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List: linux-kernel
Subject: Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling
From: Shrikanth Hegde <sshegde () linux ! ibm ! com>
Date: 2024-05-07 11:28:36
Message-ID: 34f216da-da8f-44cc-a9fc-47c8634e84c6 () linux ! ibm ! com
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On 4/27/24 12:30 AM, Ankur Arora wrote:
>
Hi Ankur,
Sorry for the delay, I was on leave last week.
There might be a delay in response as I am recovering from fever.
>
> Great. I'm guessing these tests are when running in voluntary preemption
> mode (under PREEMPT_AUTO).
>
It was run under preempt=none.
> If you haven't, could you also try full preemption? There you should see
> identical results unless something is horribly wrong.
I tried preempt=full with patch you provided below. ran the hackbench for much longer
with 100000 loops. I don't see any regression on the larger system.
I see slight improvement in some cases. I dont see any major regression with 10k ops
which was tried earlier as well.
==========================================================
1L ops.
==========================================================
Process 10 groups : 9.85, 9.87(-0.20)
Process 20 groups : 17.69, 17.32(2.09)
Process 30 groups : 25.89, 25.96(-0.27)
Process 40 groups : 34.70, 34.61(0.26)
Process 50 groups : 44.02, 43.79(0.52)
Process 60 groups : 52.72, 52.10(1.18)
Thread 10 groups : 10.50, 10.52(-0.19)
Thread 20 groups : 18.79, 18.60(1.01)
Process(Pipe) 10 groups : 10.39, 10.37(0.19)
Process(Pipe) 20 groups : 18.45, 18.54(-0.49)
Process(Pipe) 30 groups : 25.63, 25.92(-1.13)
Process(Pipe) 40 groups : 33.79, 33.49(0.89)
Process(Pipe) 50 groups : 43.15, 41.83(3.06)
Process(Pipe) 60 groups : 51.94, 50.32(3.12)
Thread(Pipe) 10 groups : 10.73, 10.85(-1.12)
Thread(Pipe) 20 groups : 19.24, 19.35(-0.57)
==========================================================
10k ops.
Process 10 groups : 1.10, 1.10(0.00)
Process 20 groups : 1.89, 1.88(0.53)
Process 30 groups : 2.82, 2.80(0.71)
Process 40 groups : 3.76, 3.76(0.00)
Process 50 groups : 4.66, 4.79(-2.79)
Process 60 groups : 5.74, 5.92(-3.14)
thread 10 groups : 1.22, 1.20(1.64)
thread 20 groups : 2.05, 2.05(0.00)
Process(Pipe) 10 groups : 1.13, 1.13(0.00)
Process(Pipe) 20 groups : 1.98, 1.93(2.53)
Process(Pipe) 30 groups : 2.91, 2.75(5.50)
Process(Pipe) 40 groups : 3.85, 3.65(5.19)
Process(Pipe) 50 groups : 4.91, 4.91(0.00)
Process(Pipe) 60 groups : 5.56, 5.90(-6.12)
thread(Pipe) 10 groups : 1.23, 1.23(0.00)
thread(Pipe) 20 groups : 1.99, 1.99(0.00)
==========================================================
Other than hackbench, I see slight improvement in unixbench and stress-ng --cpu workloads.
>
>> However, I still see 20-50%
>> regression on the larger system(320 CPUS). I will continue to debug why.
>
> Could you try this patch? This is needed because PREEMPT_AUTO turns on
> CONFIG_PREEMPTION, but not CONFIG_PREEMPT:
>
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index eca293794a1e..599410050f6b 100644
> --- a/arch/powerpc/kernel/interrupt.c
> +++ b/arch/powerpc/kernel/interrupt.c
> @@ -396,7 +396,7 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
> /* Returning to a kernel context with local irqs enabled. */
> WARN_ON_ONCE(!(regs->msr & MSR_EE));
> again:
> - if (IS_ENABLED(CONFIG_PREEMPT)) {
> + if (IS_ENABLED(CONFIG_PREEMPTION)) {
> /* Return to preemptible kernel context */
> if (unlikely(read_thread_flags() & _TIF_NEED_RESCHED)) {
> if (preempt_count() == 0)
>
>
> --
> ankur
This patch can be considered as the enablement patch for Powerpc for preempt_auto.
Michael, Nick, Do you see any concerns?
Ankur, Could you please add this patch, if there are no objections.
---
From 878a5a7c990e3459758a5d19d7697b07d8d27d0f Mon Sep 17 00:00:00 2001
From: Shrikanth Hegde <sshegde@linux.ibm.com>
Date: Tue, 7 May 2024 04:42:04 -0500
Subject: [PATCH] powerpc: add support for preempt_auto
Add PowerPC arch support for PREEMPT_AUTO by defining LAZY bits.
Since PowerPC doesn't use generic exit to functions, Add
NR_LAZY check in exit to user and exit to kernel from interrupt
routines.
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
arch/powerpc/Kconfig | 1 +
arch/powerpc/include/asm/thread_info.h | 11 ++++++++++-
arch/powerpc/kernel/interrupt.c | 6 ++++--
3 files changed, 15 insertions(+), 3 deletions(-)
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 1c4be3373686..11e7008f5dd3 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -268,6 +268,7 @@ config PPC
select HAVE_PERF_EVENTS_NMI if PPC64
select HAVE_PERF_REGS
select HAVE_PERF_USER_STACK_DUMP
+ select HAVE_PREEMPT_AUTO
select HAVE_REGS_AND_STACK_ACCESS_API
select HAVE_RELIABLE_STACKTRACE
select HAVE_RSEQ
diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
index 15c5691dd218..227b9273e2e9 100644
--- a/arch/powerpc/include/asm/thread_info.h
+++ b/arch/powerpc/include/asm/thread_info.h
@@ -118,10 +118,19 @@ void arch_setup_new_exec(void);
#define TIF_POLLING_NRFLAG 19 /* true if poll_idle() is polling TIF_NEED_RESCHED */
#define TIF_32BIT 20 /* 32 bit binary */
+#ifdef CONFIG_PREEMPT_AUTO
+#define TIF_NEED_RESCHED_LAZY 21 /* Lazy rescheduling */
+#endif
+
/* as above, but as bit values */
#define _TIF_SYSCALL_TRACE (1<<TIF_SYSCALL_TRACE)
#define _TIF_SIGPENDING (1<<TIF_SIGPENDING)
#define _TIF_NEED_RESCHED (1<<TIF_NEED_RESCHED)
+
+#ifdef CONFIG_PREEMPT_AUTO
+#define _TIF_NEED_RESCHED_LAZY (1 << TIF_NEED_RESCHED_LAZY)
+#endif
+
#define _TIF_NOTIFY_SIGNAL (1<<TIF_NOTIFY_SIGNAL)
#define _TIF_POLLING_NRFLAG (1<<TIF_POLLING_NRFLAG)
#define _TIF_32BIT (1<<TIF_32BIT)
@@ -144,7 +153,7 @@ void arch_setup_new_exec(void);
#define _TIF_USER_WORK_MASK (_TIF_SIGPENDING | _TIF_NEED_RESCHED | \
_TIF_NOTIFY_RESUME | _TIF_UPROBE | \
_TIF_RESTORE_TM | _TIF_PATCH_PENDING | \
- _TIF_NOTIFY_SIGNAL)
+ _TIF_NOTIFY_SIGNAL | _TIF_NEED_RESCHED_LAZY)
#define _TIF_PERSYSCALL_MASK (_TIF_RESTOREALL|_TIF_NOERROR)
/* Bits in local_flags */
diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index eca293794a1e..0c0b7010995a 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -185,7 +185,7 @@ interrupt_exit_user_prepare_main(unsigned long ret, struct pt_regs *regs)
ti_flags = read_thread_flags();
while (unlikely(ti_flags & (_TIF_USER_WORK_MASK & ~_TIF_RESTORE_TM))) {
local_irq_enable();
- if (ti_flags & _TIF_NEED_RESCHED) {
+ if (ti_flags & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)) {
schedule();
} else {
/*
@@ -396,7 +396,9 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
/* Returning to a kernel context with local irqs enabled. */
WARN_ON_ONCE(!(regs->msr & MSR_EE));
again:
- if (IS_ENABLED(CONFIG_PREEMPT)) {
+
+ if ((IS_ENABLED(CONFIG_PREEMPT_AUTO) && IS_ENABLED(CONFIG_PREEMPTION)) ||
+ (!IS_ENABLED(CONFIG_PREEMPT_AUTO) && IS_ENABLED(CONFIG_PREEMPT))) {
/* Return to preemptible kernel context */
if (unlikely(read_thread_flags() & _TIF_NEED_RESCHED)) {
if (preempt_count() == 0)
--
2.39.3
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