[PATCH] timekeeping:add padding in timekeeper for Unixbench pipe

[Guo Hui]

When the LLC cache line size is 128 bytes, such as Kunpeng 920,
the seq attribute and xtime attribute in the structure tk_core
are completely in the same LLC cache line,
and xtime_sec is the data protected by the seq lock
in the function ktime_get_coarse_real_ts64,
so seq and xtime_sec are in the same LLC cache line
causing the false sharing problem.

Adding padding before xtime_sec in the structure timekeeper
is based on the comment of the structure tk_read_base: "This
struct has size 56 byte on 64 bit. Together with a seqcount
it occupies a single 64byte cache line." Therefore,
seq and the structure tk_read_base
should be placed in the same 64-byte cacheline.

The performance data of Unixbench pipe on Kunpeng 920 is as follows:

Enable the LSE instruction:
seq and xtime are in the same LLC cache line:
System Benchmarks Partial Index              BASELINE       RESULT    INDEX
Pipe Throughput                               12440.0   14800574.4  11897.6
Pipe-based Context Switching                   4000.0    4357419.0  10893.5
                                                                   ========
System Benchmarks Index Score (Partial Only)                        11384.5

seq and xtime are not in the same LLC cache line:
System Benchmarks Partial Index              BASELINE       RESULT    INDEX
Pipe Throughput                               12440.0   16546306.6  13300.9
Pipe-based Context Switching                   4000.0    5654281.8  14135.7
                                                                   ========
System Benchmarks Index Score (Partial Only)                        13711.9

When the LSE instruction is enabled,
Pipe Throughput increases by 11.79%,
and Pipe-based Context Switching increases by 29.76%.

Close the LSE instruction:
seq and xtime are in the same LLC cache line:
System Benchmarks Partial Index              BASELINE       RESULT    INDEX
Pipe Throughput                               12440.0   36375286.5  29240.6
Pipe-based Context Switching                   4000.0   11994739.7  29986.8
                                                                   ========
System Benchmarks Index Score (Partial Only)                        29611.4

seq and xtime are not in the same LLC cache line:
System Benchmarks Partial Index              BASELINE       RESULT    INDEX
Pipe Throughput                               12440.0   44887148.8  36082.9
Pipe-based Context Switching                   4000.0   13666392.0  34166.0
                                                                   ========
System Benchmarks Index Score (Partial Only)                        35111.4

When the LSE instruction is disabled,
Pipe Throughput increases by 23.40%,
and Pipe-based Context Switching increases by 13.94%.

Signed-off-by: Guo Hui <guohui@xxxxxxxxxxxxx>
---
 include/linux/timekeeper_internal.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index 84ff2844d..d363cd1f3 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -92,6 +92,7 @@ struct tk_read_base {
 struct timekeeper {
 	struct tk_read_base	tkr_mono;
 	struct tk_read_base	tkr_raw;
+	u64			padding;
 	u64			xtime_sec;
 	unsigned long		ktime_sec;
 	struct timespec64	wall_to_monotonic;
-- 
2.20.1