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

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Message-ID: <20170205101844.GE22713@dhcp22.suse.cz>
Date:   Sun, 5 Feb 2017 11:18:45 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Hoeun Ryu <hoeun.ryu@...il.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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 Sat 04-02-17 11:01:32, Hoeun Ryu wrote:
> On Sat, Feb 4, 2017 at 2:52 AM, Andy Lutomirski <luto@...capital.net> wrote:
> > 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?
> >
> 
> Michal, What do you think about it. it sounds fair enough.

This is what I've tried to suggest in the other reply.
-- 
Michal Hocko
SUSE Labs