linux-kernel - [PATCH RFC 1/6] perf: Increase the maximum number of samples to 256.

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Date: Wed, 29 May 2024 19:53:32 +0100
From: Rajnesh Kanwal <rkanwal@...osinc.com>
To: linux-kernel@...r.kernel.org
Cc: linux-perf-users@...r.kernel.org,
	linux-riscv@...ts.infradead.org,
	adrian.hunter@...el.com,
	alexander.shishkin@...ux.intel.com,
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	atishp@...osinc.com,
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	tech-control-transfer-records@...ts.riscv.org,
	will@...nel.org,
	kaiwenxue1@...il.com,
	Rajnesh Kanwal <rkanwal@...osinc.com>
Subject: [PATCH RFC 1/6] perf: Increase the maximum number of samples to 256.

RISCV CTR extension support a maximum depth of 256 last branch records.
The 127 entries limit results in corrupting CTR entries for RISC-V if
configured to be 256 entries. This will not impact any other architectures
as it is just increasing maximum limit of possible entries.

Signed-off-by: Rajnesh Kanwal <rkanwal@...osinc.com>
---
 tools/perf/util/machine.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index 527517db3182..ec12f0199d46 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -2254,25 +2254,32 @@ static void save_iterations(struct iterations *iter,
 		iter->cycles += be[i].flags.cycles;
 }
 
-#define CHASHSZ 127
-#define CHASHBITS 7
-#define NO_ENTRY 0xff
+#define CHASHBITS 8
+#define NO_ENTRY 0xffU
 
-#define PERF_MAX_BRANCH_DEPTH 127
+#define PERF_MAX_BRANCH_DEPTH 256
 
 /* Remove loops. */
+/* Note: Last entry (i==ff) will never be checked against NO_ENTRY
+ * so it's safe to have an unsigned char array to process 256 entries
+ * without causing clash between last entry and NO_ENTRY value.
+ */
 static int remove_loops(struct branch_entry *l, int nr,
 			struct iterations *iter)
 {
 	int i, j, off;
-	unsigned char chash[CHASHSZ];
+	unsigned char chash[PERF_MAX_BRANCH_DEPTH];
 
 	memset(chash, NO_ENTRY, sizeof(chash));
 
-	BUG_ON(PERF_MAX_BRANCH_DEPTH > 255);
+	BUG_ON(PERF_MAX_BRANCH_DEPTH > 256);
 
 	for (i = 0; i < nr; i++) {
-		int h = hash_64(l[i].from, CHASHBITS) % CHASHSZ;
+		/* Remainder division by PERF_MAX_BRANCH_DEPTH is not
+		 * needed as hash_64 will anyway limit the hash
+		 * to CHASHBITS
+		 */
+		int h = hash_64(l[i].from, CHASHBITS);
 
 		/* no collision handling for now */
 		if (chash[h] == NO_ENTRY) {
-- 
2.34.1