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Date:   Fri, 13 Jul 2018 18:14:06 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.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 0/10] psi: pressure stall information for CPU, memory,
 and IO v2
Message-ID: <20180713221042.GA30013@cmpxchg.org>
References: <20180712172942.10094-1-hannes@cmpxchg.org>
 <20180712164422.a53cc0f9c26b078dbc7e5731@linux-foundation.org>
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On Thu, Jul 12, 2018 at 04:44:22PM -0700, Andrew Morton wrote:
> On Thu, 12 Jul 2018 13:29:32 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
> 
> >
> > ...
> >
> > The io file is similar to memory. Because the block layer doesn't have
> > a concept of hardware contention right now (how much longer is my IO
> > request taking due to other tasks?), it reports CPU potential lost on
> > all IO delays, not just the potential lost due to competition.
> 
> Probably dumb question: disks aren't the only form of IO.  Does it make
> sense to accumulate PSI for other forms of IO?  Networking comes to
> mind...

It's conceivable, although I haven't thought too much about it yet. If
that turns out to be a state we might want to track, we can easily add
a task state to identify such stalls and add /proc/pressure/net e.g.

"io" in this case means only the block layer / filesystems. I think
keeping this distinction makes sense in the interest of identifying
which type of hardware resource is posing a pressure problem.