linux-kernel - [tip: perf/core] perf/x86/amd: Warn only on new bits set

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Message-ID: <172838550950.1442.11370441097683144204.tip-bot2@tip-bot2>
Date: Tue, 08 Oct 2024 11:05:09 -0000
From: "tip-bot2 for Breno Leitao" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Paul E. McKenney" <paulmck@...nel.org>, Breno Leitao <leitao@...ian.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Sandipan Das <sandipan.das@....com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: perf/core] perf/x86/amd: Warn only on new bits set

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

Commit-ID:     de20037e1b3c2f2ca97b8c12b8c7bca8abd509a7
Gitweb:        https://git.kernel.org/tip/de20037e1b3c2f2ca97b8c12b8c7bca8abd509a7
Author:        Breno Leitao <leitao@...ian.org>
AuthorDate:    Tue, 01 Oct 2024 07:10:19 -07:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Mon, 07 Oct 2024 09:28:46 +02:00

perf/x86/amd: Warn only on new bits set

Warning at every leaking bits can cause a flood of message, triggering
various stall-warning mechanisms to fire, including CSD locks, which
makes the machine to be unusable.

Track the bits that are being leaked, and only warn when a new bit is
set.

That said, this patch will help with the following issues:

1) It will tell us which bits are being set, so, it is easy to
   communicate it back to vendor, and to do a root-cause analyzes.

2) It avoid the machine to be unusable, because, worst case
   scenario, the user gets less than 60 WARNs (one per unhandled bit).

Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Breno Leitao <leitao@...ian.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Sandipan Das <sandipan.das@....com>
Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
Link: https://lkml.kernel.org/r/20241001141020.2620361-1-leitao@debian.org
---
 arch/x86/events/amd/core.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
index 920e3a6..b4a1a25 100644
--- a/arch/x86/events/amd/core.c
+++ b/arch/x86/events/amd/core.c
@@ -943,11 +943,12 @@ static int amd_pmu_v2_snapshot_branch_stack(struct perf_branch_entry *entries, u
 static int amd_pmu_v2_handle_irq(struct pt_regs *regs)
 {
 	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+	static atomic64_t status_warned = ATOMIC64_INIT(0);
+	u64 reserved, status, mask, new_bits, prev_bits;
 	struct perf_sample_data data;
 	struct hw_perf_event *hwc;
 	struct perf_event *event;
 	int handled = 0, idx;
-	u64 reserved, status, mask;
 	bool pmu_enabled;

 	/*
@@ -1012,7 +1013,12 @@ static int amd_pmu_v2_handle_irq(struct pt_regs *regs)
 	 * the corresponding PMCs are expected to be inactive according to the
 	 * active_mask
 	 */
-	WARN_ON(status > 0);
+	if (status > 0) {
+		prev_bits = atomic64_fetch_or(status, &status_warned);
+		// A new bit was set for the very first time.
+		new_bits = status & ~prev_bits;
+		WARN(new_bits, "New overflows for inactive PMCs: %llx\n", new_bits);
+	}

 	/* Clear overflow and freeze bits */
 	amd_pmu_ack_global_status(~status);