Re: [PATCH 4/4 v4] kernel/fork.c: memory hotplug updates max_threads

[Oleg Nesterov <oleg@redhat.com>]

On 02/23, Heinrich Schuchardt wrote:
>
> On 23.02.2015 21:54, Oleg Nesterov wrote:
> >
> >> And it changes the swapper/0's rlimits. This is pointless after we fork
> >> /sbin/init.
>
> So should writing to /proc/sys/max_threads update the limits of all
> processes?

Why?

No, I think it should not touch rlimits at all.

> >> It seems to me these patches need some cleanups. Plus I am not sure the
> >> kernel should update max_threads automatically, we have the "threads-max"
> >> sysctl.
>
> The idea in the original version of fork_init is that max_threads should
> be chosen such that the memory needed to store the meta-information of
> max_threads threads should only be 1/8th of the total memory.
>
> Somebody adding or removing memory will not necessarily update
> /proc/sys/kernel/threads-max.
>
> This means that if I remove 90 % of the memory I get to a situation
> where max_threads allows so many threads to be created that the
> meta-information occupies all memory.
>
> With patch 4/4 max_threads is automatically reduced in this case.

I understand. But I think that if you remove 90 % of the memory you can
also update /proc/sys/kernel/threads-max.

And, suppose that admin specially limited max_threads, then you add more
memory. Should the kernel bump the limit silently?

And if hotplug should update max_threads, why it doesn't update, say,
files_stat.max_files?

IOW, I do not think that kernel should control max_threads after boot.
But I won't really argue. Just this looks a bit strange to me.

Oleg.