• Michal Hocko's avatar
    kernel/sysctl.c: do not override max_threads provided by userspace · b0f53dbc
    Michal Hocko authored
    Partially revert 16db3d3f ("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 6538b8ea ("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: 16db3d3f ("kernel/sysctl.c: threads-max observe limits")
    Signed-off-by: default avatarMichal Hocko <mhocko@suse.com>
    Reviewed-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
    Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    b0f53dbc
fork.c 69.7 KB