[PATCH sched_ext/for-7.1] sched_ext: Documentation: Add ops.dequeue() to task lifecycle

[PATCH sched_ext/for-7.1] sched_ext: Documentation: Add ops.dequeue() to task lifecycle

[Andrea Righi]

Document ops.dequeue() in the sched_ext task lifecycle now that its
semantics are well-defined.

Also update the pseudo-code to use task_is_runnable() consistently and
clarify the case where ops.dispatch() does not refill the time slice.

Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 Documentation/scheduler/sched-ext.rst | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
index 404b4e4c33f7e..9f03650abfeba 100644
--- a/Documentation/scheduler/sched-ext.rst
+++ b/Documentation/scheduler/sched-ext.rst
@@ -422,23 +422,29 @@ by a sched_ext scheduler:
 
         ops.runnable();         /* Task becomes ready to run */
 
-        while (task is runnable) {
+        while (task_is_runnable(task)) {
             if (task is not in a DSQ && task->scx.slice == 0) {
                 ops.enqueue();  /* Task can be added to a DSQ */
 
-                /* Any usable CPU becomes available */
+                /* Task property change (i.e., affinity, nice, etc.)? */
+                if (sched_change(task)) {
+                    ops.dequeue(); /* Exiting BPF scheduler custody */
+                    continue;
+                }
+            }
 
-                ops.dispatch(); /* Task is moved to a local DSQ */
+            /* Any usable CPU becomes available */
+
+            ops.dispatch();     /* Task is moved to a local DSQ */
+            ops.dequeue();      /* Exiting BPF scheduler custody */
 
-                ops.dequeue(); /* Exiting BPF scheduler */
-            }
             ops.running();      /* Task starts running on its assigned CPU */
 
-            while task_is_runnable(p) {
-                while (task->scx.slice > 0 && task_is_runnable(p))
-                    ops.tick();     /* Called every 1/HZ seconds */
+            while (task_is_runnable(task) && task->scx.slice > 0) {
+                ops.tick();     /* Called every 1/HZ seconds */
 
-                ops.dispatch();     /* task->scx.slice can be refilled */
+                if (task->scx.slice == 0)
+                    ops.dispatch(); /* task->scx.slice can be refilled */
             }
 
             ops.stopping();     /* Task stops running (time slice expires or wait) */
-- 
2.53.0

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add ops.dequeue() to task lifecycle

[Emil Tsalapatis]

On Mon Apr 6, 2026 at 7:47 AM EDT, Andrea Righi wrote:
> Document ops.dequeue() in the sched_ext task lifecycle now that its
> semantics are well-defined.
>
> Also update the pseudo-code to use task_is_runnable() consistently and
> clarify the case where ops.dispatch() does not refill the time slice.
>
> Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")

Is the Fixes: tag appropriate here? It's not like the original patch
introduced a bug by fixing ops.dequeue().

Otherwise the state machine looks fine to me!

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  Documentation/scheduler/sched-ext.rst | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> index 404b4e4c33f7e..9f03650abfeba 100644
> --- a/Documentation/scheduler/sched-ext.rst
> +++ b/Documentation/scheduler/sched-ext.rst
> @@ -422,23 +422,29 @@ by a sched_ext scheduler:
>  
>          ops.runnable();         /* Task becomes ready to run */
>  
> -        while (task is runnable) {
> +        while (task_is_runnable(task)) {
>              if (task is not in a DSQ && task->scx.slice == 0) {
>                  ops.enqueue();  /* Task can be added to a DSQ */
>  
> -                /* Any usable CPU becomes available */
> +                /* Task property change (i.e., affinity, nice, etc.)? */
> +                if (sched_change(task)) {
> +                    ops.dequeue(); /* Exiting BPF scheduler custody */
> +                    continue;
> +                }
> +            }
>  
> -                ops.dispatch(); /* Task is moved to a local DSQ */
> +            /* Any usable CPU becomes available */
> +
> +            ops.dispatch();     /* Task is moved to a local DSQ */
> +            ops.dequeue();      /* Exiting BPF scheduler custody */
>  
> -                ops.dequeue(); /* Exiting BPF scheduler */
> -            }
>              ops.running();      /* Task starts running on its assigned CPU */
>  
> -            while task_is_runnable(p) {
> -                while (task->scx.slice > 0 && task_is_runnable(p))
> -                    ops.tick();     /* Called every 1/HZ seconds */
> +            while (task_is_runnable(task) && task->scx.slice > 0) {
> +                ops.tick();     /* Called every 1/HZ seconds */
>  
> -                ops.dispatch();     /* task->scx.slice can be refilled */
> +                if (task->scx.slice == 0)
> +                    ops.dispatch(); /* task->scx.slice can be refilled */
>              }
>  
>              ops.stopping();     /* Task stops running (time slice expires or wait) */

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add ops.dequeue() to task lifecycle

[Andrea Righi]

Hi Emil,

On Mon, Apr 06, 2026 at 10:49:18AM -0400, Emil Tsalapatis wrote:
> On Mon Apr 6, 2026 at 7:47 AM EDT, Andrea Righi wrote:
> > Document ops.dequeue() in the sched_ext task lifecycle now that its
> > semantics are well-defined.
> >
> > Also update the pseudo-code to use task_is_runnable() consistently and
> > clarify the case where ops.dispatch() does not refill the time slice.
> >
> > Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
> 
> Is the Fixes: tag appropriate here? It's not like the original patch
> introduced a bug by fixing ops.dequeue().

Yeah, the intent here was to make sure this commit isn't applied without
ebf1ccff79c4 (otherwise the state machine would be inaccurate), but that
shouldn't happen, so it's probably reasonable to drop the Fixes line.

Thanks,
-Andrea

> 
> Otherwise the state machine looks fine to me!
> 
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
> 
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > ---
> >  Documentation/scheduler/sched-ext.rst | 24 +++++++++++++++---------
> >  1 file changed, 15 insertions(+), 9 deletions(-)
> >
> > diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> > index 404b4e4c33f7e..9f03650abfeba 100644
> > --- a/Documentation/scheduler/sched-ext.rst
> > +++ b/Documentation/scheduler/sched-ext.rst
> > @@ -422,23 +422,29 @@ by a sched_ext scheduler:
> >  
> >          ops.runnable();         /* Task becomes ready to run */
> >  
> > -        while (task is runnable) {
> > +        while (task_is_runnable(task)) {
> >              if (task is not in a DSQ && task->scx.slice == 0) {
> >                  ops.enqueue();  /* Task can be added to a DSQ */
> >  
> > -                /* Any usable CPU becomes available */
> > +                /* Task property change (i.e., affinity, nice, etc.)? */
> > +                if (sched_change(task)) {
> > +                    ops.dequeue(); /* Exiting BPF scheduler custody */
> > +                    continue;
> > +                }
> > +            }
> >  
> > -                ops.dispatch(); /* Task is moved to a local DSQ */
> > +            /* Any usable CPU becomes available */
> > +
> > +            ops.dispatch();     /* Task is moved to a local DSQ */
> > +            ops.dequeue();      /* Exiting BPF scheduler custody */
> >  
> > -                ops.dequeue(); /* Exiting BPF scheduler */
> > -            }
> >              ops.running();      /* Task starts running on its assigned CPU */
> >  
> > -            while task_is_runnable(p) {
> > -                while (task->scx.slice > 0 && task_is_runnable(p))
> > -                    ops.tick();     /* Called every 1/HZ seconds */
> > +            while (task_is_runnable(task) && task->scx.slice > 0) {
> > +                ops.tick();     /* Called every 1/HZ seconds */
> >  
> > -                ops.dispatch();     /* task->scx.slice can be refilled */
> > +                if (task->scx.slice == 0)
> > +                    ops.dispatch(); /* task->scx.slice can be refilled */
> >              }
> >  
> >              ops.stopping();     /* Task stops running (time slice expires or wait) */
> 

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add ops.dequeue() to task lifecycle

[Tejun Heo]

> sched_ext: Documentation: Add ops.dequeue() to task lifecycle

Applied to sched_ext/for-7.1 with Emil's Reviewed-by added and the
Fixes: tag dropped per his comment.

Thanks.

--
tejun

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add ops.dequeue() to task lifecycle

[Kuba Piecuch]

Hi Andrea,

On Mon Apr 6, 2026 at 11:47 AM UTC, Andrea Righi wrote:
> Document ops.dequeue() in the sched_ext task lifecycle now that its
> semantics are well-defined.
>
> Also update the pseudo-code to use task_is_runnable() consistently and
> clarify the case where ops.dispatch() does not refill the time slice.
>
> Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  Documentation/scheduler/sched-ext.rst | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> index 404b4e4c33f7e..9f03650abfeba 100644
> --- a/Documentation/scheduler/sched-ext.rst
> +++ b/Documentation/scheduler/sched-ext.rst
> @@ -422,23 +422,29 @@ by a sched_ext scheduler:
>  
>          ops.runnable();         /* Task becomes ready to run */
>  
> -        while (task is runnable) {
> +        while (task_is_runnable(task)) {
>              if (task is not in a DSQ && task->scx.slice == 0) {
>                  ops.enqueue();  /* Task can be added to a DSQ */
>  
> -                /* Any usable CPU becomes available */
> +                /* Task property change (i.e., affinity, nice, etc.)? */
> +                if (sched_change(task)) {
> +                    ops.dequeue(); /* Exiting BPF scheduler custody */

Doesn't the task also go through quiescent -> runnable here? The full path
being dequeue -> quiescent -> (actual property change) -> runnable -> enqueue.

I guess we should be accurate here since quiescent and runnable are present
elsewhere in the pseudocode.

> +                    continue;
> +                }
> +            }
>  
> -                ops.dispatch(); /* Task is moved to a local DSQ */
> +            /* Any usable CPU becomes available */
> +
> +            ops.dispatch();     /* Task is moved to a local DSQ */

s/local/terminal/?

> +            ops.dequeue();      /* Exiting BPF scheduler custody */
>  
> -                ops.dequeue(); /* Exiting BPF scheduler */
> -            }
>              ops.running();      /* Task starts running on its assigned CPU */
>  
> -            while task_is_runnable(p) {
> -                while (task->scx.slice > 0 && task_is_runnable(p))
> -                    ops.tick();     /* Called every 1/HZ seconds */
> +            while (task_is_runnable(task) && task->scx.slice > 0) {
> +                ops.tick();     /* Called every 1/HZ seconds */
>  
> -                ops.dispatch();     /* task->scx.slice can be refilled */
> +                if (task->scx.slice == 0)
> +                    ops.dispatch(); /* task->scx.slice can be refilled */
>              }
>  
>              ops.stopping();     /* Task stops running (time slice expires or wait) */

Thanks,
Kuba

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add ops.dequeue() to task lifecycle

[Andrea Righi]

Hi Kuba,

On Tue, Apr 07, 2026 at 09:54:22AM +0000, Kuba Piecuch wrote:
> Hi Andrea,
> 
> On Mon Apr 6, 2026 at 11:47 AM UTC, Andrea Righi wrote:
> > Document ops.dequeue() in the sched_ext task lifecycle now that its
> > semantics are well-defined.
> >
> > Also update the pseudo-code to use task_is_runnable() consistently and
> > clarify the case where ops.dispatch() does not refill the time slice.
> >
> > Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > ---
> >  Documentation/scheduler/sched-ext.rst | 24 +++++++++++++++---------
> >  1 file changed, 15 insertions(+), 9 deletions(-)
> >
> > diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> > index 404b4e4c33f7e..9f03650abfeba 100644
> > --- a/Documentation/scheduler/sched-ext.rst
> > +++ b/Documentation/scheduler/sched-ext.rst
> > @@ -422,23 +422,29 @@ by a sched_ext scheduler:
> >  
> >          ops.runnable();         /* Task becomes ready to run */
> >  
> > -        while (task is runnable) {
> > +        while (task_is_runnable(task)) {
> >              if (task is not in a DSQ && task->scx.slice == 0) {
> >                  ops.enqueue();  /* Task can be added to a DSQ */
> >  
> > -                /* Any usable CPU becomes available */
> > +                /* Task property change (i.e., affinity, nice, etc.)? */
> > +                if (sched_change(task)) {
> > +                    ops.dequeue(); /* Exiting BPF scheduler custody */
> 
> Doesn't the task also go through quiescent -> runnable here? The full path
> being dequeue -> quiescent -> (actual property change) -> runnable -> enqueue.
> 
> I guess we should be accurate here since quiescent and runnable are present
> elsewhere in the pseudocode.

Ah yes, we need to add ops.quiescent() and ops.runnable() here. Tejun already
applied this patch to his branch, can you send another patch on top of this?

> 
> > +                    continue;
> > +                }
> > +            }
> >  
> > -                ops.dispatch(); /* Task is moved to a local DSQ */
> > +            /* Any usable CPU becomes available */
> > +
> > +            ops.dispatch();     /* Task is moved to a local DSQ */
> 
> s/local/terminal/?

Technically it'd be correct to say "terminal", but typically we use
scx_bpf_move_to_local() here, which moves the task to a local DSQ. Then it may
fallback into SCX_DSQ_GLOBAL if something goes wrong, but, from a logical
perspective, the intention is to move the task to local DSQ at this point.

So, I'm not sure if saying "terminal" here would be more confusing than
helpful... but I don't have a strong opinion on that.

Thanks,
-Andrea

> 
> > +            ops.dequeue();      /* Exiting BPF scheduler custody */
> >  
> > -                ops.dequeue(); /* Exiting BPF scheduler */
> > -            }
> >              ops.running();      /* Task starts running on its assigned CPU */
> >  
> > -            while task_is_runnable(p) {
> > -                while (task->scx.slice > 0 && task_is_runnable(p))
> > -                    ops.tick();     /* Called every 1/HZ seconds */
> > +            while (task_is_runnable(task) && task->scx.slice > 0) {
> > +                ops.tick();     /* Called every 1/HZ seconds */
> >  
> > -                ops.dispatch();     /* task->scx.slice can be refilled */
> > +                if (task->scx.slice == 0)
> > +                    ops.dispatch(); /* task->scx.slice can be refilled */
> >              }
> >  
> >              ops.stopping();     /* Task stops running (time slice expires or wait) */
> 
> Thanks,
> Kuba

[PATCH sched_ext/for-7.1] sched_ext: Documentation: Add missing calls to quiescent(), runnable()

[Kuba Piecuch]

When a queued task has one of its scheduling properties changed
(e.g. nice, affinity), it goes through dequeue() -> quiescent() ->
(property change callback, e.g. ops.set_weight()) -> runnable() ->
enqueue().

The existing documentation only mentions dequeue() and enqueue() on that
path, so add the missing callbacks.

Fixes: a4f61f0a1afd ("sched_ext: Documentation: Add ops.dequeue() to task lifecycle")
Signed-off-by: Kuba Piecuch <jpiecuch@google.com>
---
 Documentation/scheduler/sched-ext.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
index ec594ae8086de..b5c70f4cfc352 100644
--- a/Documentation/scheduler/sched-ext.rst
+++ b/Documentation/scheduler/sched-ext.rst
@@ -429,6 +429,11 @@ by a sched_ext scheduler:
                 /* Task property change (i.e., affinity, nice, etc.)? */
                 if (sched_change(task)) {
                     ops.dequeue(); /* Exiting BPF scheduler custody */
+                    ops.quiescent();
+
+                    /* Property change callback, e.g. ops.set_weight() */
+
+                    ops.runnable();
                     continue;
                 }
             }
-- 
2.53.0.1213.gd9a14994de-goog

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add missing calls to quiescent(), runnable()

[Andrea Righi]

Hi Kuba,

On Wed, Apr 08, 2026 at 09:18:21AM +0000, Kuba Piecuch wrote:
> When a queued task has one of its scheduling properties changed
> (e.g. nice, affinity), it goes through dequeue() -> quiescent() ->
> (property change callback, e.g. ops.set_weight()) -> runnable() ->
> enqueue().
> 
> The existing documentation only mentions dequeue() and enqueue() on that
> path, so add the missing callbacks.
> 
> Fixes: a4f61f0a1afd ("sched_ext: Documentation: Add ops.dequeue() to task lifecycle")
> Signed-off-by: Kuba Piecuch <jpiecuch@google.com>
> ---
>  Documentation/scheduler/sched-ext.rst | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> index ec594ae8086de..b5c70f4cfc352 100644
> --- a/Documentation/scheduler/sched-ext.rst
> +++ b/Documentation/scheduler/sched-ext.rst
> @@ -429,6 +429,11 @@ by a sched_ext scheduler:

Looks good, but I noticed another issue, should we also change the condition up
above as following?

 Documentation/scheduler/sched-ext.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
index 29d36e248f58b..99df4cc982375 100644
--- a/Documentation/scheduler/sched-ext.rst
+++ b/Documentation/scheduler/sched-ext.rst
@@ -423,7 +423,7 @@ by a sched_ext scheduler:
         ops.runnable();         /* Task becomes ready to run */
 
         while (task_is_runnable(task)) {
-            if (task is not in a DSQ && task->scx.slice == 0) {
+            if (task is not in a DSQ || task->scx.slice == 0) {
                 ops.enqueue();  /* Task can be added to a DSQ */
 
                 /* Task property change (i.e., affinity, nice, etc.)? */

Because we trigger ops.enqueue() when the task expired its time slice or it
becomes runnable and has not been added to a DSQ.

This also represents correctly the sched_change() scenario: a task being
re-enqueued after sched_change() still has its time slice > 0, but we need to
call ops.enqueue() for it.

Thanks,
-Andrea

>                  /* Task property change (i.e., affinity, nice, etc.)? */
>                  if (sched_change(task)) {
>                      ops.dequeue(); /* Exiting BPF scheduler custody */
> +                    ops.quiescent();
> +
> +                    /* Property change callback, e.g. ops.set_weight() */
> +
> +                    ops.runnable();
>                      continue;
>                  }
>              }
> -- 
> 2.53.0.1213.gd9a14994de-goog
> 

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add missing calls to quiescent(), runnable()

[Kuba Piecuch]

Hi Andrea,

On Wed Apr 8, 2026 at 11:28 AM UTC, Andrea Righi wrote:
...
>
> Looks good, but I noticed another issue, should we also change the condition up
> above as following?
>
>  Documentation/scheduler/sched-ext.rst | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> index 29d36e248f58b..99df4cc982375 100644
> --- a/Documentation/scheduler/sched-ext.rst
> +++ b/Documentation/scheduler/sched-ext.rst
> @@ -423,7 +423,7 @@ by a sched_ext scheduler:
>          ops.runnable();         /* Task becomes ready to run */
>  
>          while (task_is_runnable(task)) {
> -            if (task is not in a DSQ && task->scx.slice == 0) {
> +            if (task is not in a DSQ || task->scx.slice == 0) {
>                  ops.enqueue();  /* Task can be added to a DSQ */
>  
>                  /* Task property change (i.e., affinity, nice, etc.)? */
>
> Because we trigger ops.enqueue() when the task expired its time slice or it
> becomes runnable and has not been added to a DSQ.
>
> This also represents correctly the sched_change() scenario: a task being
> re-enqueued after sched_change() still has its time slice > 0, but we need to
> call ops.enqueue() for it.

I agree that the condition should be changed, but I'm not sure that this is
what it should look like.

Is the "task is not in a DSQ" part of the condition there to handle direct
dispatch? Apart from direct dispatch from ops.select_cpu(), I wasn't able to
come up with a situation where we would reach this condition with the task
present on some DSQ.

A more general comment about the pseudocode: I think it can be useful to
introduce someone new to the general flow of the callbacks in sched_ext,
but the documentation should be clear that this is a simplified view that
makes assumptions about the behavior of the BPF scheduler itself (flags like
SCX_OPS_ENQ_LAST, whether the scheduler uses direct dispatch), as well as
the overall system (Can sched_ext be preempted by a higher-priority sched
class? Can scheduling properties of a task be changed while it's running?)
Without stating these assumptions clearly, we risk leaving the reader falsely
believing they have a complete understanding.

Thanks,
Kuba

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add missing calls to quiescent(), runnable()

[Andrea Righi]

On Wed, Apr 08, 2026 at 12:40:09PM +0000, Kuba Piecuch wrote:
> Hi Andrea,
> 
> On Wed Apr 8, 2026 at 11:28 AM UTC, Andrea Righi wrote:
> ...
> >
> > Looks good, but I noticed another issue, should we also change the condition up
> > above as following?
> >
> >  Documentation/scheduler/sched-ext.rst | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> > index 29d36e248f58b..99df4cc982375 100644
> > --- a/Documentation/scheduler/sched-ext.rst
> > +++ b/Documentation/scheduler/sched-ext.rst
> > @@ -423,7 +423,7 @@ by a sched_ext scheduler:
> >          ops.runnable();         /* Task becomes ready to run */
> >  
> >          while (task_is_runnable(task)) {
> > -            if (task is not in a DSQ && task->scx.slice == 0) {
> > +            if (task is not in a DSQ || task->scx.slice == 0) {
> >                  ops.enqueue();  /* Task can be added to a DSQ */
> >  
> >                  /* Task property change (i.e., affinity, nice, etc.)? */
> >
> > Because we trigger ops.enqueue() when the task expired its time slice or it
> > becomes runnable and has not been added to a DSQ.
> >
> > This also represents correctly the sched_change() scenario: a task being
> > re-enqueued after sched_change() still has its time slice > 0, but we need to
> > call ops.enqueue() for it.
> 
> I agree that the condition should be changed, but I'm not sure that this is
> what it should look like.
> 
> Is the "task is not in a DSQ" part of the condition there to handle direct
> dispatch? Apart from direct dispatch from ops.select_cpu(), I wasn't able to
> come up with a situation where we would reach this condition with the task
> present on some DSQ.

The intent is to represent the direct dispatch from ops.select_cpu(), since in
that case ops.enqueue() is skipped.

Honestly I think if we change the && to || in that condition, everything should
be pretty accurate.

> 
> A more general comment about the pseudocode: I think it can be useful to
> introduce someone new to the general flow of the callbacks in sched_ext,
> but the documentation should be clear that this is a simplified view that
> makes assumptions about the behavior of the BPF scheduler itself (flags like
> SCX_OPS_ENQ_LAST, whether the scheduler uses direct dispatch), as well as
> the overall system (Can sched_ext be preempted by a higher-priority sched
> class? Can scheduling properties of a task be changed while it's running?)
> Without stating these assumptions clearly, we risk leaving the reader falsely
> believing they have a complete understanding.

Of course this schema is not a complete representation of the entire sched_ext
state machine, if we put everything it'd become too big and complex. I think we
should just cover the most common use cases here. Maybe we can clarify this in
the description before this diagram.

Thanks,
-Andrea

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add missing calls to quiescent(), runnable()

[Kuba Piecuch]

On Wed Apr 8, 2026 at 1:49 PM UTC, Andrea Righi wrote:
> On Wed, Apr 08, 2026 at 12:40:09PM +0000, Kuba Piecuch wrote:
>> Hi Andrea,
>> 
>> On Wed Apr 8, 2026 at 11:28 AM UTC, Andrea Righi wrote:
>> ...
>> >
>> > Looks good, but I noticed another issue, should we also change the condition up
>> > above as following?
>> >
>> >  Documentation/scheduler/sched-ext.rst | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
>> > index 29d36e248f58b..99df4cc982375 100644
>> > --- a/Documentation/scheduler/sched-ext.rst
>> > +++ b/Documentation/scheduler/sched-ext.rst
>> > @@ -423,7 +423,7 @@ by a sched_ext scheduler:
>> >          ops.runnable();         /* Task becomes ready to run */
>> >  
>> >          while (task_is_runnable(task)) {
>> > -            if (task is not in a DSQ && task->scx.slice == 0) {
>> > +            if (task is not in a DSQ || task->scx.slice == 0) {
>> >                  ops.enqueue();  /* Task can be added to a DSQ */
>> >  
>> >                  /* Task property change (i.e., affinity, nice, etc.)? */
>> >
>> > Because we trigger ops.enqueue() when the task expired its time slice or it
>> > becomes runnable and has not been added to a DSQ.
>> >
>> > This also represents correctly the sched_change() scenario: a task being
>> > re-enqueued after sched_change() still has its time slice > 0, but we need to
>> > call ops.enqueue() for it.
>> 
>> I agree that the condition should be changed, but I'm not sure that this is
>> what it should look like.
>> 
>> Is the "task is not in a DSQ" part of the condition there to handle direct
>> dispatch? Apart from direct dispatch from ops.select_cpu(), I wasn't able to
>> come up with a situation where we would reach this condition with the task
>> present on some DSQ.
>
> The intent is to represent the direct dispatch from ops.select_cpu(), since in
> that case ops.enqueue() is skipped.
>
> Honestly I think if we change the && to || in that condition, everything should
> be pretty accurate.

In the case of direct dispatch from ops.select_cpu() we don't invoke
ops.dispatch() and ops.dequeue() before ops.running(), right? The current
pseudocode calls them unconditionally.

Another inaccuracy not related to direct dispatch: property changes can occur
while a task is running, while the psedocode only allows for property changes
while a task is queued.

There's also preemption by a higher sched class, which is not covered in the
loop condition (task_is_runnable(task) && task->scx.slice > 0), unless we take
task_is_runnable() to return false if there's a higher-priority sched class
with runnable tasks on the CPU, though that would be in conflict with the
actual implementation of task_is_runnable() in include/linux/sched.h.

>
>> 
>> A more general comment about the pseudocode: I think it can be useful to
>> introduce someone new to the general flow of the callbacks in sched_ext,
>> but the documentation should be clear that this is a simplified view that
>> makes assumptions about the behavior of the BPF scheduler itself (flags like
>> SCX_OPS_ENQ_LAST, whether the scheduler uses direct dispatch), as well as
>> the overall system (Can sched_ext be preempted by a higher-priority sched
>> class? Can scheduling properties of a task be changed while it's running?)
>> Without stating these assumptions clearly, we risk leaving the reader falsely
>> believing they have a complete understanding.
>
> Of course this schema is not a complete representation of the entire sched_ext
> state machine, if we put everything it'd become too big and complex. I think we
> should just cover the most common use cases here. Maybe we can clarify this in
> the description before this diagram.

Let's agree on what inaccuracies need to be fixed and I'll send a v2 with fixes
and attach an appropriate disclaimer to the pseudocode.

Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add missing calls to quiescent(), runnable()

[Andrea Righi]

Hi Kuba,

On Wed, Apr 08, 2026 at 02:17:03PM +0000, Kuba Piecuch wrote:
> On Wed Apr 8, 2026 at 1:49 PM UTC, Andrea Righi wrote:
> > On Wed, Apr 08, 2026 at 12:40:09PM +0000, Kuba Piecuch wrote:
> >> Hi Andrea,
> >> 
> >> On Wed Apr 8, 2026 at 11:28 AM UTC, Andrea Righi wrote:
> >> ...
> >> >
> >> > Looks good, but I noticed another issue, should we also change the condition up
> >> > above as following?
> >> >
> >> >  Documentation/scheduler/sched-ext.rst | 2 +-
> >> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >> >
> >> > diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> >> > index 29d36e248f58b..99df4cc982375 100644
> >> > --- a/Documentation/scheduler/sched-ext.rst
> >> > +++ b/Documentation/scheduler/sched-ext.rst
> >> > @@ -423,7 +423,7 @@ by a sched_ext scheduler:
> >> >          ops.runnable();         /* Task becomes ready to run */
> >> >  
> >> >          while (task_is_runnable(task)) {
> >> > -            if (task is not in a DSQ && task->scx.slice == 0) {
> >> > +            if (task is not in a DSQ || task->scx.slice == 0) {
> >> >                  ops.enqueue();  /* Task can be added to a DSQ */
> >> >  
> >> >                  /* Task property change (i.e., affinity, nice, etc.)? */
> >> >
> >> > Because we trigger ops.enqueue() when the task expired its time slice or it
> >> > becomes runnable and has not been added to a DSQ.
> >> >
> >> > This also represents correctly the sched_change() scenario: a task being
> >> > re-enqueued after sched_change() still has its time slice > 0, but we need to
> >> > call ops.enqueue() for it.
> >> 
> >> I agree that the condition should be changed, but I'm not sure that this is
> >> what it should look like.
> >> 
> >> Is the "task is not in a DSQ" part of the condition there to handle direct
> >> dispatch? Apart from direct dispatch from ops.select_cpu(), I wasn't able to
> >> come up with a situation where we would reach this condition with the task
> >> present on some DSQ.
> >
> > The intent is to represent the direct dispatch from ops.select_cpu(), since in
> > that case ops.enqueue() is skipped.
> >
> > Honestly I think if we change the && to || in that condition, everything should
> > be pretty accurate.
> 
> In the case of direct dispatch from ops.select_cpu() we don't invoke
> ops.dispatch() and ops.dequeue() before ops.running(), right? The current
> pseudocode calls them unconditionally.

We can move ops.dispatch() and ops.dequeue() inside the
if (task is not in a DSQ || task->scx.slice == 0) block.

> 
> Another inaccuracy not related to direct dispatch: property changes can occur
> while a task is running, while the psedocode only allows for property changes
> while a task is queued.

Sure... but again, modelling all the possible scenarios would make the
pseudocode completely unreadable.

IMHO it'd be better to give an overview of the most common use cases here and
clarify in the description that the diagram doesn't cover all the possible
scenarios. This one is a special use case that, personally, I wouldn't cover in
the pseudocode.

> 
> There's also preemption by a higher sched class, which is not covered in the
> loop condition (task_is_runnable(task) && task->scx.slice > 0), unless we take
> task_is_runnable() to return false if there's a higher-priority sched class
> with runnable tasks on the CPU, though that would be in conflict with the
> actual implementation of task_is_runnable() in include/linux/sched.h.

Ditto.

> 
> >
> >> 
> >> A more general comment about the pseudocode: I think it can be useful to
> >> introduce someone new to the general flow of the callbacks in sched_ext,
> >> but the documentation should be clear that this is a simplified view that
> >> makes assumptions about the behavior of the BPF scheduler itself (flags like
> >> SCX_OPS_ENQ_LAST, whether the scheduler uses direct dispatch), as well as
> >> the overall system (Can sched_ext be preempted by a higher-priority sched
> >> class? Can scheduling properties of a task be changed while it's running?)
> >> Without stating these assumptions clearly, we risk leaving the reader falsely
> >> believing they have a complete understanding.
> >
> > Of course this schema is not a complete representation of the entire sched_ext
> > state machine, if we put everything it'd become too big and complex. I think we
> > should just cover the most common use cases here. Maybe we can clarify this in
> > the description before this diagram.
> 
> Let's agree on what inaccuracies need to be fixed and I'll send a v2 with fixes
> and attach an appropriate disclaimer to the pseudocode.

If we move ops.dispatch() + ops.dequeue() inside the ops.enqueue() block I think
the pseudocode becomes "fairly" accurate. At least more accurate than what we
have right now. It won't be perfect, but it can help newer sched_ext devs having
an overview the task lifecycle without going too much into implementation
details.

So, to recap, what do you think about this?

    ops.init_task();            /* A new task is created */
    ops.enable();               /* Enable BPF scheduling for the task */

    while (task in SCHED_EXT) {
        if (task can migrate)
            ops.select_cpu();   /* Called on wakeup (optimization) */

        ops.runnable();         /* Task becomes ready to run */

        while (task_is_runnable(task)) {
            if (task is not in a DSQ || task->scx.slice == 0) {
                ops.enqueue();  /* Task can be added to a DSQ */

                /* Task property change (i.e., affinity, nice, etc.)? */
                if (sched_change(task)) {
                    ops.dequeue(); /* Exiting BPF scheduler custody */
                    ops.quiescent();

                    /* Property change callback, e.g. ops.set_weight() */

                    ops.runnable();
                    continue;
                }

                /* Any usable CPU becomes available */

                ops.dispatch();     /* Task is moved to a local DSQ */
                ops.dequeue();      /* Exiting BPF scheduler custody */
            }

            ops.running();      /* Task starts running on its assigned CPU */

            while (task_is_runnable(task) && task->scx.slice > 0) {
                ops.tick();     /* Called every 1/HZ seconds */

                if (task->scx.slice == 0)
                    ops.dispatch(); /* task->scx.slice can be refilled */
            }

            ops.stopping();     /* Task stops running (time slice expires or wait) */
        }

        ops.quiescent();        /* Task releases its assigned CPU (wait) */
    }

    ops.disable();              /* Disable BPF scheduling for the task */
    ops.exit_task();            /* Task is destroyed */

Thanks,
-Andrea