Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 18 Jul 2018 18:31:15 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Vinayak Menon <vinmenon@codeaurora.org>,
        Christopher Lameter <cl@linux.com>,
        Mike Galbraith <efault@gmx.de>,
        Shakeel Butt <shakeelb@google.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH 08/10] psi: pressure stall information for CPU, memory,
 and IO
Message-ID: <20180718163115.GV2494@hirez.programming.kicks-ass.net>
References: <20180712172942.10094-1-hannes@cmpxchg.org>
 <20180712172942.10094-9-hannes@cmpxchg.org>
 <20180718124627.GD2476@hirez.programming.kicks-ass.net>
 <20180718135633.GA5161@cmpxchg.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180718135633.GA5161@cmpxchg.org>
User-Agent: Mutt/1.10.0 (2018-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jul 18, 2018 at 09:56:33AM -0400, Johannes Weiner wrote:
> On Wed, Jul 18, 2018 at 02:46:27PM +0200, Peter Zijlstra wrote:

> > I'm confused by this whole MEMSTALL thing... I thought the idea was to
> > account the time we were _blocked_ because of memstall, but you seem to
> > count the time we're _running_ with PF_MEMSTALL.
> 
> Under heavy memory pressure, a lot of active CPU time is spent
> scanning and rotating through the LRU lists, which we do want to
> capture in the pressure metric. What we really want to know is the
> time in which CPU potential goes to waste due to a lack of
> resources. That's the CPU going idle due to a memstall, but it's also
> a CPU doing *work* which only occurs due to a lack of memory. We want
> to know about both to judge how productive system and workload are.

Then maybe memstall (esp. the 'stall' part of it) is a bit of a
misnomer.

> > And esp. the wait_on_page_bit_common caller seems performance sensitive,
> > and the above function is quite expensive.
> 
> Right, but we don't call it on every invocation, only when waiting for
> the IO to read back a page that was recently deactivated and evicted:
> 
> 	if (bit_nr == PG_locked &&
> 	    !PageUptodate(page) && PageWorkingset(page)) {
> 		if (!PageSwapBacked(page))
> 			delayacct_thrashing_start();
> 		psi_memstall_enter(&pflags);
> 		thrashing = true;
> 	}
> 
> That means the page cache workingset/file active list is thrashing, in
> which case the IO itself is our biggest concern, not necessarily a few
> additional cycles before going to sleep to wait on its completion.

Ah, right. PageWorkingset() is only true if we (recently) evicted that
page before, right?