[PATCH 00/14] entry: preempt_schedule_irq() callers scrub

[PATCH 00/14] entry: preempt_schedule_irq() callers scrub

[Valentin Schneider]

Hi,

This is the continuation of [1] where I'm hunting down
preempt_schedule_irq() callers because of [2].

I told myself the best way to get this moving forward wouldn't be to write
doc about it, but to go write some fixes and get some discussions going,
which is what this patch-set is about.

I've looked at users of preempt_schedule_irq(), and made sure they didn't
have one of those useless loops. The list of offenders is:

$ 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
  riscv
  s390
  sh
  sparc
  x86
  xtensa

Regarding that loop, archs seem to fall in 3 categories:
A) Those that don't have the loop
B) Those that have a small need_resched() loop around the
   preempt_schedule_irq() callsite
C) Those that branch to some more generic code further up the entry code
   and eventually branch back to preempt_schedule_irq()

arc, m68k, nios2 fall in A)
sparc, ia64, s390 fall in C)
all the others fall in B)

I've written patches for B) and C) EXCEPT for ia64 and s390 because I
haven't been able to tell if it's actually fine to kill that "long jump"
(and maybe I'm wrong on sparc). Hopefully folks who understand what goes on
in there might be able to shed some light.

Also, since I sent patches for arm & arm64 in [1] I'm not including them
here.

Boot-tested on:
- x86

Build-tested on:
- h8300
- c6x
- powerpc
- mips
- nds32
- microblaze
- sparc
- xtensa

Thanks,
Valentin

[1]: https://lore.kernel.org/lkml/20190131182339.9835-1-valentin.schneider@arm.com/
[2]: https://lore.kernel.org/lkml/cc989920-a13b-d53b-db83-1584a7f53edc@arm.com/

Valentin Schneider (14):
  sched/core: Fix preempt_schedule() interrupt return comment
  c6x: entry: Remove unneeded need_resched() loop
  csky: entry: Remove unneeded need_resched() loop
  h8300: entry: Remove unneeded need_resched() loop
  microblaze: entry: Remove unneeded need_resched() loop
  MIPS: entry: Remove unneeded need_resched() loop
  nds32: ex-exit: Remove unneeded need_resched() loop
  powerpc: entry: Remove unneeded need_resched() loop
  RISC-V: entry: Remove unneeded need_resched() loop
  sh: entry: Remove unneeded need_resched() loop
  sh64: entry: Remove unneeded need_resched() loop
  sparc64: rtrap: Remove unneeded need_resched() loop
  x86/entry: Remove unneeded need_resched() loop
  xtensa: entry: Remove unneeded need_resched() loop

 arch/c6x/kernel/entry.S        | 3 +--
 arch/csky/kernel/entry.S       | 4 ----
 arch/h8300/kernel/entry.S      | 3 +--
 arch/microblaze/kernel/entry.S | 5 -----
 arch/mips/kernel/entry.S       | 3 +--
 arch/nds32/kernel/ex-exit.S    | 4 ++--
 arch/powerpc/kernel/entry_32.S | 6 +-----
 arch/powerpc/kernel/entry_64.S | 8 +-------
 arch/riscv/kernel/entry.S      | 3 +--
 arch/sh/kernel/cpu/sh5/entry.S | 5 +----
 arch/sh/kernel/entry-common.S  | 4 +---
 arch/sparc/kernel/rtrap_64.S   | 1 -
 arch/x86/entry/entry_32.S      | 3 +--
 arch/x86/entry/entry_64.S      | 3 +--
 arch/xtensa/kernel/entry.S     | 2 +-
 kernel/sched/core.c            | 7 +++----
 16 files changed, 16 insertions(+), 48 deletions(-)

--
2.20.1

[PATCH 10/14] sh: entry: Remove unneeded need_resched() loop

[Valentin Schneider]

Since the enabling and disabling of IRQs within preempt_schedule_irq()
is contained in a need_resched() loop, we don't need the outer arch
code loop.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: linux-sh@vger.kernel.org
---
 arch/sh/kernel/entry-common.S | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/sh/kernel/entry-common.S b/arch/sh/kernel/entry-common.S
index d31f66e82ce5..65a105de52a0 100644
--- a/arch/sh/kernel/entry-common.S
+++ b/arch/sh/kernel/entry-common.S
@@ -93,7 +93,7 @@ ENTRY(resume_kernel)
 	mov.l	@(TI_PRE_COUNT,r8), r0	! current_thread_info->preempt_count
 	tst	r0, r0
 	bf	noresched
-need_resched:
+
 	mov.l	@(TI_FLAGS,r8), r0	! current_thread_info->flags
 	tst	#_TIF_NEED_RESCHED, r0	! need_resched set?
 	bt	noresched
@@ -107,8 +107,6 @@ need_resched:
 	mov.l	1f, r0
 	jsr	@r0			! call preempt_schedule_irq
 	 nop
-	bra	need_resched
-	 nop
 
 noresched:
 	bra	__restore_all
-- 
2.20.1

[PATCH 11/14] sh64: entry: Remove unneeded need_resched() loop

[Valentin Schneider]

Since the enabling and disabling of IRQs within preempt_schedule_irq()
is contained in a need_resched() loop, we don't need the outer arch
code loop.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: linux-sh@vger.kernel.org
---
 arch/sh/kernel/cpu/sh5/entry.S | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/sh/kernel/cpu/sh5/entry.S b/arch/sh/kernel/cpu/sh5/entry.S
index de68ffdfffbf..40e6d9a7a6a2 100644
--- a/arch/sh/kernel/cpu/sh5/entry.S
+++ b/arch/sh/kernel/cpu/sh5/entry.S
@@ -897,7 +897,6 @@ resume_kernel:
 	ld.l	r6, TI_PRE_COUNT, r7
 	beq/u	r7, ZERO, tr0
 
-need_resched:
 	ld.l	r6, TI_FLAGS, r7
 	movi	(1 << TIF_NEED_RESCHED), r8
 	and	r8, r7, r8
@@ -911,9 +910,7 @@ need_resched:
 	ori	r7, 1, r7
 	ptabs	r7, tr1
 	blink	tr1, LINK
-
-	pta	need_resched, tr1
-	blink	tr1, ZERO
+	blink   tr0, ZERO
 #endif
 
 	.global ret_from_syscall
-- 
2.20.1

Re: [PATCH 00/14] entry: preempt_schedule_irq() callers scrub

[Vineet Gupta]

On 3/11/19 3:47 PM, Valentin Schneider wrote:
> Hi,
> 
> This is the continuation of [1] where I'm hunting down
> preempt_schedule_irq() callers because of [2].
> 
> I told myself the best way to get this moving forward wouldn't be to write
> doc about it, but to go write some fixes and get some discussions going,
> which is what this patch-set is about.
> 
> I've looked at users of preempt_schedule_irq(), and made sure they didn't
> have one of those useless loops. The list of offenders is:
> 
> $ grep -r -I "preempt_schedule_irq" arch/ | cut -d/ -f2 | sort | uniq
> 
...

> 
> Regarding that loop, archs seem to fall in 3 categories:
> A) Those that don't have the loop

Please clarify that this is the right thing to do (since core code already has the
loop) hence no fixing is required for this "category"

> B) Those that have a small need_resched() loop around the
>    preempt_schedule_irq() callsite
> C) Those that branch to some more generic code further up the entry code
>    and eventually branch back to preempt_schedule_irq()
> 
> arc, m68k, nios2 fall in A)

> sparc, ia64, s390 fall in C)
> all the others fall in B)
> 
> I've written patches for B) and C) EXCEPT for ia64 and s390 because I
> haven't been able to tell if it's actually fine to kill that "long jump"
> (and maybe I'm wrong on sparc). Hopefully folks who understand what goes on
> in there might be able to shed some light.

Re: [PATCH 00/14] entry: preempt_schedule_irq() callers scrub

[Valentin Schneider]

On 12/03/2019 18:03, Vineet Gupta wrote:
[...]
>> Regarding that loop, archs seem to fall in 3 categories:
>> A) Those that don't have the loop
> 
> Please clarify that this is the right thing to do (since core code already has the
> loop) hence no fixing is required for this "category"
> 

Right, those don't need any change. I had a brief look at them to double
check they had the proper need_resched() gate before calling
preempt_schedule_irq() (with no loop) and they all seem fine. Also...

>> B) Those that have a small need_resched() loop around the
>>    preempt_schedule_irq() callsite
>> C) Those that branch to some more generic code further up the entry code
>>    and eventually branch back to preempt_schedule_irq()
>>
>> arc, m68k, nios2 fall in A)
> 

I forgot to include parisc in here.

[...]