Re: threads-max observe limits

[Michal Hocko]

Andrew, do you want me to send the patch or you can grab it from here?

On Sun 22-09-19 16:24:10, Eric W. Biederman wrote:
> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> 
> > From 711000fdc243b6bc68a92f9ef0017ae495086d39 Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@xxxxxxxx>
> > Date: Sun, 22 Sep 2019 08:45:28 +0200
> > Subject: [PATCH] kernel/sysctl.c: do not override max_threads provided by
> >  userspace
> >
> > Partially revert 16db3d3f1170 ("kernel/sysctl.c: threads-max observe limits")
> > because the patch is causing a regression to any workload which needs to
> > override the auto-tuning of the limit provided by kernel.
> >
> > set_max_threads is implementing a boot time guesstimate to provide a
> > sensible limit of the concurrently running threads so that runaways will
> > not deplete all the memory. This is a good thing in general but there
> > are workloads which might need to increase this limit for an application
> > to run (reportedly WebSpher MQ is affected) and that is simply not
> > possible after the mentioned change. It is also very dubious to override
> > an admin decision by an estimation that doesn't have any direct relation
> > to correctness of the kernel operation.
> >
> > Fix this by dropping set_max_threads from sysctl_max_threads so any
> > value is accepted as long as it fits into MAX_THREADS which is important
> > to check because allowing more threads could break internal robust futex
> > restriction. While at it, do not use MIN_THREADS as the lower boundary
> > because it is also only a heuristic for automatic estimation and admin
> > might have a good reason to stop new threads to be created even when
> > below this limit.
> >
> > Fixes: 16db3d3f1170 ("kernel/sysctl.c: threads-max observe limits")
> > Cc: stable
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> Reviewed-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

Thanks Eric.

> > ---
> >  kernel/fork.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 2852d0e76ea3..ef865be37e98 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -2929,7 +2929,7 @@ int sysctl_max_threads(struct ctl_table *table, int write,
> >  	struct ctl_table t;
> >  	int ret;
> >  	int threads = max_threads;
> > -	int min = MIN_THREADS;
> > +	int min = 1;
> >  	int max = MAX_THREADS;
> >  
> >  	t = *table;
> > @@ -2941,7 +2941,7 @@ int sysctl_max_threads(struct ctl_table *table, int write,
> >  	if (ret || !write)
> >  		return ret;
> >  
> > -	set_max_threads(threads);
> > +	max_threads = threads;
> >  
> >  	return 0;
> >  }
> > -- 
> > 2.20.1

-- 
Michal Hocko
SUSE Labs