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Subject: Re: [PATCH] perf: Store active software events in a hashlist
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Paul Mackerras <paulus@samba.org>
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Date: Wed, 07 Apr 2010 11:04:53 +0200
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On Tue, 2010-04-06 at 17:27 +0200, Peter Zijlstra wrote:
> 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.

Alternatively, you can simply but the hash table into the per-cpu
structure and not allocate it, its only a single page (half a page if
you use 32bit or actually use 8 bits.