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    SubjectRe: [RFC PATCH] perf: Store relevant events in a hlist
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    On Mon, 2010-03-08 at 19:35 +0100, Frederic Weisbecker wrote:
    > On Fri, Mar 05, 2010 at 10:39:29AM +0100, Peter Zijlstra wrote:
    > > On Fri, 2010-03-05 at 08:00 +0100, Frederic Weisbecker wrote:
    > > > Each time a trace event triggers, we walk through the entire
    > > > list of events from the active contexts to find the perf events
    > > > that match the current one.
    > > >
    > > > This is wasteful. To solve this, we maintain a per cpu list of
    > > > the active perf events for each running trace events and we
    > > > directly commit to these.
    > >
    > > Right, so this seems a little trace specific. I once thought about using
    > > a hash table to do this for all software events. It also keeps it all
    > > nicely inside perf_event.[ch].
    >
    >
    > What do you think about this version?
    > It builds but crashes on runtime and doesn't handle
    > cpu hotplug yet. Before spending more time in debugging/fixing,
    > I'd like to know your opinion about the general architecture.
    >
    > Thanks.
    >
    > ---
    > From c389b296a87bd38cfd28da3124508eb1e5a5d553 Mon Sep 17 00:00:00 2001
    > From: Frederic Weisbecker <fweisbec@gmail.com>
    > Date: Mon, 8 Mar 2010 19:04:02 +0100
    > Subject: [PATCH] perf: Store relevant events in a hlist
    >
    > When a software/tracepoint event triggers, we walk through the
    > entire current cpu and task context's events lists to retrieve
    > those that are concerned.
    >
    > This is wasteful. This patch proposes a hashlist to walk through
    > the relevant events only. The hash is calculated using the id of
    > the event (could be further optimized using the type of the event
    > too). Each hash map a list of distinct type:id that match the hash.
    > To these type:id nodes, we affect a list of the active events
    > matching the type:id.
    >
    > -----------------------
    > Hash 1 | Hash 2 | ....
    > ----------------------
    > |
    > swevent type:id node
    > | |
    > | ------- event 1 ---- event 2 ---- ....
    > |
    > swevent type:id node
    > |
    > --------[...]
    >
    > The hashlist is per cpu (attached to perf_cpu_context) and the
    > events in the lists gather those that are active in the cpu and
    > task context. No more per events checks are needed to guess if
    > the events are "counting" or "matching".

    I'm not quite sure why you need the node thing, you already have a
    hash-bucket to iterate, simply stick all events into the one bucket and
    walk through it with a filter and process all events that match.

    As to all those for_each_online_cpu() thingies, it might make sense to
    also have a global hash-table for events active on all cpus,... hmm was
    that the reason for the node thing, one event cannot be in multiple
    buckets?


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