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Date: Mon, 29 May 2017 17:22:54 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
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        Valery Cherepennikov <valery.cherepennikov@intel.com>,
        David Carrillo-Cisneros <davidcc@google.com>,
        Stephane Eranian <eranian@google.com>,
        Mark Rutland <mark.rutland@arm.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH]: perf/core: addressing 4x slowdown during per-process
 profiling of STREAM benchmark on Intel Xeon Phi
Message-ID: <20170529152254.wjx3b6apmatcso77@hirez.programming.kicks-ass.net>
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On Mon, May 29, 2017 at 04:43:09PM +0300, Alexey Budankov wrote:
> On 29.05.2017 15:03, Alexander Shishkin wrote:
> > Alexey Budankov <alexey.budankov@linux.intel.com> writes:

> > > +		} else if (event->cpu > node_event->cpu) {
> > > +			node = &((*node)->rb_right);
> > > +		} else {
> > > +			list_add_tail(&event->group_list_entry,
> > > +					&node_event->group_list);
> > 
> > So why is this better than simply having per-cpu event lists plus one
> > for per-thread events?
> 
> Good question. Choice of data structure and layout depends on the operations
> applied to the data so keeping groups as a tree simplifies and improves the
> implementation in terms of scalability and performance. Please ask more if
> any.

Since these lists are per context, and each task can have a context,
you'd end up with per-task-per-cpu memory, which is something we'd like
to avoid (some archs have very limited per-cpu memory space etc..).

Also, we'd like to have that tree for other reasons, like for instance
that heterogeneous PMU crud ARM has. Also, with a tree we can easier do
time based round-robin scheduling,