[PATCH] sched/fair: Fix misuse of available_idle_cpu()

[PATCH] sched/fair: Fix misuse of available_idle_cpu()

[Abel Wu]

The function available_idle_cpu() was introduced to distinguish
between the code paths that cares if the vCPU is preempted and
the ones don't care. In general, available_idle_cpu() is used in
selecting cpus for immediate use, e.g. ttwu. While idle_cpu() is
used in the paths that only cares about the cpu is idle or not,
and __update_idle_core() is one of them.

Use idle_cpu() instead in the idle path to make has_idle_core
a better hint.

Fixes: 943d355d7fee (sched/core: Distinguish between idle_cpu() calls based on desired effect, introduce available_idle_cpu())
Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index efceb670e755..5a76d814f8bc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6302,7 +6302,7 @@ void __update_idle_core(struct rq *rq)
 		if (cpu == core)
 			continue;
 
-		if (!available_idle_cpu(cpu))
+		if (!idle_cpu(cpu))
 			goto unlock;
 	}
 
-- 
2.37.3

Re: [PATCH] sched/fair: Fix misuse of available_idle_cpu()

[Mel Gorman]

On Thu, Sep 08, 2022 at 04:07:02PM +0800, Abel Wu wrote:
> The function available_idle_cpu() was introduced to distinguish
> between the code paths that cares if the vCPU is preempted and
> the ones don't care. In general, available_idle_cpu() is used in
> selecting cpus for immediate use, e.g. ttwu. While idle_cpu() is
> used in the paths that only cares about the cpu is idle or not,
> and __update_idle_core() is one of them.
> 
> Use idle_cpu() instead in the idle path to make has_idle_core
> a better hint.
> 
> Fixes: 943d355d7fee (sched/core: Distinguish between idle_cpu() calls based on desired effect, introduce available_idle_cpu())
> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>

Seems fair. As vCPU preemption is specific to virtualisation, it is very
unlikely that SMT is exposed to the guest so the impact of the patch is
minimal but I still think it's right so;

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs

Re: [PATCH] sched/fair: Fix misuse of available_idle_cpu()

[Peter Zijlstra]

On Thu, Sep 08, 2022 at 11:36:32AM +0100, Mel Gorman wrote:
> On Thu, Sep 08, 2022 at 04:07:02PM +0800, Abel Wu wrote:
> > The function available_idle_cpu() was introduced to distinguish
> > between the code paths that cares if the vCPU is preempted and
> > the ones don't care. In general, available_idle_cpu() is used in
> > selecting cpus for immediate use, e.g. ttwu. While idle_cpu() is
> > used in the paths that only cares about the cpu is idle or not,
> > and __update_idle_core() is one of them.
> > 
> > Use idle_cpu() instead in the idle path to make has_idle_core
> > a better hint.
> > 
> > Fixes: 943d355d7fee (sched/core: Distinguish between idle_cpu() calls based on desired effect, introduce available_idle_cpu())
> > Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
> 
> Seems fair. As vCPU preemption is specific to virtualisation, it is very
> unlikely that SMT is exposed to the guest so the impact of the patch is

Right; only pinned guests typically expose such topology information
(anything else would be quite broken).

> minimal but I still think it's right so;

I'm not convinced; all of select_idle_sibling() seems to use
available_idle_cpu(), and that's the only consumer of
__update_idle_core(), so in that respect the current state makes sense.

Re: [PATCH] sched/fair: Fix misuse of available_idle_cpu()

[Abel Wu]

On 9/8/22 9:17 PM, Peter Zijlstra wrote:
> On Thu, Sep 08, 2022 at 11:36:32AM +0100, Mel Gorman wrote:
>> On Thu, Sep 08, 2022 at 04:07:02PM +0800, Abel Wu wrote:
>>> The function available_idle_cpu() was introduced to distinguish
>>> between the code paths that cares if the vCPU is preempted and
>>> the ones don't care. In general, available_idle_cpu() is used in
>>> selecting cpus for immediate use, e.g. ttwu. While idle_cpu() is
>>> used in the paths that only cares about the cpu is idle or not,
>>> and __update_idle_core() is one of them.
>>>
>>> Use idle_cpu() instead in the idle path to make has_idle_core
>>> a better hint.
>>>
>>> Fixes: 943d355d7fee (sched/core: Distinguish between idle_cpu() calls based on desired effect, introduce available_idle_cpu())
>>> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
>>
>> Seems fair. As vCPU preemption is specific to virtualisation, it is very
>> unlikely that SMT is exposed to the guest so the impact of the patch is
> 
> Right; only pinned guests typically expose such topology information
> (anything else would be quite broken).
> 
>> minimal but I still think it's right so;
> 
> I'm not convinced; all of select_idle_sibling() seems to use
> available_idle_cpu(), and that's the only consumer of
> __update_idle_core(), so in that respect the current state makes sense.

Hi Peter, Mel, thanks for your reviewing!

My thought was that the preempted core can become active again before
select_idle_sibling() is called, so using available_idle_cpu() in
__update_idle_core() can potentially lose the opportunity to kick an
idle core running. While the downside of using idle_cpu() is that a
full scan can be triggered irrespective of non-preempted cores exist,
but even available_idle_cpu() can not make sure of that either.

BTW, I am also confused with select_idle_core() in which all the cpus
of a core need to be non-preempted before the core can be taken as an
idle core. IMHO, it might be enough that at least one cpu of an idle
core is non-preempted and allowed by task's taskset.

Thanks & BR,
Abel

Re: [PATCH] sched/fair: Fix misuse of available_idle_cpu()

[Abel Wu]

Ping :)

On 9/8/22 9:49 PM, Abel Wu wrote:
> On 9/8/22 9:17 PM, Peter Zijlstra wrote:
>> On Thu, Sep 08, 2022 at 11:36:32AM +0100, Mel Gorman wrote:
>>> On Thu, Sep 08, 2022 at 04:07:02PM +0800, Abel Wu wrote:
>>>> The function available_idle_cpu() was introduced to distinguish
>>>> between the code paths that cares if the vCPU is preempted and
>>>> the ones don't care. In general, available_idle_cpu() is used in
>>>> selecting cpus for immediate use, e.g. ttwu. While idle_cpu() is
>>>> used in the paths that only cares about the cpu is idle or not,
>>>> and __update_idle_core() is one of them.
>>>>
>>>> Use idle_cpu() instead in the idle path to make has_idle_core
>>>> a better hint.
>>>>
>>>> Fixes: 943d355d7fee (sched/core: Distinguish between idle_cpu() 
>>>> calls based on desired effect, introduce available_idle_cpu())
>>>> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
>>>
>>> Seems fair. As vCPU preemption is specific to virtualisation, it is very
>>> unlikely that SMT is exposed to the guest so the impact of the patch is
>>
>> Right; only pinned guests typically expose such topology information
>> (anything else would be quite broken).

Yes, and it is common in our ECS servers to use pinned guests.

>>
>>> minimal but I still think it's right so;
>>
>> I'm not convinced; all of select_idle_sibling() seems to use
>> available_idle_cpu(), and that's the only consumer of
>> __update_idle_core(), so in that respect the current state makes sense.
> 
> Hi Peter, Mel, thanks for your reviewing!
> 
> My thought was that the preempted core can become active again before
> select_idle_sibling() is called, so using available_idle_cpu() in
> __update_idle_core() can potentially lose the opportunity to kick an
> idle core running. While the downside of using idle_cpu() is that a
> full scan can be triggered irrespective of non-preempted cores exist,
> but even available_idle_cpu() can not make sure of that either.
> 
> BTW, I am also confused with select_idle_core() in which all the cpus
> of a core need to be non-preempted before the core can be taken as an
> idle core. IMHO, it might be enough that at least one cpu of an idle
> core is non-preempted and allowed by task's taskset.