linux-kernel - Re: Runtime overhead of PREEMPT

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Message-ID: <ZTKDWnLrSnPs9VUi@ariel>
Date:   Fri, 20 Oct 2023 15:40:42 +0200
From:   Emanuele Rocca <ema@...ian.org>
To:     Mike Galbraith <efault@....de>
Cc:     linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.com>
Subject: Re: Runtime overhead of PREEMPT_DYNAMIC

Hi Mike,

On 2023-10-20 02:42, Mike Galbraith wrote:
> It's kinda hard to imagine the author's mailbox surviving submission
> day were it _that_ horrid, surely something is amiss.

Thanks for trying to reproduce! Let's see if we can find out what's
wrong in my setup then.

> voluntary (my usual)
> CONFIG_PREEMPT_VOLUNTARY_BUILD=y
> # CONFIG_PREEMPT_NONE is not set
> CONFIG_PREEMPT_VOLUNTARY=y
> # CONFIG_PREEMPT is not set
> # CONFIG_PREEMPT_DYNAMIC is not set
> 
> dynamic (same config, just flip dynamic switch)
> CONFIG_PREEMPT_BUILD=y
> # CONFIG_PREEMPT_NONE is not set
> CONFIG_PREEMPT_VOLUNTARY=y
> # CONFIG_PREEMPT is not set
> CONFIG_PREEMPT_COUNT=y
> CONFIG_PREEMPTION=y
> CONFIG_PREEMPT_DYNAMIC=y

Here is the full diff between the kernel configurations I used. The only
change I made was setting CONFIG_PREEMPT_DYNAMIC=n, everything else was
a consequence of that AFAICT, but please do let me know if you see
anything that shouldn't be there.

--- config-6.5.0-0.preempt-dynamic-amd64	2023-10-11 15:30:02.000000000 +0200
+++ config-6.5.0-0.a.test-amd64	2023-10-11 14:30:02.000000000 +0200
@@ -125,13 +125,11 @@
 CONFIG_BPF_LSM=y
 # end of BPF subsystem
 
-CONFIG_PREEMPT_BUILD=y
+CONFIG_PREEMPT_VOLUNTARY_BUILD=y
 # CONFIG_PREEMPT_NONE is not set
 CONFIG_PREEMPT_VOLUNTARY=y
 # CONFIG_PREEMPT is not set
-CONFIG_PREEMPT_COUNT=y
-CONFIG_PREEMPTION=y
-CONFIG_PREEMPT_DYNAMIC=y
+# CONFIG_PREEMPT_DYNAMIC is not set
 # CONFIG_SCHED_CORE is not set
 
 #
@@ -156,11 +154,9 @@
 # RCU Subsystem
 #
 CONFIG_TREE_RCU=y
-CONFIG_PREEMPT_RCU=y
 # CONFIG_RCU_EXPERT is not set
 CONFIG_TREE_SRCU=y
 CONFIG_TASKS_RCU_GENERIC=y
-CONFIG_TASKS_RCU=y
 CONFIG_TASKS_RUDE_RCU=y
 CONFIG_TASKS_TRACE_RCU=y
 CONFIG_RCU_STALL_COMMON=y
@@ -1009,7 +1005,11 @@
 CONFIG_PREEMPT_NOTIFIERS=y
 CONFIG_PADATA=y
 CONFIG_ASN1=y
-CONFIG_UNINLINE_SPIN_UNLOCK=y
+CONFIG_INLINE_SPIN_UNLOCK_IRQ=y
+CONFIG_INLINE_READ_UNLOCK=y
+CONFIG_INLINE_READ_UNLOCK_IRQ=y
+CONFIG_INLINE_WRITE_UNLOCK=y
+CONFIG_INLINE_WRITE_UNLOCK_IRQ=y
 CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=y
 CONFIG_MUTEX_SPIN_ON_OWNER=y
 CONFIG_RWSEM_SPIN_ON_OWNER=y
@@ -5395,7 +5395,6 @@
 CONFIG_MEDIA_CEC_SUPPORT=y
 # CONFIG_CEC_CH7322 is not set
 # CONFIG_CEC_CROS_EC is not set
-# CONFIG_CEC_GPIO is not set
 CONFIG_CEC_SECO=m
 # CONFIG_CEC_SECO_RC is not set
 CONFIG_USB_PULSE8_CEC=m
@@ -10597,7 +10596,6 @@
 # end of Scheduler Debugging
 
 # CONFIG_DEBUG_TIMEKEEPING is not set
-CONFIG_DEBUG_PREEMPT=y
 
 #
 # Lock Debugging (spinlocks, mutexes, etc...)
@@ -10696,7 +10694,6 @@
 # CONFIG_FUNCTION_PROFILER is not set
 CONFIG_STACK_TRACER=y
 # CONFIG_IRQSOFF_TRACER is not set
-# CONFIG_PREEMPT_TRACER is not set
 # CONFIG_SCHED_TRACER is not set
 # CONFIG_HWLAT_TRACER is not set
 # CONFIG_OSNOISE_TRACER is not set