diff mbox series

[v12,2/6] mm/vmstat: Use vmstat_dirty to track CPU-specific vmstat discrepancies

Message ID 20221227121341.981235373@redhat.com
State New
Headers
Series Ensure quiet_vmstat() is called when returning to userpace and when idle tick is stopped |

Commit Message

Marcelo Tosatti Dec. 27, 2022, 12:11 p.m. UTC 
  From: Aaron Tomlin <atomlin@atomlin.com>

This patch will now use the previously introduced CPU-specific variable
namely vmstat_dirty to indicate if a vmstat differential/or imbalance is
present for a given CPU. So, at the appropriate time, vmstat processing can
be initiated. The hope is that this particular approach is "cheaper" when
compared to need_update(). The idea is based on Marcelo's patch [1].

[1]: https://lore.kernel.org/lkml/20220204173554.763888172@fedora.localdomain/

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
---
 mm/vmstat.c |   48 ++++++++++++++----------------------------------
 1 file changed, 14 insertions(+), 34 deletions(-)

Comments

Frederic Weisbecker Dec. 30, 2022, 1:21 p.m. UTC | #1 
On Tue, Dec 27, 2022 at 09:11:39AM -0300, Marcelo Tosatti wrote:
> @@ -606,6 +608,7 @@ static inline void mod_zone_state(struct
>  
>  	if (z)
>  		zone_page_state_add(z, zone, item);
> +	vmstat_mark_dirty();
>  }
>  
>  void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
> @@ -674,6 +677,7 @@ static inline void mod_node_state(struct
>  
>  	if (z)
>  		node_page_state_add(z, pgdat, item);
> +	vmstat_mark_dirty();

Looking at this further, about the two above chunks, there is a risk to
mark the wrong CPU dirty because those functions are preemptible and rely
on this_cpu_cmpxchg() to deal with preemption.

Thanks.
Marcelo Tosatti Jan. 1, 2023, 12:38 a.m. UTC | #2 
On Fri, Dec 30, 2022 at 02:21:32PM +0100, Frederic Weisbecker wrote:
> On Tue, Dec 27, 2022 at 09:11:39AM -0300, Marcelo Tosatti wrote:
> > @@ -606,6 +608,7 @@ static inline void mod_zone_state(struct
> >  
> >  	if (z)
> >  		zone_page_state_add(z, zone, item);
> > +	vmstat_mark_dirty();
> >  }
> >  
> >  void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
> > @@ -674,6 +677,7 @@ static inline void mod_node_state(struct
> >  
> >  	if (z)
> >  		node_page_state_add(z, pgdat, item);
> > +	vmstat_mark_dirty();
> 
> Looking at this further, about the two above chunks, there is a risk to
> mark the wrong CPU dirty because those functions are preemptible and rely
> on this_cpu_cmpxchg() to deal with preemption.
> 
> Thanks.

Hi Frederic,

Yes, good catch: if the CPU is preempted after this_cpu_cmpxchg(),
but before vmstat_mark_dirty, then one ends up with a CPU with
per-CPU vm counters dirty and the per-CPU vmstat dirty bit unset.

This could cause a CPU to remain with the per-CPU vm counters dirty
for longer than sysctl_stat_interval.

Should move vmstat_mark_dirty() above "if (z)", then do
preempt_disable() on function entry and preempt_enable()
after vmstat_mark_dirty. Luckily preempt_disable()/preempt_enable()
is much cheaper than local_irq_disable()/local_irq_enable().
Aaron Tomlin Jan. 4, 2023, 1:34 p.m. UTC | #3 
On Fri 2022-12-30 14:21 +0100, Frederic Weisbecker wrote:
> On Tue, Dec 27, 2022 at 09:11:39AM -0300, Marcelo Tosatti wrote:
> > @@ -606,6 +608,7 @@ static inline void mod_zone_state(struct
> >  
> >  	if (z)
> >  		zone_page_state_add(z, zone, item);
> > +	vmstat_mark_dirty();
> >  }
> >  
> >  void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
> > @@ -674,6 +677,7 @@ static inline void mod_node_state(struct
> >  
> >  	if (z)
> >  		node_page_state_add(z, pgdat, item);
> > +	vmstat_mark_dirty();
> 
> Looking at this further, about the two above chunks, there is a risk to
> mark the wrong CPU dirty because those functions are preemptible and rely
> on this_cpu_cmpxchg() to deal with preemption.

Hi Frederic,

Agreed.


Kind regards,
diff mbox series

Patch

Index: linux-2.6/mm/vmstat.c
===================================================================
--- linux-2.6.orig/mm/vmstat.c
+++ linux-2.6/mm/vmstat.c
@@ -381,6 +381,7 @@  void __mod_zone_page_state(struct zone *
 		x = 0;
 	}
 	__this_cpu_write(*p, x);
+	vmstat_mark_dirty();
 
 	preempt_enable_nested();
 }
@@ -417,6 +418,7 @@  void __mod_node_page_state(struct pglist
 		x = 0;
 	}
 	__this_cpu_write(*p, x);
+	vmstat_mark_dirty();
 
 	preempt_enable_nested();
 }
@@ -606,6 +608,7 @@  static inline void mod_zone_state(struct
 
 	if (z)
 		zone_page_state_add(z, zone, item);
+	vmstat_mark_dirty();
 }
 
 void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
@@ -674,6 +677,7 @@  static inline void mod_node_state(struct
 
 	if (z)
 		node_page_state_add(z, pgdat, item);
+	vmstat_mark_dirty();
 }
 
 void mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
@@ -828,6 +832,14 @@  static int refresh_cpu_vm_stats(bool do_
 	int global_node_diff[NR_VM_NODE_STAT_ITEMS] = { 0, };
 	int changes = 0;
 
+	/*
+	 * Clear vmstat_dirty before clearing the percpu vmstats.
+	 * If interrupts are enabled, it is possible that an interrupt
+	 * or another task modifies a percpu vmstat, which will
+	 * set vmstat_dirty to true.
+	 */
+	vmstat_clear_dirty();
+
 	for_each_populated_zone(zone) {
 		struct per_cpu_zonestat __percpu *pzstats = zone->per_cpu_zonestats;
 #ifdef CONFIG_NUMA
@@ -1957,35 +1969,6 @@  static void vmstat_update(struct work_st
 }
 
 /*
- * Check if the diffs for a certain cpu indicate that
- * an update is needed.
- */
-static bool need_update(int cpu)
-{
-	pg_data_t *last_pgdat = NULL;
-	struct zone *zone;
-
-	for_each_populated_zone(zone) {
-		struct per_cpu_zonestat *pzstats = per_cpu_ptr(zone->per_cpu_zonestats, cpu);
-		struct per_cpu_nodestat *n;
-
-		/*
-		 * The fast way of checking if there are any vmstat diffs.
-		 */
-		if (memchr_inv(pzstats->vm_stat_diff, 0, sizeof(pzstats->vm_stat_diff)))
-			return true;
-
-		if (last_pgdat == zone->zone_pgdat)
-			continue;
-		last_pgdat = zone->zone_pgdat;
-		n = per_cpu_ptr(zone->zone_pgdat->per_cpu_nodestats, cpu);
-		if (memchr_inv(n->vm_node_stat_diff, 0, sizeof(n->vm_node_stat_diff)))
-			return true;
-	}
-	return false;
-}
-
-/*
  * Switch off vmstat processing and then fold all the remaining differentials
  * until the diffs stay at zero. The function is used by NOHZ and can only be
  * invoked when tick processing is not active.
@@ -1995,10 +1978,7 @@  void quiet_vmstat(void)
 	if (system_state != SYSTEM_RUNNING)
 		return;
 
-	if (!delayed_work_pending(this_cpu_ptr(&vmstat_work)))
-		return;
-
-	if (!need_update(smp_processor_id()))
+	if (!is_vmstat_dirty())
 		return;
 
 	/*
@@ -2029,7 +2009,7 @@  static void vmstat_shepherd(struct work_
 	for_each_online_cpu(cpu) {
 		struct delayed_work *dw = &per_cpu(vmstat_work, cpu);
 
-		if (!delayed_work_pending(dw) && need_update(cpu))
+		if (!delayed_work_pending(dw) && per_cpu(vmstat_dirty, cpu))
 			queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
 
 		cond_resched();