linux-kernel - [RFC 1/2] x86/bugs: Disable coresched on hardware that does not need it

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Date:   Wed, 11 Nov 2020 16:10:10 -0500
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, torvalds@...ux-foundation.org,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, joel@...lfernandes.org,
        vineeth@...byteword.org, Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        amistry@...gle.com, Borislav Petkov <bp@...en8.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>, Mike Rapoport <rppt@...nel.org>,
        thomas.lendacky@....com, Tony Luck <tony.luck@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        x86@...nel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT))
Subject: [RFC 1/2] x86/bugs: Disable coresched on hardware that does not need it

Some hardware such as certain AMD variants don't have cross-HT MDS/L1TF
issues. Detect this and don't enable core scheduling as it can
needlessly slow the device done.

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 arch/x86/kernel/cpu/bugs.c | 8 ++++++++
 kernel/sched/core.c        | 7 +++++++
 kernel/sched/sched.h       | 5 +++++
 3 files changed, 20 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index dece79e4d1e9..0e6e61e49b23 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -152,6 +152,14 @@ void __init check_bugs(void)
 #endif
 }
 
+/*
+ * Do not need core scheduling if CPU does not have MDS/L1TF vulnerability.
+ */
+int arch_allow_core_sched(void)
+{
+	return boot_cpu_has_bug(X86_BUG_MDS) || boot_cpu_has_bug(X86_BUG_L1TF);
+}
+
 void
 x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest)
 {
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 64c559192634..c6158b4959fe 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -319,6 +319,13 @@ static void __sched_core_enable(void)
 	for_each_online_cpu(cpu)
 		BUG_ON(!sched_core_empty(cpu_rq(cpu)));
 
+	/*
+	 * Some architectures may not want coresched. (ex, AMD does not have
+	 * MDS/L1TF issues so it wants SMT completely on).
+	 */
+	if (!arch_allow_core_sched())
+		return;
+
 	static_branch_enable(&__sched_core_enabled);
 	stop_machine(__sched_core_stopper, (void *)true, NULL);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3cf08c77b678..a1b39764a6ed 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1203,6 +1203,11 @@ int cpu_core_tag_color_write_u64(struct cgroup_subsys_state *css,
 
 bool cfs_prio_less(struct task_struct *a, struct task_struct *b, bool fi);
 
+int __weak arch_allow_core_sched(void)
+{
+	return true;
+}
+
 #else /* !CONFIG_SCHED_CORE */
 
 static inline bool sched_core_enqueued(struct task_struct *task) { return false; }
-- 
2.29.2.222.g5d2a92d10f8-goog