linux-kernel - Re: [PATCH] perf: Store active software events in a hashlist

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Message-ID: <1270567653.20295.11.camel@laptop>
Date:	Tue, 06 Apr 2010 17:27:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH] perf: Store active software events in a hashlist

On Mon, 2010-04-05 at 16:08 +0200, Frederic Weisbecker wrote:
> Each time a software event triggers, we need to walk through
> the entire list of events from the current cpu and task contexts
> to retrieve a running perf event that matches.
> We also need to check a matching perf event is actually counting.
> 
> This walk is wasteful and makes the event fast path scaling
> down with a growing number of events running on the same
> contexts.
> 
> To solve this, we store the running perf events in a hashlist to
> get an immediate access to them against their type:event_id when
> they trigger. 

So we have a hash-table per-cpu, each event takes a ref on the hash
table, when the thing is empty we free it.

When the event->cpu == -1 (all cpus) we take a ref on all possible cpu's
hash-table (should be online I figure, but that requires adding a
hotplug handler).

Then on event enable/disable we actually add the event to the hash-table
belonging to the cpu the event/task gets scheduled on, since each event
can only ever be active on one cpu.

Right?

So looks good, altough I think we want to do that online/hotplug thing.

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