Re: [PATCH] kvm: x86: mmu: Make NX huge page recovery period configurable

Paolo Bonzini <pbonzini@xxxxxxxxxx> · Sat, 16 Oct 2021 09:00:22 +0200

On 16/10/21 02:50, Junaid Shahid wrote:
Currently, the NX huge page recovery thread wakes up every minute and
zaps 1/nx_huge_pages_recovery_ratio of the total number of split NX
huge pages at a time. This is intended to ensure that only a
relatively small number of pages get zapped at a time. But for very
large VMs (or more specifically, VMs with a large number of
executable pages), a period of 1 minute could still result in this
number being too high (unless the ratio is changed significantly,
but that can result in split pages lingering on for too long).

This change makes the period configurable instead of fixing it at
1 minute (though that is still kept as the default). Users of
large VMs can then adjust both the ratio and the period together to
reduce the number of pages zapped at one time while still
maintaining the same overall duration for cycling through the
entire list (e.g. the period could be set to 1 second and the ratio
to 3600 to maintain the overall cycling period of 1 hour).

The patch itself looks good, but perhaps the default (corresponding to a 
period of 0) could be 3600000 / ratio, so that it's enough to adjust the 
ratio?

Paolo


Signed-off-by: Junaid Shahid <junaids@xxxxxxxxxx>
---
  .../admin-guide/kernel-parameters.txt         |  8 ++++-
  arch/x86/kvm/mmu/mmu.c                        | 33 +++++++++++++------
  2 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 91ba391f9b32..8e2b426726e5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2353,7 +2353,13 @@
  			[KVM] Controls how many 4KiB pages are periodically zapped
  			back to huge pages.  0 disables the recovery, otherwise if
  			the value is N KVM will zap 1/Nth of the 4KiB pages every
-			minute.  The default is 60.
+			period (see below).  The default is 60.
+
+	kvm.nx_huge_pages_recovery_period_ms=
+			[KVM] Controls the time period at which KVM zaps 4KiB pages
+			back to huge pages. 0 disables the recovery, otherwise if
+			the value is N, KVM will zap a portion (see ratio above) of
+			the pages every N msecs. The default is 60000 (i.e. 1 min).
  
  	kvm-amd.nested=	[KVM,AMD] Allow nested virtualization in KVM/SVM.
  			Default is 1 (enabled)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 24a9f4c3f5e7..47e113fc05ab 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -61,28 +61,33 @@ int __read_mostly nx_huge_pages = -1;
  #ifdef CONFIG_PREEMPT_RT
  /* Recovery can cause latency spikes, disable it for PREEMPT_RT.  */
  static uint __read_mostly nx_huge_pages_recovery_ratio = 0;
+static uint __read_mostly nx_huge_pages_recovery_period_ms = 0;
  #else
  static uint __read_mostly nx_huge_pages_recovery_ratio = 60;
+static uint __read_mostly nx_huge_pages_recovery_period_ms = 60000;
  #endif
  
  static int set_nx_huge_pages(const char *val, const struct kernel_param *kp);
-static int set_nx_huge_pages_recovery_ratio(const char *val, const struct kernel_param *kp);
+static int set_nx_huge_pages_recovery_param(const char *val, const struct kernel_param *kp);
  
  static const struct kernel_param_ops nx_huge_pages_ops = {
  	.set = set_nx_huge_pages,
  	.get = param_get_bool,
  };
  
-static const struct kernel_param_ops nx_huge_pages_recovery_ratio_ops = {
-	.set = set_nx_huge_pages_recovery_ratio,
+static const struct kernel_param_ops nx_huge_pages_recovery_param_ops = {
+	.set = set_nx_huge_pages_recovery_param,
  	.get = param_get_uint,
  };
  
  module_param_cb(nx_huge_pages, &nx_huge_pages_ops, &nx_huge_pages, 0644);
  __MODULE_PARM_TYPE(nx_huge_pages, "bool");
-module_param_cb(nx_huge_pages_recovery_ratio, &nx_huge_pages_recovery_ratio_ops,
+module_param_cb(nx_huge_pages_recovery_ratio, &nx_huge_pages_recovery_param_ops,
  		&nx_huge_pages_recovery_ratio, 0644);
  __MODULE_PARM_TYPE(nx_huge_pages_recovery_ratio, "uint");
+module_param_cb(nx_huge_pages_recovery_period_ms, &nx_huge_pages_recovery_param_ops,
+		&nx_huge_pages_recovery_period_ms, 0644);
+__MODULE_PARM_TYPE(nx_huge_pages_recovery_period_ms, "uint");
  
  static bool __read_mostly force_flush_and_sync_on_reuse;
  module_param_named(flush_on_reuse, force_flush_and_sync_on_reuse, bool, 0644);
@@ -6088,18 +6093,23 @@ void kvm_mmu_module_exit(void)
  	mmu_audit_disable();
  }
  
-static int set_nx_huge_pages_recovery_ratio(const char *val, const struct kernel_param *kp)
+static int set_nx_huge_pages_recovery_param(const char *val, const struct kernel_param *kp)
  {
-	unsigned int old_val;
+	bool was_recovery_enabled, is_recovery_enabled;
  	int err;
  
-	old_val = nx_huge_pages_recovery_ratio;
+	was_recovery_enabled = nx_huge_pages_recovery_ratio &&
+			       nx_huge_pages_recovery_period_ms;
+
  	err = param_set_uint(val, kp);
  	if (err)
  		return err;
  
+	is_recovery_enabled = nx_huge_pages_recovery_ratio &&
+			      nx_huge_pages_recovery_period_ms;
+
  	if (READ_ONCE(nx_huge_pages) &&
-	    !old_val && nx_huge_pages_recovery_ratio) {
+	    !was_recovery_enabled && is_recovery_enabled) {
  		struct kvm *kvm;
  
  		mutex_lock(&kvm_lock);
@@ -6162,8 +6172,11 @@ static void kvm_recover_nx_lpages(struct kvm *kvm)
  
  static long get_nx_lpage_recovery_timeout(u64 start_time)
  {
-	return READ_ONCE(nx_huge_pages) && READ_ONCE(nx_huge_pages_recovery_ratio)
-		? start_time + 60 * HZ - get_jiffies_64()
+	uint ratio = READ_ONCE(nx_huge_pages_recovery_ratio);
+	uint period = READ_ONCE(nx_huge_pages_recovery_period_ms);
+
+	return READ_ONCE(nx_huge_pages) && ratio && period
+		? start_time + msecs_to_jiffies(period) - get_jiffies_64()
  		: MAX_SCHEDULE_TIMEOUT;
  }
  








