[RFC PATCH 0/3] arm/arm64: entry: Remove need_resched() loop

[Valentin Schneider <valentin.schneider@arm.com>]

A while back (before [1]), i386 had this in the tail of its irq
handling code:

  need_resched:
	  movl TI_flags(%ebp), %ecx       # need_resched set ?
	  testb $_TIF_NEED_RESCHED, %cl
	  jz restore_all
	  testl $IF_MASK,EFLAGS(%esp)     # interrupts off (exception path) ?
	  jz restore_all
	  sti
	  call preempt_schedule
	  cli
	  movl $0,TI_preempt_count(%ebp)
	  jmp need_resched

preempt_schedule() already had an inner need_resched() loop introduced by

  commit 4d0b85ea4610 ("[PATCH] kernel preemption bits (2/2)")

and the outer loop was needed since, as the above commit explains, it
was possible to return from preempt_schedule(), get an interrupt and
have need_resched() true.

This was eventually changed when preempt_schedule_irq() was introduced
by [1]:

  commit b268264c6299 ("[PATCH] sched: fix preemption race (Core/i386)")

which moved the enabling & disabling of interrupts inside the then
new preempt_schedule_irq(). From then on, the arch-code loop was no
longer necessary, providing preempt_schedule_irq() was used. This was
talked over on LKML in [2].

It's worth noting that it seems most archs calling
preempt_schedule_irq() have that outer loop, and for most of them it
really looks like they could get rid of it as well.

FWIW the suspects are

$ grep -r -I "preempt_schedule_irq" arch/ | cut -d/ -f2 | sort | uniq

  arc
  arm
  arm64
  c6x
  csky
  h8300
  ia64
  m68k
  microblaze
  mips
  nds32
  nios2
  parisc
  powerpc
  s390
  sh
  sparc
  x86
  xtensa


- Patches 1-2 remove the loop for arm & arm64
- Patch 3 adds a bit of documentation to point out the loop isn't needed
  to try and spread the word.

[2]: https://lore.kernel.org/lkml/cc989920-a13b-d53b-db83-1584a7f53edc@arm.com/

Valentin Schneider (3):
  arm64: entry: Remove unneeded need_resched() loop
  ARM: entry: Remove unneeded need_resched() loop
  sched/Documentation: Point out use of preempt_schedule_irq()

 Documentation/scheduler/sched-arch.txt | 10 ++++++++++
 arch/arm/kernel/entry-armv.S           | 12 +-----------
 arch/arm64/kernel/entry.S              | 11 +----------
 3 files changed, 12 insertions(+), 21 deletions(-)

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2.20.1