On Thu 17-10-19 12:59:40, Pavel Machek wrote:
> Hi!
>
> > From: Michal Hocko <mhocko@xxxxxxxx>
> >
> > commit b0f53dbc4bc4c371f38b14c391095a3bb8a0bb40 upstream.
> >
> > 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.
>
> Ok, why not, but I smell followup work could be done:
>
> > @@ -2635,7 +2635,7 @@ int sysctl_max_threads(struct ctl_table
> > if (ret || !write)
> > return ret;
> >
> > - set_max_threads(threads);
> > + max_threads = threads;
> >
>
> AFAICT set_max_threads can now become __init.
Yes. Care to send a patch?
> Plus, I don't see any locking here, should this be WRITE_ONCE() at
> minimum?
Why would that matter? Do you expect several root processes race to set
the value?
--
Michal Hocko
SUSE Labs
