linux-kernel - Re: [PATCH v7 00/13] fold per-CPU vmstats remotely

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Date:   Wed, 19 Apr 2023 08:15:56 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Theurer <atheurer@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Russell King <linux@...linux.org.uk>,
        Huacai Chen <chenhuacai@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>, x86@...nel.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v7 00/13] fold per-CPU vmstats remotely

On Wed, Apr 19, 2023 at 08:14:09AM -0300, Marcelo Tosatti wrote:
> On Tue, Apr 18, 2023 at 03:02:00PM -0700, Andrew Morton wrote:
> > On Mon, 20 Mar 2023 15:03:32 -0300 Marcelo Tosatti <mtosatti@...hat.com> wrote:
> > 
> > > This patch series addresses the following two problems:
> > > 
> > > 1. A customer provided evidence indicating that a process
> > >    was stalled in direct reclaim:
> > > 
> > > ...
> > >
> > >  2. With a task that busy loops on a given CPU,
> > >     the kworker interruption to execute vmstat_update
> > >     is undesired and may exceed latency thresholds
> > >     for certain applications.
> > > 
> > 
> > I don't think I'll be sending this upstream in the next merge window. 
> > Because it isn't clear that the added complexity in vmstat handling is
> > justified.
> 
> From my POV this is an incorrect statement (that the complexity in
> vmstat handling is not justified).
> 
> Andrew, this is the 3rd attempt to fix this problem:
> 
> First try:  https://lore.kernel.org/lkml/20220127173037.318440631@fedora.localdomain/
> 
> Second try: https://patchew.org/linux/20230105125218.031928326@redhat.com/
> 
> Third try: syncing vmstats remotely from vmstat_shepherd (this
> patchset).
> 
> And also, can you please explain: what is so complicated about the
> vmstat handling? cmpxchg has been around and is used all over the
> kernel, and nobody considers "excessively complicated".
> 
> > - Michal's request for more clarity on the end-user requirements
> >   seems reasonable.
> 
> And i explained to Michal in great detail where the end-user 
> requirements come from. For virtualized workloads, there are two
> types of use-cases:
> 
> 1) For example, for the MAC scheduler processing must occur every 1ms,
> and a certain amount of computation takes place (and must finish before
> the next 1ms timeframe). A > 50us latency spike as observed by cyclictest
> is considered a "failure".
> 
> I showed him a 7us trace caused by, and explained that will extend to >
> 50us in the case of virtualized vCPU.
> 
> 2) PLCs. These workloads will also suffer > 50us latency spikes
> which is undesirable.
> 
> Can you please explain what additional clarity is required?
> 
> RH's performance team, for example, has been performing packet
> latency tests and waiting for this issue to be fixed for about 2
> years now.
> 
> Andrew Theurer, can you please explain what problem is the vmstat_work
> interruption causing in your testing?

+CC Andrew.