* [RFC PATCH] mm, swap: don't disable preemption while taking the per-CPU cache
@ 2017-06-23 10:12 Sebastian Andrzej Siewior
2017-06-23 10:34 ` Michal Hocko
0 siblings, 1 reply; 6+ messages in thread
From: Sebastian Andrzej Siewior @ 2017-06-23 10:12 UTC (permalink / raw)
To: Tim Chen; +Cc: tglx, ying.huang, Michal Hocko, linux-mm
get_cpu_var() disables preemption and returns the per-CPU version of the
variable. Disabling preemption is useful to ensure atomic access to the
variable within the critical section.
In this case however, after the per-CPU version of the variable is
obtained the ->free_lock is acquired. For that reason it seems the raw
accessor could be used. It only seems that ->slots_ret should be
retested (because with disabled preemption this variable can not be set
to NULL otherwise).
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
mm/swap_slots.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/mm/swap_slots.c b/mm/swap_slots.c
index 58f6c78f1dad..51c304477482 100644
--- a/mm/swap_slots.c
+++ b/mm/swap_slots.c
@@ -272,11 +272,11 @@ int free_swap_slot(swp_entry_t entry)
{
struct swap_slots_cache *cache;
- cache = &get_cpu_var(swp_slots);
+ cache = raw_cpu_ptr(&swp_slots);
if (use_swap_slot_cache && cache->slots_ret) {
spin_lock_irq(&cache->free_lock);
/* Swap slots cache may be deactivated before acquiring lock */
- if (!use_swap_slot_cache) {
+ if (!use_swap_slot_cache || !cache->slots_ret) {
spin_unlock_irq(&cache->free_lock);
goto direct_free;
}
@@ -296,7 +296,6 @@ int free_swap_slot(swp_entry_t entry)
direct_free:
swapcache_free_entries(&entry, 1);
}
- put_cpu_var(swp_slots);
return 0;
}
--
2.13.1
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^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [RFC PATCH] mm, swap: don't disable preemption while taking the per-CPU cache
2017-06-23 10:12 [RFC PATCH] mm, swap: don't disable preemption while taking the per-CPU cache Sebastian Andrzej Siewior
@ 2017-06-23 10:34 ` Michal Hocko
2017-06-23 11:47 ` [PATCH] " Sebastian Andrzej Siewior
0 siblings, 1 reply; 6+ messages in thread
From: Michal Hocko @ 2017-06-23 10:34 UTC (permalink / raw)
To: Sebastian Andrzej Siewior; +Cc: Tim Chen, tglx, ying.huang, linux-mm
On Fri 23-06-17 12:12:54, Sebastian Andrzej Siewior wrote:
> get_cpu_var() disables preemption and returns the per-CPU version of the
> variable. Disabling preemption is useful to ensure atomic access to the
> variable within the critical section.
> In this case however, after the per-CPU version of the variable is
> obtained the ->free_lock is acquired. For that reason it seems the raw
> accessor could be used. It only seems that ->slots_ret should be
> retested (because with disabled preemption this variable can not be set
> to NULL otherwise).
The changelog doesn't explain, why does this change matter. Disabling
preemption shortly before taking a spinlock shouldn't make much
difference. I suspect you care because of RT, right? In that case spell
that in the changelog and explain why it matters.
Other than hat the patch looks good to me.
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> mm/swap_slots.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/mm/swap_slots.c b/mm/swap_slots.c
> index 58f6c78f1dad..51c304477482 100644
> --- a/mm/swap_slots.c
> +++ b/mm/swap_slots.c
> @@ -272,11 +272,11 @@ int free_swap_slot(swp_entry_t entry)
> {
> struct swap_slots_cache *cache;
>
> - cache = &get_cpu_var(swp_slots);
> + cache = raw_cpu_ptr(&swp_slots);
> if (use_swap_slot_cache && cache->slots_ret) {
> spin_lock_irq(&cache->free_lock);
> /* Swap slots cache may be deactivated before acquiring lock */
> - if (!use_swap_slot_cache) {
> + if (!use_swap_slot_cache || !cache->slots_ret) {
> spin_unlock_irq(&cache->free_lock);
> goto direct_free;
> }
> @@ -296,7 +296,6 @@ int free_swap_slot(swp_entry_t entry)
> direct_free:
> swapcache_free_entries(&entry, 1);
> }
> - put_cpu_var(swp_slots);
>
> return 0;
> }
> --
> 2.13.1
>
--
Michal Hocko
SUSE Labs
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^ permalink raw reply [flat|nested] 6+ messages in thread* [PATCH] mm, swap: don't disable preemption while taking the per-CPU cache
2017-06-23 10:34 ` Michal Hocko
@ 2017-06-23 11:47 ` Sebastian Andrzej Siewior
2017-06-23 12:02 ` Michal Hocko
0 siblings, 1 reply; 6+ messages in thread
From: Sebastian Andrzej Siewior @ 2017-06-23 11:47 UTC (permalink / raw)
To: Michal Hocko; +Cc: Tim Chen, tglx, ying.huang, linux-mm, Andrew Morton
get_cpu_var() disables preemption and returns the per-CPU version of the
variable. Disabling preemption is useful to ensure atomic access to the
variable within the critical section.
In this case however, after the per-CPU version of the variable is
obtained the ->free_lock is acquired. For that reason it seems the raw
accessor could be used. It only seems that ->slots_ret should be
retested (because with disabled preemption this variable can not be set
to NULL otherwise).
This popped up during PREEMPT-RT testing because it tries to take
spinlocks in a preempt disabled section.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
On 2017-06-23 12:34:23 [+0200], Michal Hocko wrote:
> The changelog doesn't explain, why does this change matter. Disabling
> preemption shortly before taking a spinlock shouldn't make much
> difference. I suspect you care because of RT, right? In that case spell
> that in the changelog and explain why it matters.
yes, it is bad for RT. I added the RT pieces as explanation.
> Other than hat the patch looks good to me.
Thank you. +akpm.
mm/swap_slots.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/mm/swap_slots.c b/mm/swap_slots.c
index 58f6c78f1dad..51c304477482 100644
--- a/mm/swap_slots.c
+++ b/mm/swap_slots.c
@@ -272,11 +272,11 @@ int free_swap_slot(swp_entry_t entry)
{
struct swap_slots_cache *cache;
- cache = &get_cpu_var(swp_slots);
+ cache = raw_cpu_ptr(&swp_slots);
if (use_swap_slot_cache && cache->slots_ret) {
spin_lock_irq(&cache->free_lock);
/* Swap slots cache may be deactivated before acquiring lock */
- if (!use_swap_slot_cache) {
+ if (!use_swap_slot_cache || !cache->slots_ret) {
spin_unlock_irq(&cache->free_lock);
goto direct_free;
}
@@ -296,7 +296,6 @@ int free_swap_slot(swp_entry_t entry)
direct_free:
swapcache_free_entries(&entry, 1);
}
- put_cpu_var(swp_slots);
return 0;
}
--
2.13.1
--
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^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] mm, swap: don't disable preemption while taking the per-CPU cache
2017-06-23 11:47 ` [PATCH] " Sebastian Andrzej Siewior
@ 2017-06-23 12:02 ` Michal Hocko
2017-06-23 12:08 ` Sebastian Andrzej Siewior
0 siblings, 1 reply; 6+ messages in thread
From: Michal Hocko @ 2017-06-23 12:02 UTC (permalink / raw)
To: Sebastian Andrzej Siewior
Cc: Tim Chen, tglx, ying.huang, linux-mm, Andrew Morton
On Fri 23-06-17 13:47:55, Sebastian Andrzej Siewior wrote:
> get_cpu_var() disables preemption and returns the per-CPU version of the
> variable. Disabling preemption is useful to ensure atomic access to the
> variable within the critical section.
> In this case however, after the per-CPU version of the variable is
> obtained the ->free_lock is acquired. For that reason it seems the raw
> accessor could be used. It only seems that ->slots_ret should be
> retested (because with disabled preemption this variable can not be set
> to NULL otherwise).
> This popped up during PREEMPT-RT testing because it tries to take
> spinlocks in a preempt disabled section.
Ohh, because the spinlock can sleep with PREEMPT-RT right? Don't we have
much more places like that? It is perfectly valid to take a spinlock
while the preemption is disabled. E.g. we do take ptl lock inside
kmap_atomic sections which disables preemption on 32b systems.
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Michal Hocko <mhocko@suse.com>
> ---
> On 2017-06-23 12:34:23 [+0200], Michal Hocko wrote:
> > The changelog doesn't explain, why does this change matter. Disabling
> > preemption shortly before taking a spinlock shouldn't make much
> > difference. I suspect you care because of RT, right? In that case spell
> > that in the changelog and explain why it matters.
>
> yes, it is bad for RT. I added the RT pieces as explanation.
>
> > Other than hat the patch looks good to me.
>
> Thank you. +akpm.
>
> mm/swap_slots.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/mm/swap_slots.c b/mm/swap_slots.c
> index 58f6c78f1dad..51c304477482 100644
> --- a/mm/swap_slots.c
> +++ b/mm/swap_slots.c
> @@ -272,11 +272,11 @@ int free_swap_slot(swp_entry_t entry)
> {
> struct swap_slots_cache *cache;
>
> - cache = &get_cpu_var(swp_slots);
> + cache = raw_cpu_ptr(&swp_slots);
> if (use_swap_slot_cache && cache->slots_ret) {
> spin_lock_irq(&cache->free_lock);
> /* Swap slots cache may be deactivated before acquiring lock */
> - if (!use_swap_slot_cache) {
> + if (!use_swap_slot_cache || !cache->slots_ret) {
> spin_unlock_irq(&cache->free_lock);
> goto direct_free;
> }
> @@ -296,7 +296,6 @@ int free_swap_slot(swp_entry_t entry)
> direct_free:
> swapcache_free_entries(&entry, 1);
> }
> - put_cpu_var(swp_slots);
>
> return 0;
> }
> --
> 2.13.1
>
--
Michal Hocko
SUSE Labs
--
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^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] mm, swap: don't disable preemption while taking the per-CPU cache
2017-06-23 12:02 ` Michal Hocko
@ 2017-06-23 12:08 ` Sebastian Andrzej Siewior
2017-06-23 12:13 ` Michal Hocko
0 siblings, 1 reply; 6+ messages in thread
From: Sebastian Andrzej Siewior @ 2017-06-23 12:08 UTC (permalink / raw)
To: Michal Hocko; +Cc: Tim Chen, tglx, ying.huang, linux-mm, Andrew Morton
On 2017-06-23 14:02:33 [+0200], Michal Hocko wrote:
> On Fri 23-06-17 13:47:55, Sebastian Andrzej Siewior wrote:
> > get_cpu_var() disables preemption and returns the per-CPU version of the
> > variable. Disabling preemption is useful to ensure atomic access to the
> > variable within the critical section.
> > In this case however, after the per-CPU version of the variable is
> > obtained the ->free_lock is acquired. For that reason it seems the raw
> > accessor could be used. It only seems that ->slots_ret should be
> > retested (because with disabled preemption this variable can not be set
> > to NULL otherwise).
> > This popped up during PREEMPT-RT testing because it tries to take
> > spinlocks in a preempt disabled section.
>
> Ohh, because the spinlock can sleep with PREEMPT-RT right? Don't we have
yup.
> much more places like that? It is perfectly valid to take a spinlock
well we have more than just this one patch to fix things like that :)
The easy/simple things (like this one which is valid in RT and !RT) I
try to push upstream asap and the other remain in the RT tree.
> while the preemption is disabled. E.g. we do take ptl lock inside
> kmap_atomic sections which disables preemption on 32b systems.
we don't disable preemption in kmap_atomic(). It would be bad :)
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>
> Acked-by: Michal Hocko <mhocko@suse.com>
Thanks.
Sebastian
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] mm, swap: don't disable preemption while taking the per-CPU cache
2017-06-23 12:08 ` Sebastian Andrzej Siewior
@ 2017-06-23 12:13 ` Michal Hocko
0 siblings, 0 replies; 6+ messages in thread
From: Michal Hocko @ 2017-06-23 12:13 UTC (permalink / raw)
To: Sebastian Andrzej Siewior
Cc: Tim Chen, tglx, ying.huang, linux-mm, Andrew Morton
On Fri 23-06-17 14:08:42, Sebastian Andrzej Siewior wrote:
> On 2017-06-23 14:02:33 [+0200], Michal Hocko wrote:
> > On Fri 23-06-17 13:47:55, Sebastian Andrzej Siewior wrote:
> > > get_cpu_var() disables preemption and returns the per-CPU version of the
> > > variable. Disabling preemption is useful to ensure atomic access to the
> > > variable within the critical section.
> > > In this case however, after the per-CPU version of the variable is
> > > obtained the ->free_lock is acquired. For that reason it seems the raw
> > > accessor could be used. It only seems that ->slots_ret should be
> > > retested (because with disabled preemption this variable can not be set
> > > to NULL otherwise).
> > > This popped up during PREEMPT-RT testing because it tries to take
> > > spinlocks in a preempt disabled section.
> >
> > Ohh, because the spinlock can sleep with PREEMPT-RT right? Don't we have
> yup.
>
> > much more places like that? It is perfectly valid to take a spinlock
> well we have more than just this one patch to fix things like that :)
> The easy/simple things (like this one which is valid in RT and !RT) I
> try to push upstream asap and the other remain in the RT tree.
yeah, makes sense to me.
> > while the preemption is disabled. E.g. we do take ptl lock inside
> > kmap_atomic sections which disables preemption on 32b systems.
> we don't disable preemption in kmap_atomic(). It would be bad :)
Ohh, I didn't know about that.
> > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> >
> > Acked-by: Michal Hocko <mhocko@suse.com>
> Thanks.
>
> Sebastian
--
Michal Hocko
SUSE Labs
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^ permalink raw reply [flat|nested] 6+ messages in thread
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2017-06-23 10:12 [RFC PATCH] mm, swap: don't disable preemption while taking the per-CPU cache Sebastian Andrzej Siewior
2017-06-23 10:34 ` Michal Hocko
2017-06-23 11:47 ` [PATCH] " Sebastian Andrzej Siewior
2017-06-23 12:02 ` Michal Hocko
2017-06-23 12:08 ` Sebastian Andrzej Siewior
2017-06-23 12:13 ` Michal Hocko
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