linux-kernel - [tip: perf/core] perf/x86/amd/lbr: Adjust LBR regardless of filtering

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Date:   Fri, 30 Sep 2022 09:31:00 -0000
From:   "tip-bot2 for Stephane Eranian" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Stephane Eranian <eranian@...gle.com>,
        "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/lbr: Adjust LBR regardless of filtering

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

Commit-ID:     3f9a1b3591003b122a6ea2d69f89a0fd96ec58b9
Gitweb:        https://git.kernel.org/tip/3f9a1b3591003b122a6ea2d69f89a0fd96ec58b9
Author:        Stephane Eranian <eranian@...gle.com>
AuthorDate:    Wed, 28 Sep 2022 11:40:43 -07:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 29 Sep 2022 12:20:57 +02:00

perf/x86/amd/lbr: Adjust LBR regardless of filtering

In case of fused compare and taken branch instructions, the AMD LBR points to
the compare instruction instead of the branch. Users of LBR usually expects
the from address to point to a branch instruction. The kernel has code to
adjust the from address via get_branch_type_fused(). However this correction
is only applied when a branch filter is applied. That means that if no
filter is present, the quality of the data is lower.

Fix the problem by applying the adjustment regardless of the filter setting,
bringing the AMD LBR to the same level as other LBR implementations.

Fixes: 245268c19f70 ("perf/x86/amd/lbr: Use fusion-aware branch classifier")
Signed-off-by: Stephane Eranian <eranian@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Sandipan Das <sandipan.das@....com>
Link: https://lore.kernel.org/r/20220928184043.408364-3-eranian@google.com
---
 arch/x86/events/amd/lbr.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
index 2e1c157..38a7521 100644
--- a/arch/x86/events/amd/lbr.c
+++ b/arch/x86/events/amd/lbr.c
@@ -99,12 +99,13 @@ static void amd_pmu_lbr_filter(void)
 	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
 	int br_sel = cpuc->br_sel, offset, type, i, j;
 	bool compress = false;
+	bool fused_only = false;
 	u64 from, to;
 
 	/* If sampling all branches, there is nothing to filter */
 	if (((br_sel & X86_BR_ALL) == X86_BR_ALL) &&
 	    ((br_sel & X86_BR_TYPE_SAVE) != X86_BR_TYPE_SAVE))
-		return;
+		fused_only = true;
 
 	for (i = 0; i < cpuc->lbr_stack.nr; i++) {
 		from = cpuc->lbr_entries[i].from;
@@ -116,8 +117,11 @@ static void amd_pmu_lbr_filter(void)
 		 * fusion where it points to an instruction preceding the
 		 * actual branch
 		 */
-		if (offset)
+		if (offset) {
 			cpuc->lbr_entries[i].from += offset;
+			if (fused_only)
+				continue;
+		}
 
 		/* If type does not correspond, then discard */
 		if (type == X86_BR_NONE || (br_sel & type) != type) {