[patch V4 0/2] signals: Allow caching one sigqueue object per task

[Thomas Gleixner <tglx@linutronix.de>]

This is a follow up to the V2/V3 submission which can be found here:

  https://lore.kernel.org/r/20210311132036.228542540@linutronix.de

Signal sending requires a kmem cache allocation at the sender side and the
receiver hands it back to the kmem cache when consuming the signal.

This works pretty well even for realtime workloads except for the case when
the kmem cache allocation has to go into the slow path which is rare but
happens.

Preempt-RT carries a patch which allows caching of one sigqueue object per
task. The object is not preallocated. It's cached when the task receives a
signal. The cache is freed when the task exits.

The memory overhead for a standard distro setup is pretty small. After boot
there are less than 10 objects cached in about 1500 tasks. The speedup for
sending a signal from a cached sigqueue object is small (~3us) per signal
and almost invisible, but for signal heavy workloads it's definitely
measurable and for the targeted realtime workloads it's solving a real
world latency issue.

Changes vs V2/3:

   - Drop the previous wrapper function and explicitly drop
     the sigqueue cache at the end of __exit_signals() to
     handle the self reaping case correctly

Thanks,

	tglx
---
 include/linux/sched.h  |    1 
 include/linux/signal.h |    1 
 kernel/exit.c          |    1 
 kernel/fork.c          |    1 
 kernel/signal.c        |   55 +++++++++++++++++++++++++++++++++++++++----------
 5 files changed, 48 insertions(+), 11 deletions(-)