[PATCH] kernel/sched/fair: Fix the issue of virtual runtime conversion error

[PATCH] kernel/sched/fair: Fix the issue of virtual runtime conversion error

[Li kunyu]

The logic of the function here should be to convert the virtual runtime
into actual time by combining it with the weight of the scheduling
entity.
However, it cannot be converted by simply converting actual time into
virtual running time. A reverse conversion is required to obtain the
correct time.

Signed-off-by: Li kunyu <likunyu10@163.com>
---
 kernel/sched/fair.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 69361c63353a..456cc6104310 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7033,7 +7033,9 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
 			resched_curr(rq);
 		return;
 	}
-	delta = (se->load.weight * vdelta) / NICE_0_LOAD;
+
+	if (unlikely(se->load.weight != NICE_0_LOAD))
+		delta = (NICE_0_LOAD * vdelta) / se->load.weight;
 
 	/*
 	 * Correct for instantaneous load of other classes.
-- 
2.47.3

Re: [PATCH] kernel/sched/fair: Fix the issue of virtual runtime conversion error

[kernel test robot]

Hi Li,

kernel test robot noticed the following build errors:

[auto build test ERROR on tip/sched/core]
[also build test ERROR on peterz-queue/sched/core linus/master v7.1-rc5 next-20260526]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Li-kunyu/kernel-sched-fair-Fix-the-issue-of-virtual-runtime-conversion-error/20260526-151853
base:   tip/sched/core
patch link:    https://lore.kernel.org/r/20260526071537.17460-1-likunyu10%40163.com
patch subject: [PATCH] kernel/sched/fair: Fix the issue of virtual runtime conversion error
config: i386-allnoconfig-bpf (https://download.01.org/0day-ci/archive/20260527/202605270352.wVynZOtB-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260527/202605270352.wVynZOtB-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202605270352.wVynZOtB-lkp@intel.com/

All errors (new ones prefixed by >>):

   ld: kernel/sched/fair.o: in function `hrtick_start_fair':
>> kernel/sched/fair.c:7633:(.text+0x701): undefined reference to `__udivdi3'


vim +7633 kernel/sched/fair.c

  7603	
  7604	/**************************************************
  7605	 * CFS operations on tasks:
  7606	 */
  7607	
  7608	#ifdef CONFIG_SCHED_HRTICK
  7609	static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
  7610	{
  7611		struct sched_entity *se = &p->se;
  7612		unsigned long scale = 1024;
  7613		unsigned long util = 0;
  7614		u64 vdelta;
  7615		u64 delta;
  7616	
  7617		WARN_ON_ONCE(task_rq(p) != rq);
  7618	
  7619		if (rq->cfs.h_nr_queued <= 1)
  7620			return;
  7621	
  7622		/*
  7623		 * Compute time until virtual deadline
  7624		 */
  7625		vdelta = se->deadline - se->vruntime;
  7626		if ((s64)vdelta < 0) {
  7627			if (task_current_donor(rq, p))
  7628				resched_curr(rq);
  7629			return;
  7630		}
  7631	
  7632		if (unlikely(se->load.weight != NICE_0_LOAD))
> 7633			delta = (NICE_0_LOAD * vdelta) / se->load.weight;
  7634	
  7635		/*
  7636		 * Correct for instantaneous load of other classes.
  7637		 */
  7638		util += cpu_util_irq(rq);
  7639		if (util && util < 1024) {
  7640			scale *= 1024;
  7641			scale /= (1024 - util);
  7642		}
  7643	
  7644		hrtick_start(rq, (scale * delta) / 1024);
  7645	}
  7646	

--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] kernel/sched/fair: Fix the issue of virtual runtime conversion error

[Geert Uytterhoeven]

On Wed, 27 May 2026 at 03:17, kernel test robot <lkp@intel.com> wrote:
> kernel test robot noticed the following build errors:
>
> [auto build test ERROR on tip/sched/core]
> [also build test ERROR on peterz-queue/sched/core linus/master v7.1-rc5 next-20260526]
> [If your patch is applied to the wrong git tree, kindly drop us a note.
> And when submitting patch, we suggest to use '--base' as documented in
> https://git-scm.com/docs/git-format-patch#_base_tree_information]
>
> url:    https://github.com/intel-lab-lkp/linux/commits/Li-kunyu/kernel-sched-fair-Fix-the-issue-of-virtual-runtime-conversion-error/20260526-151853
> base:   tip/sched/core
> patch link:    https://lore.kernel.org/r/20260526071537.17460-1-likunyu10%40163.com
> patch subject: [PATCH] kernel/sched/fair: Fix the issue of virtual runtime conversion error
> config: i386-allnoconfig-bpf (https://download.01.org/0day-ci/archive/20260527/202605270352.wVynZOtB-lkp@intel.com/config)
> compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260527/202605270352.wVynZOtB-lkp@intel.com/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202605270352.wVynZOtB-lkp@intel.com/
>
> All errors (new ones prefixed by >>):
>
>    ld: kernel/sched/fair.o: in function `hrtick_start_fair':
> >> kernel/sched/fair.c:7633:(.text+0x701): undefined reference to `__udivdi3'
>
>
> vim +7633 kernel/sched/fair.c
>
>   7603
>   7604  /**************************************************
>   7605   * CFS operations on tasks:
>   7606   */
>   7607
>   7608  #ifdef CONFIG_SCHED_HRTICK
>   7609  static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
>   7610  {
>   7611          struct sched_entity *se = &p->se;
>   7612          unsigned long scale = 1024;
>   7613          unsigned long util = 0;
>   7614          u64 vdelta;
>   7615          u64 delta;
>   7616
>   7617          WARN_ON_ONCE(task_rq(p) != rq);
>   7618
>   7619          if (rq->cfs.h_nr_queued <= 1)
>   7620                  return;
>   7621
>   7622          /*
>   7623           * Compute time until virtual deadline
>   7624           */
>   7625          vdelta = se->deadline - se->vruntime;
>   7626          if ((s64)vdelta < 0) {
>   7627                  if (task_current_donor(rq, p))
>   7628                          resched_curr(rq);
>   7629                  return;
>   7630          }
>   7631
>   7632          if (unlikely(se->load.weight != NICE_0_LOAD))
> > 7633                  delta = (NICE_0_LOAD * vdelta) / se->load.weight;

vdelta is u64, load_weight.weight is unsigned long, so please use
div64_ul() instead of an open-coded division.

>   7634
>   7635          /*
>   7636           * Correct for instantaneous load of other classes.
>   7637           */
>   7638          util += cpu_util_irq(rq);
>   7639          if (util && util < 1024) {
>   7640                  scale *= 1024;
>   7641                  scale /= (1024 - util);
>   7642          }
>   7643
>   7644          hrtick_start(rq, (scale * delta) / 1024);
>   7645  }
>   7646

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

[PATCH v2] kernel/sched/fair: Fix the issue of virtual runtime conversion error

[Li kunyu]

The logic of the function here should be to convert the virtual runtime
into actual time by combining it with the weight of the scheduling
entity.
However, it cannot be converted by simply converting actual time into
virtual running time. A reverse conversion is required to obtain the
correct time.

Signed-off-by: Li kunyu <likunyu10@163.com>
---
v2: Use the div64_ul function to perform division operation

 kernel/sched/fair.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 69361c63353a..53f3373c1735 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7033,7 +7033,9 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
 			resched_curr(rq);
 		return;
 	}
-	delta = (se->load.weight * vdelta) / NICE_0_LOAD;
+
+	if (unlikely(se->load.weight != NICE_0_LOAD))
+		delta = div64_ul(NICE_0_LOAD * vdelta, se->load.weight);
 
 	/*
 	 * Correct for instantaneous load of other classes.
-- 
2.47.3

Re: [PATCH v2] kernel/sched/fair: Fix the issue of virtual runtime conversion error

[Peter Zijlstra]

On Wed, May 27, 2026 at 03:21:13PM +0800, Li kunyu wrote:
> The logic of the function here should be to convert the virtual runtime
> into actual time by combining it with the weight of the scheduling
> entity.
> However, it cannot be converted by simply converting actual time into
> virtual running time. A reverse conversion is required to obtain the
> correct time.
> 
> Signed-off-by: Li kunyu <likunyu10@163.com>
> ---
> v2: Use the div64_ul function to perform division operation
> 
>  kernel/sched/fair.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69361c63353a..53f3373c1735 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7033,7 +7033,9 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
>  			resched_curr(rq);
>  		return;
>  	}
> -	delta = (se->load.weight * vdelta) / NICE_0_LOAD;
> +
> +	if (unlikely(se->load.weight != NICE_0_LOAD))
> +		delta = div64_ul(NICE_0_LOAD * vdelta, se->load.weight);
>  
>  	/*
>  	 * Correct for instantaneous load of other classes.

This seems wrong. Note that calc_delta_fair() does: 

  vdelta := (delta * NICE_0_LOAD) / se->load.weight

which gets us time -> vtime. Therefore the vtime -> time transform is
the inverse of that:

  delta := (vdelta * se->load.weight) / NICE_0_LOAD;

which is exactly what the current code does.

Specifically:

                  NICE_0_LOAD      se->load.weight
       (delta * ---------------) * --------------- = delta
                se->load.weight      NICE_0_LOAD   

Re: [PATCH v2] kernel/sched/fair: Fix the issue of virtual runtime conversion error

[Li kunyu]

Hello maintainer:
  Sorry, I didn't pay close attention to the brackets when I was looking at the notes. It's indeed correct here.
  I am preparing to resubmit a patch that adds a nice0 judgment.

Re: [PATCH v2] kernel/sched/fair: Fix the issue of virtual runtime conversion error

[kernel test robot]

Hi Li,

kernel test robot noticed the following build warnings:

[auto build test WARNING on tip/sched/core]
[also build test WARNING on peterz-queue/sched/core linus/master v7.1-rc5 next-20260527]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Li-kunyu/kernel-sched-fair-Fix-the-issue-of-virtual-runtime-conversion-error/20260527-152554
base:   tip/sched/core
patch link:    https://lore.kernel.org/r/20260527072113.359604-1-likunyu10%40163.com
patch subject: [PATCH v2] kernel/sched/fair: Fix the issue of virtual runtime conversion error
config: x86_64-kexec (https://download.01.org/0day-ci/archive/20260528/202605281756.0qWgdGW5-lkp@intel.com/config)
compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260528/202605281756.0qWgdGW5-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202605281756.0qWgdGW5-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> kernel/sched/fair.c:7632:6: warning: variable 'delta' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
    7632 |         if (unlikely(se->load.weight != NICE_0_LOAD))
         |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   include/linux/compiler.h:77:22: note: expanded from macro 'unlikely'
      77 | # define unlikely(x)    __builtin_expect(!!(x), 0)
         |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~
   kernel/sched/fair.c:7644:28: note: uninitialized use occurs here
    7644 |         hrtick_start(rq, (scale * delta) / 1024);
         |                                   ^~~~~
   kernel/sched/fair.c:7632:2: note: remove the 'if' if its condition is always true
    7632 |         if (unlikely(se->load.weight != NICE_0_LOAD))
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    7633 |                 delta = div64_ul(NICE_0_LOAD * vdelta, se->load.weight);
   kernel/sched/fair.c:7615:11: note: initialize the variable 'delta' to silence this warning
    7615 |         u64 delta;
         |                  ^
         |                   = 0
   1 warning generated.


vim +7632 kernel/sched/fair.c

  7603	
  7604	/**************************************************
  7605	 * CFS operations on tasks:
  7606	 */
  7607	
  7608	#ifdef CONFIG_SCHED_HRTICK
  7609	static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
  7610	{
  7611		struct sched_entity *se = &p->se;
  7612		unsigned long scale = 1024;
  7613		unsigned long util = 0;
  7614		u64 vdelta;
  7615		u64 delta;
  7616	
  7617		WARN_ON_ONCE(task_rq(p) != rq);
  7618	
  7619		if (rq->cfs.h_nr_queued <= 1)
  7620			return;
  7621	
  7622		/*
  7623		 * Compute time until virtual deadline
  7624		 */
  7625		vdelta = se->deadline - se->vruntime;
  7626		if ((s64)vdelta < 0) {
  7627			if (task_current_donor(rq, p))
  7628				resched_curr(rq);
  7629			return;
  7630		}
  7631	
> 7632		if (unlikely(se->load.weight != NICE_0_LOAD))
  7633			delta = div64_ul(NICE_0_LOAD * vdelta, se->load.weight);
  7634	
  7635		/*
  7636		 * Correct for instantaneous load of other classes.
  7637		 */
  7638		util += cpu_util_irq(rq);
  7639		if (util && util < 1024) {
  7640			scale *= 1024;
  7641			scale /= (1024 - util);
  7642		}
  7643	
  7644		hrtick_start(rq, (scale * delta) / 1024);
  7645	}
  7646	

--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki