From: Michal Hocko mhocko@suse.com Subject: 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.

This became more severe when we switched x86 from 4k to 8k kernel stacks. Starting since 6538b8ea886e ("x86_64: expand kernel stack to 16K") (3.16) we use THREAD_SIZE_ORDER = 2 and that halved the auto-tuned value.

In the particular case 3.12 kernel.threads-max = 515561

4.4 kernel.threads-max = 200000

Neither of the two values is really insane on 32GB machine.

I am not sure we want/need to tune the max_thread value further. If anything the tuning should be removed altogether if proven not useful in general. But we definitely need a way to override this auto-tuning.

Link: http://lkml.kernel.org/r/20190922065801.GB18814@dhcp22.suse.cz Fixes: 16db3d3f1170 ("kernel/sysctl.c: threads-max observe limits") Signed-off-by: Michal Hocko mhocko@suse.com Reviewed-by: "Eric W. Biederman" ebiederm@xmission.com Cc: Heinrich Schuchardt xypron.glpk@gmx.de Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

kernel/fork.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/fork.c~kernel-sysctlc-do-not-override-max_threads-provided-by-userspace +++ a/kernel/fork.c @@ -2925,7 +2925,7 @@ int sysctl_max_threads(struct ctl_table struct ctl_table t; int ret; int threads = max_threads; - int min = MIN_THREADS; + int min = 1; int max = MAX_THREADS;

t = *table; @@ -2937,7 +2937,7 @@ int sysctl_max_threads(struct ctl_table if (ret || !write) return ret;

- set_max_threads(threads); + max_threads = threads;

return 0; } _