Date: Fri, 3 Feb 2017 09:52:57 -0800 From: Andy Lutomirski <luto@...capital.net> To: Hoeun Ryu <hoeun.ryu@...il.com> Cc: Michal Hocko <mhocko@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>, Kees Cook <keescook@...omium.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, Mateusz Guzik <mguzik@...hat.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH 1/3] fork: dynamically allocate cache array for vmapped stacks using cpuhp On Fri, Feb 3, 2017 at 8:42 AM, Hoeun Ryu <hoeun.ryu@...il.com> wrote: > On Sat, Feb 4, 2017 at 12:39 AM, Michal Hocko <mhocko@...nel.org> 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
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