mm: vmscan: support setting of kswapd cpu affinity

Allow the kswapd cpu affinity to be configured.
There can be power benefits on certain targets when limiting kswapd
to run only on certain cores.

CRs-fixed: 752344
Change-Id: I8a83337ff313a7e0324361140398226a09f8be0f
Signed-off-by: Liam Mark <lmark@codeaurora.org>
[imaund@codeaurora.org: Resolved trivial context conflicts.]
Signed-off-by: Ian Maund <imaund@codeaurora.org>

mm/Kconfig

@@ -628,6 +628,19 @@ config BALANCE_ANON_FILE_RECLAIM
 	  Swapping anonymous pages out to memory can be efficient enough to justify
 	  treating anonymous and file backed pages equally.
 
+config KSWAPD_CPU_AFFINITY_MASK
+	string "kswapd cpu affinity mask"
+	depends on SMP
+	help
+	  Set the cpu affinity for the kswapd task.
+	  There can be power benefits on certain targets when limiting kswapd
+	  to run only on certain cores.
+	  The cpu affinity bitmask is represented by a hex string where commas
+	  group hex digits into chunks.  Each chunk defines exactly 32 bits of
+	  the resultant bitmask.
+	  For example to limit kswapd to the first 4 cores use the following:
+	  CONFIG_KSWAPD_CPU_AFFINITY_MASK="f"
+
 # For architectures that support deferred memory initialisation
 config ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
 	bool

mm/vmscan.c

@@ -147,6 +147,12 @@ int vm_swappiness = 60;
  */
 unsigned long vm_total_pages;
 
+#ifdef CONFIG_KSWAPD_CPU_AFFINITY_MASK
+char *kswapd_cpu_mask = CONFIG_KSWAPD_CPU_AFFINITY_MASK;
+#else
+char *kswapd_cpu_mask = NULL;
+#endif
+
 static LIST_HEAD(shrinker_list);
 static DECLARE_RWSEM(shrinker_rwsem);
 
@@ -3485,7 +3491,7 @@ static int kswapd(void *p)
 
 	lockdep_set_current_reclaim_state(GFP_KERNEL);
 
-	if (!cpumask_empty(cpumask))
+	if (kswapd_cpu_mask == NULL && !cpumask_empty(cpumask))
 		set_cpus_allowed_ptr(tsk, cpumask);
 	current->reclaim_state = &reclaim_state;
 
@@ -3655,6 +3661,22 @@ static int cpu_callback(struct notifier_block *nfb, unsigned long action,
 	return NOTIFY_OK;
 }
 
+static int set_kswapd_cpu_mask(pg_data_t *pgdat)
+{
+	int ret = 0;
+	cpumask_t tmask;
+
+	if (!kswapd_cpu_mask)
+		return 0;
+
+	cpumask_clear(&tmask);
+	ret = cpumask_parse(kswapd_cpu_mask, &tmask);
+	if (ret)
+		return ret;
+
+	return set_cpus_allowed_ptr(pgdat->kswapd, &tmask);
+}
+
 /*
  * This kswapd start function will be called by init and node-hot-add.
  * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
@@ -3674,6 +3696,9 @@ int kswapd_run(int nid)
 		pr_err("Failed to start kswapd on node %d\n", nid);
 		ret = PTR_ERR(pgdat->kswapd);
 		pgdat->kswapd = NULL;
+	} else if (kswapd_cpu_mask) {
+		if (set_kswapd_cpu_mask(pgdat))
+			pr_warn("error setting kswapd cpu affinity mask\n");
 	}
 	return ret;
 }
@@ -3699,7 +3724,8 @@ static int __init kswapd_init(void)
 	swap_setup();
 	for_each_node_state(nid, N_MEMORY)
  		kswapd_run(nid);
-	hotcpu_notifier(cpu_callback, 0);
+	if (kswapd_cpu_mask == NULL)
+		hotcpu_notifier(cpu_callback, 0);
 	return 0;
 }