linux-kernel - Re: [PATCH v2 1/3] sched/numa: advanced per-cgroup numa statistic

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Date:   Thu, 28 Nov 2019 16:58:18 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     王贇 <yun.wang@...ux.alibaba.com>
Cc:     Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [PATCH v2 1/3] sched/numa: advanced per-cgroup numa statistic

On Thu, Nov 28, 2019 at 09:41:37PM +0800, 王贇 <yun.wang@...ux.alibaba.com> wrote:
> There are used to be a discussion on this, Peter mentioned we no longer
> expose raw ticks into userspace and micro seconds could be fine.
I don't mean the unit presented but the precision.

> Basically we use this to calculate percentages, for which jiffy could be
> accurate enough :-)
You also report the raw times.

Ad percentages (or raw times precision), on average, it should be fine
but can't there be any "aliasing" artifacts when only an unimportant
task is regularly sampled, hence not capturing the real pattern on the
CPU? (Again, I'm not confident I'm not missing anything that prevents
that behavior.)

> But still, what if folks don't use v2... any good suggestions?
(Note this applies to exectimes not locality.) On v1, they can add up
per CPU values from cpuacct. (So it's v2 that's missing the records.)


> Yes, since they don't have NUMA balancing to do optimization, and
> generally they are not that much.
Aha, I didn't realize that.

> Sorry but I don't get it... at first it was 10 regions, as Peter suggested
> we pick 8, but now to insert member 'jiffies' it become 7,
See, there are various arguments for different values :-)

I meant that the currently chosen one is imprinted into the API file.
That is IMO fixable by documenting (e.g. the number of bands may change,
assume uniform division) or making all this just a debug API. Or, see
below.

> Yes, here what I try to highlight is the similar usage, but not the way of
> monitoring ;-) as the docs tell, we monitoring increments.
I see, the docs give me an idea what's the supposed use case.

What about exposing only the counters for local, remote and let the user
do their monitoring based on Δlocal/(Δlocal + Δremote)?

That would avoid the partitioning question completely, exposed values
would be simple numbers and provided information should be equal. A
drawback is that such a sampling would be slower (but sufficient for the
illustrating example).

Michal

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