Re: [RFC PATCH 10/10] psi: aggregate ongoing stall events when somebody reads pressure

From: Suren Baghdasaryan <surenb@google.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Tejun Heo <tj@kernel.org>,
	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: [RFC PATCH 10/10] psi: aggregate ongoing stall events when somebody reads pressure
Date: Fri, 13 Jul 2018 16:34:59 -0700	[thread overview]
Message-ID: <CAJuCfpFe-tfcK3BPZ2Y9AEC56PtYpCc04YPGF+fs=e=RqkW-XA@mail.gmail.com> (raw)
In-Reply-To: <20180713224920.GA31566@cmpxchg.org>

On Fri, Jul 13, 2018 at 3:49 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Fri, Jul 13, 2018 at 03:13:07PM -0700, Suren Baghdasaryan wrote:
>> On Thu, Jul 12, 2018 at 10:29 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
>> > might want to know about and react to stall states before they have
>> > even concluded (e.g. a prolonged reclaim cycle).
>> >
>> > This patches the procfs/cgroupfs interface such that when the pressure
>> > metrics are read, the current per-cpu states, if any, are taken into
>> > account as well.
>> >
>> > Any ongoing states are concluded, their time snapshotted, and then
>> > restarted. This requires holding the rq lock to avoid corruption. It
>> > could use some form of rq lock ratelimiting or avoidance.
>> >
>> > Requested-by: Suren Baghdasaryan <surenb@google.com>
>> > Not-yet-signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
>> > ---
>>
>> IMHO this description is a little difficult to understand. In essence,
>> PSI information is being updated periodically every 2secs and without
>> this patch the data can be stale at the time when we read it (because
>> it was last updated up to 2secs ago). To avoid this we update the PSI
>> "total" values when data is being read.
>
> That fix I actually folded into the main patch. We now always update
> the total= field at the time the user reads to include all concluded
> events, even if we sampled less than 2s ago. Only the running averages
> are still bound to the 2s sampling window.
>
> What this patch adds on top is for total= to include any *ongoing*
> stall events that might be happening on a CPU at the time of reading
> from the interface, like a reclaim cycle that hasn't finished yet.

Ok, I see now what you mean. So ondemand flag controls whether
*ongoing* stall events are accounted for or not. Nit: maybe rename
that flag to better explain it's function?