linux-kernel - Re: [RFC PATCH] perf: Store relevant events in a hlist

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Date:	Wed, 10 Mar 2010 20:34:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Paul Mackerras <paulus@...ba.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Jason Baron <jbaron@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [RFC PATCH] perf: Store relevant events in a hlist

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@...il.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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