linux-kernel - [PATCH 2/3] x86,tlb: add tracepoint for TLB flush IPI to stale CPU

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Message-ID: <20241109003727.3958374-3-riel@surriel.com>
Date: Fri,  8 Nov 2024 19:27:49 -0500
From: Rik van Riel <riel@...riel.com>
To: linux-kernel@...r.kernel.org
Cc: dave.hansen@...ux.intel.com,
	luto@...nel.org,
	peterz@...radead.org,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	x86@...nel.org,
	kernel-team@...a.com,
	hpa@...or.com,
	Rik van Riel <riel@...riel.com>,
	Dave Hansen <dave.hansen@...el.com>
Subject: [PATCH 2/3] x86,tlb: add tracepoint for TLB flush IPI to stale CPU

Add a tracepoint when we send a TLB flush IPI to a CPU that used
to be in the mm_cpumask, but isn't any more.

This can be used to evaluate whether there any workloads where
we end up in this path problematically often. Hopefully they
don't exist.

Suggested-by: Dave Hansen <dave.hansen@...el.com>
Signed-off-by: Rik van Riel <riel@...riel.com>
---
 arch/x86/mm/tlb.c        | 1 +
 include/linux/mm_types.h | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index f19f6378cabf..9d0d34576928 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -760,6 +760,7 @@ static void flush_tlb_func(void *info)
 		/* Can only happen on remote CPUs */
 		if (f->mm && f->mm != loaded_mm) {
 			cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(f->mm));
+			trace_tlb_flush(TLB_REMOTE_WRONG_CPU, 0);
 			return;
 		}
 	}
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6e3bdf8e38bc..6b6f05404304 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1335,6 +1335,7 @@ enum tlb_flush_reason {
 	TLB_LOCAL_SHOOTDOWN,
 	TLB_LOCAL_MM_SHOOTDOWN,
 	TLB_REMOTE_SEND_IPI,
+	TLB_REMOTE_WRONG_CPU,
 	NR_TLB_FLUSH_REASONS,
 };
 
-- 
2.45.2