xen/events: fix setting irq affinity

[PATCH] xen/events: fix setting irq affinity

Posted by Juergen Gross 3 years ago

The backport of upstream patch 25da4618af240fbec61 ("xen/events: don't
unmask an event channel when an eoi is pending") introduced a
regression for stable kernels 5.10 and older: setting IRQ affinity for
IRQs related to interdomain events would no longer work, as moving the
IRQ to its new cpu was not included in the irq_ack callback for those
events.

Fix that by adding the needed call.

Note that kernels 5.11 and later don't need the explicit moving of the
IRQ to the target cpu in the irq_ack callback, due to a rework of the
affinity setting in kernel 5.11.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
This patch should be applied to all stable kernel branches up to
(including) linux-5.10.y, where upstream patch 25da4618af240fbec61 has
been added.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 drivers/xen/events/events_base.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
index 7bd03f6e0422..ee5269331406 100644
--- a/drivers/xen/events/events_base.c
+++ b/drivers/xen/events/events_base.c
@@ -1809,7 +1809,7 @@ static void lateeoi_ack_dynirq(struct irq_data *data)
 
 	if (VALID_EVTCHN(evtchn)) {
 		do_mask(info, EVT_MASK_REASON_EOI_PENDING);
-		event_handler_exit(info);
+		ack_dynirq(data);
 	}
 }
 
@@ -1820,7 +1820,7 @@ static void lateeoi_mask_ack_dynirq(struct irq_data *data)
 
 	if (VALID_EVTCHN(evtchn)) {
 		do_mask(info, EVT_MASK_REASON_EXPLICIT);
-		event_handler_exit(info);
+		ack_dynirq(data);
 	}
 }
 
-- 
2.26.2

Re: [PATCH] xen/events: fix setting irq affinity

Posted by Jan Beulich 3 years ago

On 12.04.2021 08:28, Juergen Gross wrote:
> The backport of upstream patch 25da4618af240fbec61 ("xen/events: don't
> unmask an event channel when an eoi is pending") introduced a
> regression for stable kernels 5.10 and older: setting IRQ affinity for
> IRQs related to interdomain events would no longer work, as moving the
> IRQ to its new cpu was not included in the irq_ack callback for those
> events.
> 
> Fix that by adding the needed call.
> 
> Note that kernels 5.11 and later don't need the explicit moving of the
> IRQ to the target cpu in the irq_ack callback, due to a rework of the
> affinity setting in kernel 5.11.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> This patch should be applied to all stable kernel branches up to
> (including) linux-5.10.y, where upstream patch 25da4618af240fbec61 has
> been added.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>

This looks functionally correct to me, so:
Reviewed-by: Jan Beulich <jbeulich@suse.com>

But I have remarks / questions:

> --- a/drivers/xen/events/events_base.c
> +++ b/drivers/xen/events/events_base.c
> @@ -1809,7 +1809,7 @@ static void lateeoi_ack_dynirq(struct irq_data *data)
>  
>  	if (VALID_EVTCHN(evtchn)) {
>  		do_mask(info, EVT_MASK_REASON_EOI_PENDING);
> -		event_handler_exit(info);
> +		ack_dynirq(data);
>  	}
>  }
>  
> @@ -1820,7 +1820,7 @@ static void lateeoi_mask_ack_dynirq(struct irq_data *data)
>  
>  	if (VALID_EVTCHN(evtchn)) {
>  		do_mask(info, EVT_MASK_REASON_EXPLICIT);
> -		event_handler_exit(info);
> +		ack_dynirq(data);
>  	}
>  }
>  

Can EVT_MASK_REASON_EOI_{PENDING,EXPLICIT} be cleared in a way racing
event_handler_exit() and (if it was called directly from here)
irq_move_masked_irq()? If not, the extra do_mask() / do_unmask() pair
(granted living on an "unlikely" path) could be avoided.

Even leaving aside the extra overhead in ack_dynirq()'s unlikely code
path, there's now some extra (redundant) processing. I guess this is
assumed to be within noise?

Possibly related, but first of all seeing the redundancy between
eoi_pirq() and ack_dynirq(): Wouldn't it make sense to break out the
common part into a helper? (Really the former could simply call the
latter as it seems.)

Jan

Re: [PATCH] xen/events: fix setting irq affinity

Posted by Juergen Gross 3 years ago

On 12.04.21 11:32, Jan Beulich wrote:
> On 12.04.2021 08:28, Juergen Gross wrote:
>> The backport of upstream patch 25da4618af240fbec61 ("xen/events: don't
>> unmask an event channel when an eoi is pending") introduced a
>> regression for stable kernels 5.10 and older: setting IRQ affinity for
>> IRQs related to interdomain events would no longer work, as moving the
>> IRQ to its new cpu was not included in the irq_ack callback for those
>> events.
>>
>> Fix that by adding the needed call.
>>
>> Note that kernels 5.11 and later don't need the explicit moving of the
>> IRQ to the target cpu in the irq_ack callback, due to a rework of the
>> affinity setting in kernel 5.11.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> This patch should be applied to all stable kernel branches up to
>> (including) linux-5.10.y, where upstream patch 25da4618af240fbec61 has
>> been added.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
> 
> This looks functionally correct to me, so:
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> 
> But I have remarks / questions:
> 
>> --- a/drivers/xen/events/events_base.c
>> +++ b/drivers/xen/events/events_base.c
>> @@ -1809,7 +1809,7 @@ static void lateeoi_ack_dynirq(struct irq_data *data)
>>   
>>   	if (VALID_EVTCHN(evtchn)) {
>>   		do_mask(info, EVT_MASK_REASON_EOI_PENDING);
>> -		event_handler_exit(info);
>> +		ack_dynirq(data);
>>   	}
>>   }
>>   
>> @@ -1820,7 +1820,7 @@ static void lateeoi_mask_ack_dynirq(struct irq_data *data)
>>   
>>   	if (VALID_EVTCHN(evtchn)) {
>>   		do_mask(info, EVT_MASK_REASON_EXPLICIT);
>> -		event_handler_exit(info);
>> +		ack_dynirq(data);
>>   	}
>>   }
>>   
> 
> Can EVT_MASK_REASON_EOI_{PENDING,EXPLICIT} be cleared in a way racing
> event_handler_exit() and (if it was called directly from here)
> irq_move_masked_irq()? If not, the extra do_mask() / do_unmask() pair
> (granted living on an "unlikely" path) could be avoided.

No, they can't race. And yes, the path is really unlikely, so I didn't
want to optimize this rare case.

> Even leaving aside the extra overhead in ack_dynirq()'s unlikely code
> path, there's now some extra (redundant) processing. I guess this is
> assumed to be within noise?

Yes. All required data should be in the caches already, and the extra
processing is only a few instructions.

> Possibly related, but first of all seeing the redundancy between
> eoi_pirq() and ack_dynirq(): Wouldn't it make sense to break out the
> common part into a helper? (Really the former could simply call the
> latter as it seems.)

In theory, yes. OTOH this no longer applies to upstream, so i dind't
bother doing that for stable.


Juergen

Re: [PATCH] xen/events: fix setting irq affinity

Posted by Jan Beulich 3 years ago

On 12.04.2021 11:39, Juergen Gross wrote:
> On 12.04.21 11:32, Jan Beulich wrote:
>> Possibly related, but first of all seeing the redundancy between
>> eoi_pirq() and ack_dynirq(): Wouldn't it make sense to break out the
>> common part into a helper? (Really the former could simply call the
>> latter as it seems.)
> 
> In theory, yes. OTOH this no longer applies to upstream, so i dind't
> bother doing that for stable.

Oh, I guess I should have check the tip of the tree first...

Jan

Re: [PATCH] xen/events: fix setting irq affinity

Posted by Greg KH 3 years ago

On Mon, Apr 12, 2021 at 08:28:45AM +0200, Juergen Gross wrote:
> The backport of upstream patch 25da4618af240fbec61 ("xen/events: don't
> unmask an event channel when an eoi is pending") introduced a
> regression for stable kernels 5.10 and older: setting IRQ affinity for
> IRQs related to interdomain events would no longer work, as moving the
> IRQ to its new cpu was not included in the irq_ack callback for those
> events.
> 
> Fix that by adding the needed call.
> 
> Note that kernels 5.11 and later don't need the explicit moving of the
> IRQ to the target cpu in the irq_ack callback, due to a rework of the
> affinity setting in kernel 5.11.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> This patch should be applied to all stable kernel branches up to
> (including) linux-5.10.y, where upstream patch 25da4618af240fbec61 has
> been added.

Now queued up, thanks.

greg k-h

Patch "xen/events: fix setting irq affinity" has been added to the 4.4-stable tree