linux-kernel - Re: [PATCH 1/1] perf, core: Use sample period avg as child event's initial period

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Message-ID: <20141216123558.GX3337@twins.programming.kicks-ass.net>
Date:	Tue, 16 Dec 2014 13:35:58 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"eranian@...gle.com" <eranian@...gle.com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH 1/1] perf, core: Use sample period avg as child event's
 initial period

On Mon, Dec 15, 2014 at 09:17:33PM +0000, Liang, Kan wrote:
> > This doesn't seem to make any kind of sense, and its weirdly implemented.
> >
> > So why would you push anything to the original parent? Your description
> > states that the parent event usually has 1, and then you argue about fixing
> > that by using the orig parent, but then you need to update the orig parent.
> > Did you go in circles and confuse yourself? Why not push things into the
> > regular parent event if you're going to push things up.
> 
> My thought is that the original parent is the root of the tree.

No parent is the root; I thought your orig parent thing was the event
you forked from, but now I see its not.

See inherit_event(), event->parent is the root event.

> If there is an
> average sample period for nodes, it should be kept in the root node, since
> it's the only node everyone knows.

Right, but, that's also contention central..

> > Also, since you can have multiple child events, on many CPUs local64_t is
> > the wrong data type, furthermore its going to be a scalability issue on big
> > hardware.
> 
> I'd like to have avg_sample_period for each CPU. The similar usage is
> period_left in hw_perf_event.

Well, some events are per cpu, some are per task. The per task events do
not have per-cpu storage and their parent can be on whatever cpu.

> We don't need to share the avg_sample_period between CPUs, after all
> it's only a reference.

Right, some smarts are needed to avoid the worst contention there. Maybe
a jiffy timestamp and don't update more than once every HZ jiffies or
so.
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