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Date:   Thu, 25 May 2023 08:47:57 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     Christoph Lameter <cl@linux.com>,
        Aaron Tomlin <atomlin@atomlin.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Russell King <linux@armlinux.org.uk>,
        Huacai Chen <chenhuacai@kernel.org>,
        Heiko Carstens <hca@linux.ibm.com>, x86@kernel.org,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v8 00/13] fold per-CPU vmstats remotely
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References: <20230515180015.016409657@redhat.com>
 <ZG4Ia4fQROyynSVG@dhcp22.suse.cz>
 <ZG4W01AcwhD5AiQU@tpad>
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On Wed 24-05-23 10:53:23, Marcelo Tosatti wrote:
> On Wed, May 24, 2023 at 02:51:55PM +0200, Michal Hocko wrote:
> > [Sorry for a late response but I was conferencing last two weeks and now
> > catching up]
> > 
> > On Mon 15-05-23 15:00:15, Marcelo Tosatti wrote:
> > [...]
> > > v8
> > > - Add summary of discussion on -v7 to cover letter
> > 
> > Thanks this is very useful! This helps to frame the further discussion.
> > 
> > I believe the most important question to answer is this in fact
> > > I think what needs to be done is to avoid new queue_work_on()
> > > users from being introduced in the tree (the number of
> > > existing ones is finite and can therefore be fixed).
> > > 
> > > Agree with the criticism here, however, i can't see other
> > > options than the following:
> > > 
> > >         1) Given an activity, which contains a sequence of instructions
> > >            to execute on a CPU, to change the algorithm
> > >            to execute that code remotely (therefore avoid interrupting a CPU),
> > >            or to avoid the interruption somehow (which must be dealt with
> > >            on a case-by-case basis).
> > > 
> > >         2) To block that activity from happening in the first place,
> > >            for the sites where it can be blocked (that return errors to
> > >            userspace, for example).
> > > 
> > >         3) Completly isolate the CPU from the kernel (off-line it).
> > 
> > I agree that a reliable cpu isolation implementation needs to address
> > queue_work_on problem. And it has to do that _realiably_. This cannot by
> > achieved by an endless whack-a-mole and chasing each new instance. There
> > must be a more systematic approach. One way would be to change the
> > semantic of schedule_work_on and fail call for an isolated CPU. The
> > caller would have a way to fallback and handle the operation by other
> > means. E.g. vmstat could simply ignore folding pcp data because an
> > imprecision shouldn't really matter. Other callers might chose to do the
> > operation remotely. This is a lot of work, no doubt about that, but it
> > is a long term maintainable solution that doesn't give you new surprises
> > with any new released kernel. There are likely other remote interfaces
> > that would need to follow that scheme.
> > 
> > If the cpu isolation is not planned to be worth that time investment
> > then I do not think it is also worth reducing a highly optimized vmstat
> > code. These stats are invoked from many hot paths and per-cpu
> > implementation has been optimized for that case.
> 
> It is exactly the same code, but now with a "LOCK" prefix for CMPXCHG
> instruction. Which should not cost much due to cache locking (these are
> per-CPU variables anyway).

Sorry but just a LOCK prefix for a hot path is not a serious argument.
 
> > If your workload would
> > like to avoid that as disturbing then you already have a quiet_vmstat
> > precedence so find a way how to use it for your workload instead.
> >  
> > -- 
> > Michal Hocko
> > SUSE Labs
> 
> OK so an alternative solution is to completly disable vmstat updates
> for isolated CPUs. Are you OK with that ?

Yes, the number of events should be reasonably small and those places in
the kernel which really need a precise value need to do a per-cpu walk
anyway. IIRC /proc/vmstat et al also do accumulate pcp state.

But let me reiterate. Even with vmstat updates out of the game, you have
so many other sources of disruption that your isolated workload will be
fragile until you actually try to deal with the problem on a more
fundamental level.
-- 
Michal Hocko
SUSE Labs