dynamically allocate tasks' perf_counter_context struct |
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* Paul Mackerras <paulus@samba.org> wrote:
> Ingo Molnar writes: > > > * Paul Mackerras <paulus@samba.org> wrote: > > > > > This replaces the struct perf_counter_context in the task_struct > > > with a pointer to a dynamically allocated perf_counter_context > > > struct. The main reason for doing is this is to allow us to > > > transfer a perf_counter_context from one task to another when we > > > do lazy PMU switching in a later patch. > > > > Hm, i'm not sure how far this gets us towards lazy PMU switching. > > > > In fact i'd say that the term "lazy PMU switching" is probably > > misleading, we should use: "equivalent PMU context switching" or > > instead. > > Yes, that's what I mean. > > As you say, we need to be able to detect when two tasks have > equivalent contexts - that is, when their counters are all > inherited from a common ancestor. My idea is that in that > situation we simply swap the contexts: move the context of the > outgoing task onto the incoming task, and give the incoming task's > context to the outgoing task. With my patch, that involves simply > swapping the pointers over and adjusting the task pointers in the > two contexts. > > That means that all the counters get transferred over to the > incoming task, so there is nothing in the PMU or the arch code > that needs to changed or adjusted. The outgoing task still has a > perfectly valid context, so it doesn't matter if it migrates to > another CPU. The nice thing is that there is nothing special or > unusual about the state after we have swapped the contexts - > nothing that needs to be remembered or undone later.
Yeah, agreed - your scheme is simpler than the scheme i thought of!
Ingo
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