linux-kernel - [PATCH 3/3] IPI: Avoid to use 2 cache lines for one call_single

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Date:   Wed,  2 Aug 2017 16:52:20 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Huang Ying <ying.huang@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juergen Gross <jgross@...e.com>, Aaron Lu <aaron.lu@...el.com>
Subject: [PATCH 3/3] IPI: Avoid to use 2 cache lines for one call_single_data

From: Huang Ying <ying.huang@...el.com>

struct call_single_data is used in IPI to transfer information between
CPUs.  Its size is bigger than sizeof(unsigned long) and less than
cache line size.  Now, it is allocated with no any alignment
requirement.  This makes it possible for allocated call_single_data to
cross 2 cache lines.  So that double the number of the cache lines
that need to be transferred among CPUs.  This is resolved by aligning
the allocated call_single_data with cache line size.

To test the effect of the patch, we use the vm-scalability multiple
thread swap test case (swap-w-seq-mt).  The test will create multiple
threads and each thread will eat memory until all RAM and part of swap
is used, so that huge number of IPI will be triggered when unmapping
memory.  In the test, the throughput of memory writing improves ~5%
compared with misaligned call_single_data because of faster IPI.

Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Borislav Petkov <bp@...e.de>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Juergen Gross <jgross@...e.com>
Cc: Aaron Lu <aaron.lu@...el.com>
---
 kernel/smp.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 3061483cb3ad..81d9ae08eb6e 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -51,7 +51,7 @@ int smpcfd_prepare_cpu(unsigned int cpu)
 		free_cpumask_var(cfd->cpumask);
 		return -ENOMEM;
 	}
-	cfd->csd = alloc_percpu(struct call_single_data);
+	cfd->csd = alloc_percpu_aligned(struct call_single_data);
 	if (!cfd->csd) {
 		free_cpumask_var(cfd->cpumask);
 		free_cpumask_var(cfd->cpumask_ipi);
@@ -269,7 +269,9 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
 			     int wait)
 {
 	struct call_single_data *csd;
-	struct call_single_data csd_stack = { .flags = CSD_FLAG_LOCK | CSD_FLAG_SYNCHRONOUS };
+	struct call_single_data csd_stack ____cacheline_aligned = {
+		.flags = CSD_FLAG_LOCK | CSD_FLAG_SYNCHRONOUS
+	};
 	int this_cpu;
 	int err;

-- 
2.13.2