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

From: Wei Yang <richardw.yang@linux.intel.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Wei Yang <richardw.yang@linux.intel.com>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, rientjes@google.com
Subject: Re: [PATCH v2] mm/vmscan.c: only adjust related kswapd cpu affinity when online cpu
Date: Tue, 18 Feb 2020 08:24:56 +0800	[thread overview]
Message-ID: <20200218002456.GA16623@richard> (raw)
In-Reply-To: <20200217093124.GH31531@dhcp22.suse.cz>

On Mon, Feb 17, 2020 at 10:31:24AM +0100, Michal Hocko wrote:
>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.

I am fine with this suggestion. Let me prepare v3.

>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me