From: Ingo Molnar <mingo@xxxxxxx> Subject: mm/vmstat: protect per cpu variables with preempt disable on RT Disable preemption on -RT for the vmstat code. On vanila the code runs in IRQ-off regions while on -RT it may not when stats are updated under a local_lock. "preempt_disable" ensures that the same resources is not updated in parallel due to preemption. This patch differs from the preempt-rt version where __count_vm_event and __count_vm_events are also protected. The counters are explicitly "allowed to be to be racy" so there is no need to protect them from preemption. Only the accurate page stats that are updated by a read-modify-write need protection. This patch also differs in that a preempt_[en|dis]able_rt helper is not used. As vmstat is the only user of the helper, it was suggested that it be open-coded in vmstat.c instead of risking the helper being used in unnecessary contexts. Link: https://lkml.kernel.org/r/20210805160019.1137-2-mgorman@xxxxxxxxxxxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/vmstat.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 48 insertions(+) --- a/mm/vmstat.c~mm-vmstat-protect-per-cpu-variables-with-preempt-disable-on-rt +++ a/mm/vmstat.c @@ -319,6 +319,16 @@ void __mod_zone_page_state(struct zone * long x; long t; + /* + * Accurate vmstat updates require a RMW. On !PREEMPT_RT kernels, + * atomicity is provided by IRQs being disabled -- either explicitly + * or via local_lock_irq. On PREEMPT_RT, local_lock_irq only disables + * CPU migrations and preemption potentially corrupts a counter so + * disable preemption. + */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_disable(); + x = delta + __this_cpu_read(*p); t = __this_cpu_read(pcp->stat_threshold); @@ -328,6 +338,9 @@ void __mod_zone_page_state(struct zone * x = 0; } __this_cpu_write(*p, x); + + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_enable(); } EXPORT_SYMBOL(__mod_zone_page_state); @@ -350,6 +363,10 @@ void __mod_node_page_state(struct pglist delta >>= PAGE_SHIFT; } + /* See __mod_node_page_state */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_disable(); + x = delta + __this_cpu_read(*p); t = __this_cpu_read(pcp->stat_threshold); @@ -359,6 +376,9 @@ void __mod_node_page_state(struct pglist x = 0; } __this_cpu_write(*p, x); + + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_enable(); } EXPORT_SYMBOL(__mod_node_page_state); @@ -391,6 +411,10 @@ void __inc_zone_state(struct zone *zone, s8 __percpu *p = pcp->vm_stat_diff + item; s8 v, t; + /* See __mod_node_page_state */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_disable(); + v = __this_cpu_inc_return(*p); t = __this_cpu_read(pcp->stat_threshold); if (unlikely(v > t)) { @@ -399,6 +423,9 @@ void __inc_zone_state(struct zone *zone, zone_page_state_add(v + overstep, zone, item); __this_cpu_write(*p, -overstep); } + + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_enable(); } void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item) @@ -409,6 +436,10 @@ void __inc_node_state(struct pglist_data VM_WARN_ON_ONCE(vmstat_item_in_bytes(item)); + /* See __mod_node_page_state */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_disable(); + v = __this_cpu_inc_return(*p); t = __this_cpu_read(pcp->stat_threshold); if (unlikely(v > t)) { @@ -417,6 +448,9 @@ void __inc_node_state(struct pglist_data node_page_state_add(v + overstep, pgdat, item); __this_cpu_write(*p, -overstep); } + + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_enable(); } void __inc_zone_page_state(struct page *page, enum zone_stat_item item) @@ -437,6 +471,10 @@ void __dec_zone_state(struct zone *zone, s8 __percpu *p = pcp->vm_stat_diff + item; s8 v, t; + /* See __mod_node_page_state */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_disable(); + v = __this_cpu_dec_return(*p); t = __this_cpu_read(pcp->stat_threshold); if (unlikely(v < - t)) { @@ -445,6 +483,9 @@ void __dec_zone_state(struct zone *zone, zone_page_state_add(v - overstep, zone, item); __this_cpu_write(*p, overstep); } + + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_enable(); } void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item) @@ -455,6 +496,10 @@ void __dec_node_state(struct pglist_data VM_WARN_ON_ONCE(vmstat_item_in_bytes(item)); + /* See __mod_node_page_state */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_disable(); + v = __this_cpu_dec_return(*p); t = __this_cpu_read(pcp->stat_threshold); if (unlikely(v < - t)) { @@ -463,6 +508,9 @@ void __dec_node_state(struct pglist_data node_page_state_add(v - overstep, pgdat, item); __this_cpu_write(*p, overstep); } + + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + preempt_enable(); } void __dec_zone_page_state(struct page *page, enum zone_stat_item item) _
