Re: [PATCH 1/3] fork: dynamically allocate cache array for vmapped stacks using cpuhp

[Andy Lutomirski]

On Fri, Feb 3, 2017 at 8:42 AM, Hoeun Ryu <hoeun.ryu@xxxxxxxxx> wrote:
> On Sat, Feb 4, 2017 at 12:39 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>> On Sat 04-02-17 00:30:05, Hoeun Ryu wrote:
>>>  Using virtually mapped stack, kernel stacks are allocated via vmalloc.
>>> In the current implementation, two stacks per cpu can be cached when
>>> tasks are freed and the cached stacks are used again in task duplications.
>>> but the array for the cached stacks is statically allocated by per-cpu api.
>>>  In this new implementation, the array for the cached stacks are dynamically
>>> allocted and freed by cpu hotplug callbacks and the cached stacks are freed
>>> when cpu is down. setup for cpu hotplug is established in fork_init().
>>
>> Why do we want this? I can see that the follow up patch makes the number
>> configurable but the changelog doesn't describe the motivation for that.
>> Which workload would benefit from a higher value?
>>
>
> The key difference of this implementation, the cached stacks for a cpu
> is freed when a cpu is down.
> so the cached stacks are no longer wasted.
> In the current implementation, the cached stacks for a cpu still
> remain on the system when a cpu is down.
> I think we could imagine what if a machine has many cpus and someone
> wants to have bigger size of stack caches.

Then how about just registering a simple hotplug hook to free the
stacks without worrying about freeing the tiny array as well?

--Andy