linux-kernel - Re: [PATCH] perf/x86: Add cond_resched() when allocate/release DS buffers

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Message-ID: <20251028152747.GR4068168@noisy.programming.kicks-ass.net>
Date: Tue, 28 Oct 2025 16:27:47 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Ingo Molnar <mingo@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH] perf/x86: Add cond_resched() when allocate/release DS
 buffers

On Tue, Oct 28, 2025 at 03:15:18PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 27, 2025 at 02:27:24PM -0700, Namhyung Kim wrote:
> > On large systems, it's possible to trigger sched latency warning during
> > the DS buffer allocation or release.  Add cond_resched() to avoid it.
> 
> We're >.< close to deleting cond_resched(), it makes absolutely no sense
> adding more.

Specifically, IIRC the plan was to do something like the below after the
next LTS release, and then continue to remove VOLUNTARY in subsequent
releases, leaving NONE the only option for the legacy architectures that
do not support preemption.


diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index da326800c1c9..db4ae53c1d49 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -16,11 +16,12 @@ config ARCH_HAS_PREEMPT_LAZY
 
 choice
 	prompt "Preemption Model"
-	default PREEMPT_NONE
+	default PREEMPT_LAZY
 
 config PREEMPT_NONE
 	bool "No Forced Preemption (Server)"
 	depends on !PREEMPT_RT
+	depends on ARCH_NO_PREEMPT
 	select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC
 	help
 	  This is the traditional Linux preemption model, geared towards
@@ -35,8 +36,8 @@ config PREEMPT_NONE
 
 config PREEMPT_VOLUNTARY
 	bool "Voluntary Kernel Preemption (Desktop)"
-	depends on !ARCH_NO_PREEMPT
 	depends on !PREEMPT_RT
+	depends on ARCH_NO_PREEMPT
 	select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC
 	help
 	  This option reduces the latency of the kernel by adding more