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

From: Peter Zijlstra <peterz@infradead.org>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com
Subject: Re: [PATCH 3/3] mm/sched: memdelay: memory health interface for systems and workloads
Date: Mon, 31 Jul 2017 10:31:11 +0200	[thread overview]
Message-ID: <20170731083111.tgjgkwge5dgt5m2e@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170730152813.GA26672@cmpxchg.org>

On Sun, Jul 30, 2017 at 11:28:13AM -0400, Johannes Weiner wrote:
> On Sat, Jul 29, 2017 at 11:10:55AM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 27, 2017 at 11:30:10AM -0400, Johannes Weiner wrote:
> > > +static void domain_cpu_update(struct memdelay_domain *md, int cpu,
> > > +			      int old, int new)
> > > +{
> > > +	enum memdelay_domain_state state;
> > > +	struct memdelay_domain_cpu *mdc;
> > > +	unsigned long now, delta;
> > > +	unsigned long flags;
> > > +
> > > +	mdc = per_cpu_ptr(md->mdcs, cpu);
> > > +	spin_lock_irqsave(&mdc->lock, flags);
> > 
> > Afaict this is inside scheduler locks, this cannot be a spinlock. Also,
> > do we really want to add more atomics there?
> 
> I think we should be able to get away without an additional lock and
> rely on the rq lock instead. schedule, enqueue, dequeue already hold
> it, memdelay_enter/leave could be added. I need to think about what to
> do with try_to_wake_up in order to get the cpu move accounting inside
> the locked section of ttwu_queue(), but that should be doable too.

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?