linux-kernel - Re: [PATCH v2] mm/vmscan.c: only adjust related kswapd cpu affinity when online cpu

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Message-ID: <20200217093124.GH31531@dhcp22.suse.cz>
Date:   Mon, 17 Feb 2020 10:31:24 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Yang <richardw.yang@...ux.intel.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, rientjes@...gle.com
Subject: Re: [PATCH v2] mm/vmscan.c: only adjust related kswapd cpu affinity
 when online cpu

On Sat 15-02-20 08:37:53, Wei Yang wrote:
> On Fri, Feb 14, 2020 at 09:51:13AM +0100, Michal Hocko wrote:
> >On Fri 14-02-20 15:33:20, Wei Yang wrote:
> >> When onlining a cpu, kswapd_cpu_online() is called to adjust kswapd cpu
> >> affinity.
> >> 
> >> Current routine does like this:
> >> 
> >>   a) Iterate all the numa node
> >>   b) Adjust cpu affinity when node has an online cpu
> >> 
> >> For a) this is not necessary, since the particular online cpu belongs to
> >> a particular numa node. So it is not necessary to iterate on every nodes
> >> on the system. This new onlined cpu just affect kswapd cpu affinity of
> >> this particular node.
> >> 
> >> For b) several cpumask operation is used to check whether the node has
> >> an online CPU. Since at this point we are sure one of our CPU onlined,
> >> we can set the cpu affinity directly to current cpumask_of_node().
> >> 
> >> This patch simplifies the logic by set cpu affinity of the affected
> >> kswapd.
> >
> >How have you tested this patch?
> >
> 
> I online one cpu and confirm the "cpu" is the one we just onlined.
> 
> If my understanding is correct, this is the expected behavior.
> 
> >Also this is an old code and quite convoluted but does it still work as
> >inteded? I mean, I do not see any cpu offline callback to reduce the
> >cpu mask as all the CPUs for the given node go offline? Wouldn't the
> 
> You are right, I didn't see the counterpart for cpu offline. This is the
> question I want to ask. Seems we didn't handle it at the very beginning.
> 
> >scheduler simply go and fallback to no affinity if that happens?
> >In other words what is the value of kswapd_cpu_online in the first
> >place?
> 
> Some cases may this function be useful.
> 
> If we have a memory node which doesn't have any online cpu, the default
> cpumask is not set. After one of the cpu online, we want to change cpu
> affinity.
> 
> Or we want to add more cpu to the system, we could allow kswapd use more cpu
> resources. Otherwise, kswapd would be limited to those original cpus.

OK, so the usecase is when a NUMA node gains a new CPU which wasn't
there at the time when the node got onlined. Is this a scenario we
really do care about? While not completely impossible I haven't seen
a system which would allow such a runtime configurability. Maybe it
would be simply easier to drop the callback for now until we have a real
world usecase to support it and have it documented.
-- 
Michal Hocko
SUSE Labs