linux-kernel - [tip: sched/core] sched,x86: Fix L2 cache mask

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Message-ID: <163492030498.626.18148681011754446836.tip-bot2@tip-bot2>
Date:   Fri, 22 Oct 2021 16:31:44 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched,x86: Fix L2 cache mask

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     55409ac5c371c6403012d5f4df5e7c6cf0e7dce6
Gitweb:        https://git.kernel.org/tip/55409ac5c371c6403012d5f4df5e7c6cf0e7dce6
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Fri, 22 Oct 2021 17:49:53 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 22 Oct 2021 18:21:28 +02:00

sched,x86: Fix L2 cache mask

Currently AMD/Hygon do not populate l2c_id, this means that for SMT
enabled systems they report an L2 per thread. This is ofcourse not
true but was harmless so far.

However, since commit: 66558b730f25 ("sched: Add cluster scheduler
level for x86") the scheduler topology setup requires:

  SMT <= L2 <= LLC

Which leads to noisy warnings and possibly weird behaviour on affected
chips.

Therefore change the topology generation such that if l2c_id is not
populated it follows the SMT topology, thereby satisfying the
constraint.

Fixes: 66558b730f25 ("sched: Add cluster scheduler level for x86")
Reported-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Tested-by: Tom Lendacky <thomas.lendacky@....com>
---
 arch/x86/kernel/smpboot.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 5094ab0..f80f459 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -470,9 +470,9 @@ static bool match_l2c(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
 {
 	int cpu1 = c->cpu_index, cpu2 = o->cpu_index;
 
-	/* Do not match if we do not have a valid APICID for cpu: */
+	/* If the arch didn't set up l2c_id, fall back to SMT */
 	if (per_cpu(cpu_l2c_id, cpu1) == BAD_APICID)
-		return false;
+		return match_smt(c, o);
 
 	/* Do not match if L2 cache id does not match: */
 	if (per_cpu(cpu_l2c_id, cpu1) != per_cpu(cpu_l2c_id, cpu2))