linux-kernel - Re: [PATCH 3/3] mm/sched: memdelay: memory health interface for systems and workloads

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Date:   Tue, 1 Aug 2017 08:26:34 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 3/3] mm/sched: memdelay: memory health interface for
 systems and workloads

On Tue, Aug 01, 2017 at 09:57:28AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 31, 2017 at 02:41:42PM -0400, Johannes Weiner wrote:
> > On Mon, Jul 31, 2017 at 10:31:11AM +0200, Peter Zijlstra wrote:
> 
> > > So could you start by describing what actual statistics we need? Because
> > > as is the scheduler already does a gazillion stats and why can't re
> > > repurpose some of those?
> > 
> > If that's possible, that would be great of course.
> > 
> > We want to be able to tell how many tasks in a domain (the system or a
> > memory cgroup) are inside a memdelay section as opposed to how many
> 
> And you haven't even defined wth a memdelay section is yet..

It's what a task is in after it calls memdelay_enter() and before it
calls memdelay_leave().

Tasks mark themselves to be in a memory section when they know to
perform work that is necessary due to a lack of memory, such as
waiting for a refault or a direct reclaim invocation.

>From the patch:

+/**
+ * memdelay_enter - mark the beginning of a memory delay section
+ * @flags: flags to handle nested memdelay sections
+ *
+ * Marks the calling task as being delayed due to a lack of memory,
+ * such as waiting for a workingset refault or performing reclaim.
+ */

+/**
+ * memdelay_leave - mark the end of a memory delay section
+ * @flags: flags to handle nested memdelay sections
+ *
+ * Marks the calling task as no longer delayed due to memory.
+ */

where a reclaim callsite looks like this (decluttered):

	memdelay_enter()
	nr_reclaimed = do_try_to_free_pages()
	memdelay_leave()

That's what defines the "unproductive due to lack of memory" state of
a task. Time spent in that state weighed against time spent while the
task is productive - runnable or in iowait while not in a memdelay
section - gives the memory health of the task. And the system and
cgroup states/health can be derived from task states as described:

> > are in a "productive" state such as runnable or iowait. Then derive
> > from that whether the domain as a whole is unproductive (all non-idle
> > tasks memdelayed), or partially unproductive (some delayed, but CPUs
> > are productive or there are iowait tasks). Then derive the percentages
> > of walltime the domain spends partially or fully unproductive.
> > 
> > For that we need per-domain counters for
> > 
> > 	1) nr of tasks in memdelay sections
> > 	2) nr of iowait or runnable/queued tasks that are NOT inside
> > 	   memdelay sections