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kernel/sysctl.c: do not override max_threads provided by userspace
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Partially revert 16db3d3 ("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 6538b8e ("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/[email protected]
Fixes: 16db3d3 ("kernel/sysctl.c: threads-max observe limits")
Signed-off-by: Michal Hocko <[email protected]>
Reviewed-by: "Eric W. Biederman" <[email protected]>
Cc: Heinrich Schuchardt <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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Michal Hocko authored and torvalds committed Oct 7, 2019
1 parent 08d1d0e commit b0f53db
Showing 1 changed file with 2 additions and 2 deletions.
4 changes: 2 additions & 2 deletions kernel/fork.c
Original file line number Diff line number Diff line change
Expand Up @@ -2925,7 +2925,7 @@ 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;
Expand All @@ -2937,7 +2937,7 @@ int sysctl_max_threads(struct ctl_table *table, int write,
if (ret || !write)
return ret;

set_max_threads(threads);
max_threads = threads;

return 0;
}

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