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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Tue, 1 Nov 2022 05:49:27 +0100
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        rostedt@goodmis.org
Subject: Re: [PATCH rcu 13/14] workqueue: Make queue_rcu_work() use
 call_rcu_flush()
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> On Mon, Oct 31, 2022 at 9:21 AM Uladzislau Rezki <urezki@gmail.com> wrote:
> >
> > On Fri, Oct 28, 2022 at 09:23:47PM +0000, Joel Fernandes wrote:
> > > On Mon, Oct 24, 2022 at 09:48:19AM -0700, Paul E. McKenney wrote:
> > > > On Mon, Oct 24, 2022 at 06:25:30PM +0200, Uladzislau Rezki wrote:
> > > > > >
> > > > > > You guys might need to agree on the definition of "good" here.  Or maybe
> > > > > > understand the differences in your respective platforms' definitions of
> > > > > > "good".  ;-)
> > > > > >
> > > > > Indeed. Bad is when once per-millisecond infinitely :) At least in such use
> > > > > workload a can detect a power delta and power gain. Anyway, below is a new
> > > > > trace where i do not use "flush" variant for the kvfree_rcu():
> > > > >
> > > > > <snip>
> > > > > 1. Home screen swipe:
> [...]
> > > > > 2. App launches:
> [...]
> > > > > <snip>
> > > > >
> > > > > it is much more better. But. As i wrote earlier there is a patch that i have submitted
> > > > > some time ago improving kvfree_rcu() batching:
> > > > >
> > > > > <snip>
> > > > > commit 51824b780b719c53113dc39e027fbf670dc66028
> > > > > Author: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > > > Date:   Thu Jun 30 18:33:35 2022 +0200
> > > > >
> > > > >     rcu/kvfree: Update KFREE_DRAIN_JIFFIES interval
> > > > >
> > > > >     Currently the monitor work is scheduled with a fixed interval of HZ/20,
> > > > >     which is roughly 50 milliseconds. The drawback of this approach is
> > > > >     low utilization of the 512 page slots in scenarios with infrequence
> > > > >     kvfree_rcu() calls.  For example on an Android system:
> > > > > <snip>
> > > > >
> > > > > The trace that i posted was taken without it.
> > > >
> > > > And if I am not getting too confused, that patch is now in mainline.
> > > > So it does make sense to rely on it, then.  ;-)
> > >
> > > Vlad's patch to change the KFREE_DRAIN_JIFFIES to 5 seconds seems reasonable
> > > to me. However, can we unify KFREE_DRAIN_JIFFIES and LAZY_FLUSH_JIFFIES ?
> > >
> > This is very good.
> >
> > Below is a plot that i have taken during one use-case. It is about three
> > apps usage in parallel. It was done by running "monkey" test:
> >
> > wget ftp://vps418301.ovh.net/incoming/monkey_3_apps_slab_usage_5_minutes.png
> >
> > i set up three apps as usage scenario: Google Chrome, YoTube and Camera.
> > I logged the Slab metric from the /proc/meminfo. Sampling rate is 0.1 second.
> >
> > Please have a look at results. It reflects what i am saying. non-flush
> > kvfree RCU variant makes a memory usage higher. What is not acceptable
> > for our mobile devices and workloads.
> 
> That does look higher, though honestly about ~5%. But that's just the
> effect of more "laziness". The graph itself does not show a higher
> number of shrinker invocations, in fact I think shrinker invocations
> are not happening much that's why the slab holds more memory. The
> system may not be under memory pressure?
> 
The idea is to minimize a possibility of entering into a low memory
condition mode. This is bad from a sluggishness point of view for users.
I am saying it in a context of android devices.

> Anyway, I agree with your point of view and I think my concern does
> not even occur with the latest patch on avoiding RCU that I posted
> [1], so I come in peace.
> 
> [1] https://lore.kernel.org/rcu/20221029132856.3752018-1-joel@joelfernandes.org/
> 
I will have a look at it.

>
> I am going to start merging all the lazy patches to ChromeOS 5.10 now
> including your kfree updates, except for [1] while we discuss it.
>
Good for ChromeOS users :)

--
Uladzislau Rezki