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From:   Guo Hui <guohui@uniontech.com>
To:     peterz@infradead.org, mingo@redhat.com, will@kernel.org,
        longman@redhat.com, boqun.feng@gmail.com
Cc:     linux-kernel@vger.kernel.org, wangxiaohua@uniontech.com,
        Guo Hui <guohui@uniontech.com>
Subject: [PATCH v2] locking/osq_lock: Fix false sharing of optimistic_spin_node in osq_lock
Date:   Thu, 15 Jun 2023 20:01:52 +0800
Message-Id: <20230615120152.20836-1-guohui@uniontech.com>
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For the performance of osq_lock,
I have made a patch before:

https://lore.kernel.org/lkml/20220628161251.21950-1-guohui@uniontech.com/

the analysis conclusion is due to the memory access
of the following code caused performance degradation:

cpu = node->cpu - 1;

The instructions corresponding to the C code are:
mov 0x14(%rax),%edi
sub $0x1,%edi

in the X86 operating environment,
causing high cache-misses and degrading performance.

The memory access instructions that cause performance degradation are
further analyzed.The cache-misses of the above instructions are caused
by a large number of cache-line false sharing
when accessing non-local-CPU variables,as follows:

         ---------------------------------
         |   struct optimistic_spin_node |
         ---------------------------------
         | next | prev | locked |  cpu   |
         ---------------------------------
         |        cache line             |
         ---------------------------------
         |          CPU0                 |
         ---------------------------------

When a CPU other than CPU0 reads the value of
optimistic_spin_node->cpu of CPU0,CPU0 frequently modifies
the data of the cache line,which will cause false sharing
on the currently accessing CPU,and the variable of
the structure optimistic_spin_node type will be
defined as a cacheline alignmented per cpu variable,
each optimistic_spin_node variable is bound to the corresponding CPU core:

    DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);

Therefore, the value of optimistic_spin_node->cpu is usually unchanged,
so the false sharing caused by access to optimistic_spin_node->cpu
is caused by frequent modification of the other three attributes
of optimistic_spin_node.

There are two solutions as follows:

solution 1:
Put the cpu attribute of optimistic_spin_node into a cacheline separately.
The patch is as follows：

 struct optimistic_spin_node {
        struct optimistic_spin_node *next, *prev;
        int locked; /* 1 if lock acquired */
-       int cpu; /* encoded CPU # + 1 value */
+       int cpu ____cacheline_aligned; /* encoded CPU # + 1 value */
 };

Unixbench full-core performance data is as follows:
Machine: Hygon X86, 128 cores
                                    with patch   without patch  promote
Dhrystone 2 using register variables   194923.07    195091      -0.09%
Double-Precision Whetstone             79885.47     79838.87    +0.06%
Execl Throughput                       2327.17      2272.1      +2.42%
File Copy 1024 bufsize 2000 maxblocks  742.1        687.53      +7.94%
File Copy 256 bufsize 500 maxblocks    462.73       428.03      +8.11%
File Copy 4096 bufsize 8000 maxblocks  1600.37      1520.53     +5.25%
Pipe Throughput                        79815.33     79522.13    +0.37%
Pipe-based Context Switching           28962.9      27987.8     +3.48%
Process Creation                       3084.4       2999.1      +2.84%
Shell Scripts 1 concurrent             11687.1      11394.67    +2.57%
Shell Scripts 8 concurrent             10787.1      10496.17    +2.77%
System Call Overhead                   4322.77      4322.23     +0.01%
System Benchmarks Index Score          8079.4       7848.37     +3.0%

solution 2:
The core idea of osq lock is that
the lock applicant spins on the local-CPU variable
to eliminate cache-line bouncing.Therefore,
the same method is used for the above degradation.
For the optimistic_spin_node of the current CPU,
the cpu attribute of its predecessor optimistic_spin_node is
non-local-CPU variables,the cpu attribute of
the predecessor optimistic_spin_node is cached
in the optimistic_spin_node of the current CPU
to eliminate performance degradation
caused by non-local-CPU variable access, as follows:

        bool osq_lock(struct optimistic_spin_queue *lock)
        {
                [... ...]

                node->prev = prev;
                node->prev_cpu = prev->cpu; --------------- A

                [... ...]

                WRITE_ONCE(next->prev, prev);
                WRITE_ONCE(prev->next, next);
                WRITE_ONCE(next->prev_cpu, prev->cpu); -------------- B

                [... ...]
        }

        static inline int node_cpu(struct optimistic_spin_node *node)
        {
                return node->prev_cpu - 1; ----------------------------- C
        }

While setting the prev attribute of the optimistic_spin_node of
the current CPU,the current patch also caches the prev cpu attribute
in the prev_cpu attribute of the optimistic_spin_node of the current CPU,
as in the above code lines A and B,where node is a per cpu variable,
so each one node corresponds to a CPU core,and the cpu attribute of
the node corresponding to the CPU core will not change.
Only when the prev attribute of the node is set,
the prev_cpu of the node may change with the change of prev.
At other times, the prev attribute of the node will not change.
so the prev_cpu of node will not change.
This patch greatly reduces the non-local-CPU variable
access at code line C and improves performance.

Unixbench full-core performance data is as follows:
Machine: Hygon X86, 128 cores
                                    with patch   without patch   promote
Dhrystone 2 using register variables  194818.7     195091        -0.14%
Double-Precision Whetstone            79847.57     79838.87      +0.01%
Execl Throughput                      2372.83      2272.1        +4.43%
File Copy 1024 bufsize 2000 maxblocks 765          687.53        +11.27%
File Copy 256 bufsize 500 maxblocks   472.13       428.03        +10.30%
File Copy 4096 bufsize 8000 maxblocks 1658.13      1520.53       +9.05%
Pipe Throughput                       79634.17     79522.13      +0.14%
Pipe-based Context Switching          28584.7      27987.8       +2.13%
Process Creation                      3020.27      2999.1        +0.71%
Shell Scripts 1 concurrent            11890.87     11394.67      +4.35%
Shell Scripts 8 concurrent            10912.9      10496.17      +3.97%
System Call Overhead                  4320.63      4322.23       -0.04%
System Benchmarks Index Score         8144.43      7848.37       +4.0%

In summary, the performance of solution 2 is better than solution 1.
Especially use cases: execl, file copy, shell1, shell8,
great improvement,because solution 1 still has the possibility of
remote memory access across NUMA nodes,
and solution 2 completely accesses local-CPU variables,
so solution 2 is better than solution 1.

Both solutions also have a great improvement in the X86 virtual machine.

The current patch also uses solution 2.

Signed-off-by: Guo Hui <guohui@uniontech.com>
---
 include/linux/osq_lock.h  | 1 +
 kernel/locking/osq_lock.c | 8 +++++---
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
index 5581dbd3bd34..8a1bb36f4a07 100644
--- a/include/linux/osq_lock.h
+++ b/include/linux/osq_lock.h
@@ -10,6 +10,7 @@ struct optimistic_spin_node {
 	struct optimistic_spin_node *next, *prev;
 	int locked; /* 1 if lock acquired */
 	int cpu; /* encoded CPU # + 1 value */
+	int prev_cpu; /* Only for optimizing false sharing */
 };
 
 struct optimistic_spin_queue {
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index d5610ad52b92..ea1fdd10ab3e 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -22,9 +22,9 @@ static inline int encode_cpu(int cpu_nr)
 	return cpu_nr + 1;
 }
 
-static inline int node_cpu(struct optimistic_spin_node *node)
+static inline int node_prev_cpu(struct optimistic_spin_node *node)
 {
-	return node->cpu - 1;
+	return READ_ONCE(node->prev_cpu) - 1;
 }
 
 static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
@@ -110,6 +110,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 
 	prev = decode_cpu(old);
 	node->prev = prev;
+	node->prev_cpu = prev->cpu;
 
 	/*
 	 * osq_lock()			unqueue
@@ -141,7 +142,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 * polling, be careful.
 	 */
 	if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
-				  vcpu_is_preempted(node_cpu(node->prev))))
+				  vcpu_is_preempted(node_prev_cpu(node))))
 		return true;
 
 	/* unqueue */
@@ -200,6 +201,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 
 	WRITE_ONCE(next->prev, prev);
 	WRITE_ONCE(prev->next, next);
+	WRITE_ONCE(next->prev_cpu, prev->cpu);
 
 	return false;
 }
-- 
2.20.1