linux-kernel - Re: [PATCH v6 1/3] perf/core: use rb trees for pinned/flexible groups

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Message-ID: <20170807165711.lqv2yjobqskruvpx@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Aug 2017 18:57:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>,
        Valery Cherepennikov <valery.cherepennikov@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        Stephane Eranian <eranian@...gle.com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/3] perf/core: use rb trees for pinned/flexible groups

On Mon, Aug 07, 2017 at 07:27:30PM +0300, Alexey Budankov wrote:
> On 07.08.2017 18:55, Peter Zijlstra wrote:

> > In the extreme, if you construct your program such that you'll never get
> > hit by the tick (this used to be a popular measure to hide yourself from
> > time accounting)
> 
> Well, some weird thing for me. Never run longer than one tick? 
> I could imaging some I/O bound code that would fast serve some short 
> messages, all the other time waiting for incoming requests.
> Not sure if CPU events monitoring is helpful in this case.

Like I said, in extreme. Typically its less weird.

Another example is scheduling a very constrained counter/group along
with a bunch of simple events such that the group will only succeed to
schedule when its the first. In this case it will get only 1/nr_events
time with RR, as opposed to the other/simple events that will get
nr_counters/nr_events time.

By making it runtime based, the constrained thing will more often be
head of list and acquire equal total runtime to the other events.