[kernel-sched-cputime] question about probable bug in cputime_adjust()

[kernel-sched-cputime] question about probable bug in cputime_adjust()

[Gustavo A. R. Silva]

Hello everybody,

While looking into Coverity ID 1371643 I ran into the following piece  
of code at kernel/sched/cputime.c:568:

568/*
569 * Adjust tick based cputime random precision against scheduler runtime
570 * accounting.
571 *
572 * Tick based cputime accounting depend on random scheduling  
timeslices of a
573 * task to be interrupted or not by the timer.  Depending on these
574 * circumstances, the number of these interrupts may be over or
575 * under-optimistic, matching the real user and system cputime with  
a variable
576 * precision.
577 *
578 * Fix this by scaling these tick based values against the total runtime
579 * accounted by the CFS scheduler.
580 *
581 * This code provides the following guarantees:
582 *
583 *   stime + utime == rtime
584 *   stime_i+1 >= stime_i, utime_i+1 >= utime_i
585 *
586 * Assuming that rtime_i+1 >= rtime_i.
587 */
588static void cputime_adjust(struct task_cputime *curr,
589                           struct prev_cputime *prev,
590                           u64 *ut, u64 *st)
591{
592        u64 rtime, stime, utime;
593        unsigned long flags;
594
595        /* Serialize concurrent callers such that we can honour our  
guarantees */
596        raw_spin_lock_irqsave(&prev->lock, flags);
597        rtime = curr->sum_exec_runtime;
598
599        /*
600         * This is possible under two circumstances:
601         *  - rtime isn't monotonic after all (a bug);
602         *  - we got reordered by the lock.
603         *
604         * In both cases this acts as a filter such that the rest  
of the code
605         * can assume it is monotonic regardless of anything else.
606         */
607        if (prev->stime + prev->utime >= rtime)
608                goto out;
609
610        stime = curr->stime;
611        utime = curr->utime;
612
613        /*
614         * If either stime or both stime and utime are 0, assume  
all runtime is
615         * userspace. Once a task gets some ticks, the monotonicy code at
616         * 'update' will ensure things converge to the observed ratio.
617         */
618        if (stime == 0) {
619                utime = rtime;
620                goto update;
621        }
622
623        if (utime == 0) {
624                stime = rtime;
625                goto update;
626        }
627
628        stime = scale_stime(stime, rtime, stime + utime);
629
630update:
631        /*
632         * Make sure stime doesn't go backwards; this preserves  
monotonicity
633         * for utime because rtime is monotonic.
634         *
635         *  utime_i+1 = rtime_i+1 - stime_i
636         *            = rtime_i+1 - (rtime_i - utime_i)
637         *            = (rtime_i+1 - rtime_i) + utime_i
638         *            >= utime_i
639         */
640        if (stime < prev->stime)
641                stime = prev->stime;
642        utime = rtime - stime;
643
644        /*
645         * Make sure utime doesn't go backwards; this still preserves
646         * monotonicity for stime, analogous argument to above.
647         */
648        if (utime < prev->utime) {
649                utime = prev->utime;
650                stime = rtime - utime;
651        }
652
653        prev->stime = stime;
654        prev->utime = utime;
655out:
656        *ut = prev->utime;
657        *st = prev->stime;
658        raw_spin_unlock_irqrestore(&prev->lock, flags);
659}


The issue here is that the value assigned to variable utime at line  
619 is overwritten at line 642, which would make such variable  
assignment useless.
But I'm suspicious that such assignment is actually correct and that  
line 642 should be included into the IF block at line 640. Something  
similar to the following patch:

--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -637,9 +637,10 @@ static void cputime_adjust(struct task_cputime *curr,
          *            = (rtime_i+1 - rtime_i) + utime_i
          *            >= utime_i
          */
-       if (stime < prev->stime)
+       if (stime < prev->stime) {
                 stime = prev->stime;
-       utime = rtime - stime;
+               utime = rtime - stime;
+       }


If you confirm this, I will send a patch in a full and proper form.

I'd really appreciate your comments.

Thank you!
--
Gustavo A. R. Silva

Re: [kernel-sched-cputime] question about probable bug in cputime_adjust()

[Frans Klaver]

On Wed, Jun 28, 2017 at 1:03 AM, Gustavo A. R. Silva
<garsilva@embeddedor.com> wrote:
>
> Hello everybody,
>
> While looking into Coverity ID 1371643 I ran into the following piece of
> code at kernel/sched/cputime.c:568:
>
> 568/*
> 569 * Adjust tick based cputime random precision against scheduler runtime
> 570 * accounting.
> 571 *
> 572 * Tick based cputime accounting depend on random scheduling timeslices
> of a
> 573 * task to be interrupted or not by the timer.  Depending on these
> 574 * circumstances, the number of these interrupts may be over or
> 575 * under-optimistic, matching the real user and system cputime with a
> variable
> 576 * precision.
> 577 *
> 578 * Fix this by scaling these tick based values against the total runtime
> 579 * accounted by the CFS scheduler.
> 580 *
> 581 * This code provides the following guarantees:
> 582 *
> 583 *   stime + utime == rtime
> 584 *   stime_i+1 >= stime_i, utime_i+1 >= utime_i
> 585 *
> 586 * Assuming that rtime_i+1 >= rtime_i.
> 587 */
> 588static void cputime_adjust(struct task_cputime *curr,
> 589                           struct prev_cputime *prev,
> 590                           u64 *ut, u64 *st)
> 591{
> 592        u64 rtime, stime, utime;
> 593        unsigned long flags;
> 594
> 595        /* Serialize concurrent callers such that we can honour our
> guarantees */
> 596        raw_spin_lock_irqsave(&prev->lock, flags);
> 597        rtime = curr->sum_exec_runtime;
> 598
> 599        /*
> 600         * This is possible under two circumstances:
> 601         *  - rtime isn't monotonic after all (a bug);
> 602         *  - we got reordered by the lock.
> 603         *
> 604         * In both cases this acts as a filter such that the rest of the
> code
> 605         * can assume it is monotonic regardless of anything else.
> 606         */
> 607        if (prev->stime + prev->utime >= rtime)
> 608                goto out;
> 609
> 610        stime = curr->stime;
> 611        utime = curr->utime;
> 612
> 613        /*
> 614         * If either stime or both stime and utime are 0, assume all
> runtime is
> 615         * userspace. Once a task gets some ticks, the monotonicy code at
> 616         * 'update' will ensure things converge to the observed ratio.
> 617         */
> 618        if (stime == 0) {
> 619                utime = rtime;
> 620                goto update;
> 621        }
> 622
> 623        if (utime == 0) {
> 624                stime = rtime;
> 625                goto update;
> 626        }
> 627
> 628        stime = scale_stime(stime, rtime, stime + utime);
> 629
> 630update:
> 631        /*
> 632         * Make sure stime doesn't go backwards; this preserves
> monotonicity
> 633         * for utime because rtime is monotonic.
> 634         *
> 635         *  utime_i+1 = rtime_i+1 - stime_i
> 636         *            = rtime_i+1 - (rtime_i - utime_i)
> 637         *            = (rtime_i+1 - rtime_i) + utime_i
> 638         *            >= utime_i
> 639         */
> 640        if (stime < prev->stime)
> 641                stime = prev->stime;
> 642        utime = rtime - stime;
> 643
> 644        /*
> 645         * Make sure utime doesn't go backwards; this still preserves
> 646         * monotonicity for stime, analogous argument to above.
> 647         */
> 648        if (utime < prev->utime) {
> 649                utime = prev->utime;
> 650                stime = rtime - utime;
> 651        }
> 652
> 653        prev->stime = stime;
> 654        prev->utime = utime;
> 655out:
> 656        *ut = prev->utime;
> 657        *st = prev->stime;
> 658        raw_spin_unlock_irqrestore(&prev->lock, flags);
> 659}
>
>
> The issue here is that the value assigned to variable utime at line 619 is
> overwritten at line 642, which would make such variable assignment useless.

It isn't completely useless, since utime is used in line 628 to calculate stime.


> But I'm suspicious that such assignment is actually correct and that line
> 642 should be included into the IF block at line 640. Something similar to
> the following patch:
>
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -637,9 +637,10 @@ static void cputime_adjust(struct task_cputime *curr,
>          *            = (rtime_i+1 - rtime_i) + utime_i
>          *            >= utime_i
>          */
> -       if (stime < prev->stime)
> +       if (stime < prev->stime) {
>                 stime = prev->stime;
> -       utime = rtime - stime;
> +               utime = rtime - stime;
> +       }
>
>
> If you confirm this, I will send a patch in a full and proper form.
>
> I'd really appreciate your comments.

If you do that, how would you meet the guarantee made in line 583?

Frans

Re: [kernel-sched-cputime] question about probable bug in cputime_adjust()

[Frans Klaver]

On Wed, Jun 28, 2017 at 7:35 AM, Frans Klaver <fransklaver@gmail.com> wrote:
> On Wed, Jun 28, 2017 at 1:03 AM, Gustavo A. R. Silva
> <garsilva@embeddedor.com> wrote:
>>
>> Hello everybody,
>>
>> While looking into Coverity ID 1371643 I ran into the following piece of
>> code at kernel/sched/cputime.c:568:
>>
>> 568/*
>> 569 * Adjust tick based cputime random precision against scheduler runtime
>> 570 * accounting.
>> 571 *
>> 572 * Tick based cputime accounting depend on random scheduling timeslices
>> of a
>> 573 * task to be interrupted or not by the timer.  Depending on these
>> 574 * circumstances, the number of these interrupts may be over or
>> 575 * under-optimistic, matching the real user and system cputime with a
>> variable
>> 576 * precision.
>> 577 *
>> 578 * Fix this by scaling these tick based values against the total runtime
>> 579 * accounted by the CFS scheduler.
>> 580 *
>> 581 * This code provides the following guarantees:
>> 582 *
>> 583 *   stime + utime == rtime
>> 584 *   stime_i+1 >= stime_i, utime_i+1 >= utime_i
>> 585 *
>> 586 * Assuming that rtime_i+1 >= rtime_i.
>> 587 */
>> 588static void cputime_adjust(struct task_cputime *curr,
>> 589                           struct prev_cputime *prev,
>> 590                           u64 *ut, u64 *st)
>> 591{
>> 592        u64 rtime, stime, utime;
>> 593        unsigned long flags;
>> 594
>> 595        /* Serialize concurrent callers such that we can honour our
>> guarantees */
>> 596        raw_spin_lock_irqsave(&prev->lock, flags);
>> 597        rtime = curr->sum_exec_runtime;
>> 598
>> 599        /*
>> 600         * This is possible under two circumstances:
>> 601         *  - rtime isn't monotonic after all (a bug);
>> 602         *  - we got reordered by the lock.
>> 603         *
>> 604         * In both cases this acts as a filter such that the rest of the
>> code
>> 605         * can assume it is monotonic regardless of anything else.
>> 606         */
>> 607        if (prev->stime + prev->utime >= rtime)
>> 608                goto out;
>> 609
>> 610        stime = curr->stime;
>> 611        utime = curr->utime;
>> 612
>> 613        /*
>> 614         * If either stime or both stime and utime are 0, assume all
>> runtime is
>> 615         * userspace. Once a task gets some ticks, the monotonicy code at
>> 616         * 'update' will ensure things converge to the observed ratio.
>> 617         */
>> 618        if (stime == 0) {
>> 619                utime = rtime;
>> 620                goto update;
>> 621        }
>> 622
>> 623        if (utime == 0) {
>> 624                stime = rtime;
>> 625                goto update;
>> 626        }
>> 627
>> 628        stime = scale_stime(stime, rtime, stime + utime);
>> 629
>> 630update:
>> 631        /*
>> 632         * Make sure stime doesn't go backwards; this preserves
>> monotonicity
>> 633         * for utime because rtime is monotonic.
>> 634         *
>> 635         *  utime_i+1 = rtime_i+1 - stime_i
>> 636         *            = rtime_i+1 - (rtime_i - utime_i)
>> 637         *            = (rtime_i+1 - rtime_i) + utime_i
>> 638         *            >= utime_i
>> 639         */
>> 640        if (stime < prev->stime)
>> 641                stime = prev->stime;
>> 642        utime = rtime - stime;
>> 643
>> 644        /*
>> 645         * Make sure utime doesn't go backwards; this still preserves
>> 646         * monotonicity for stime, analogous argument to above.
>> 647         */
>> 648        if (utime < prev->utime) {
>> 649                utime = prev->utime;
>> 650                stime = rtime - utime;
>> 651        }
>> 652
>> 653        prev->stime = stime;
>> 654        prev->utime = utime;
>> 655out:
>> 656        *ut = prev->utime;
>> 657        *st = prev->stime;
>> 658        raw_spin_unlock_irqrestore(&prev->lock, flags);
>> 659}
>>
>>
>> The issue here is that the value assigned to variable utime at line 619 is
>> overwritten at line 642, which would make such variable assignment useless.
>
> It isn't completely useless, since utime is used in line 628 to calculate stime.

Oh, I missed 'goto update'. Never mind about this one.


>> But I'm suspicious that such assignment is actually correct and that line
>> 642 should be included into the IF block at line 640. Something similar to
>> the following patch:
>>
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -637,9 +637,10 @@ static void cputime_adjust(struct task_cputime *curr,
>>          *            = (rtime_i+1 - rtime_i) + utime_i
>>          *            >= utime_i
>>          */
>> -       if (stime < prev->stime)
>> +       if (stime < prev->stime) {
>>                 stime = prev->stime;
>> -       utime = rtime - stime;
>> +               utime = rtime - stime;
>> +       }
>>
>>
>> If you confirm this, I will send a patch in a full and proper form.
>>
>> I'd really appreciate your comments.
>
> If you do that, how would you meet the guarantee made in line 583?
>
> Frans

Re: [kernel-sched-cputime] question about probable bug in cputime_adjust()

[Gustavo A. R. Silva]

Hi Frans,

Quoting Frans Klaver <fransklaver@gmail.com>:

> On Wed, Jun 28, 2017 at 7:35 AM, Frans Klaver <fransklaver@gmail.com> wrote:
>> On Wed, Jun 28, 2017 at 1:03 AM, Gustavo A. R. Silva
>> <garsilva@embeddedor.com> wrote:
>>>
>>> Hello everybody,
>>>
>>> While looking into Coverity ID 1371643 I ran into the following piece of
>>> code at kernel/sched/cputime.c:568:
>>>
>>> 568/*
>>> 569 * Adjust tick based cputime random precision against scheduler runtime
>>> 570 * accounting.
>>> 571 *
>>> 572 * Tick based cputime accounting depend on random scheduling timeslices
>>> of a
>>> 573 * task to be interrupted or not by the timer.  Depending on these
>>> 574 * circumstances, the number of these interrupts may be over or
>>> 575 * under-optimistic, matching the real user and system cputime with a
>>> variable
>>> 576 * precision.
>>> 577 *
>>> 578 * Fix this by scaling these tick based values against the total runtime
>>> 579 * accounted by the CFS scheduler.
>>> 580 *
>>> 581 * This code provides the following guarantees:
>>> 582 *
>>> 583 *   stime + utime == rtime
>>> 584 *   stime_i+1 >= stime_i, utime_i+1 >= utime_i
>>> 585 *
>>> 586 * Assuming that rtime_i+1 >= rtime_i.
>>> 587 */
>>> 588static void cputime_adjust(struct task_cputime *curr,
>>> 589                           struct prev_cputime *prev,
>>> 590                           u64 *ut, u64 *st)
>>> 591{
>>> 592        u64 rtime, stime, utime;
>>> 593        unsigned long flags;
>>> 594
>>> 595        /* Serialize concurrent callers such that we can honour our
>>> guarantees */
>>> 596        raw_spin_lock_irqsave(&prev->lock, flags);
>>> 597        rtime = curr->sum_exec_runtime;
>>> 598
>>> 599        /*
>>> 600         * This is possible under two circumstances:
>>> 601         *  - rtime isn't monotonic after all (a bug);
>>> 602         *  - we got reordered by the lock.
>>> 603         *
>>> 604         * In both cases this acts as a filter such that the rest of the
>>> code
>>> 605         * can assume it is monotonic regardless of anything else.
>>> 606         */
>>> 607        if (prev->stime + prev->utime >= rtime)
>>> 608                goto out;
>>> 609
>>> 610        stime = curr->stime;
>>> 611        utime = curr->utime;
>>> 612
>>> 613        /*
>>> 614         * If either stime or both stime and utime are 0, assume all
>>> runtime is
>>> 615         * userspace. Once a task gets some ticks, the  
>>> monotonicy code at
>>> 616         * 'update' will ensure things converge to the observed ratio.
>>> 617         */
>>> 618        if (stime == 0) {
>>> 619                utime = rtime;
>>> 620                goto update;
>>> 621        }
>>> 622
>>> 623        if (utime == 0) {
>>> 624                stime = rtime;
>>> 625                goto update;
>>> 626        }
>>> 627
>>> 628        stime = scale_stime(stime, rtime, stime + utime);
>>> 629
>>> 630update:
>>> 631        /*
>>> 632         * Make sure stime doesn't go backwards; this preserves
>>> monotonicity
>>> 633         * for utime because rtime is monotonic.
>>> 634         *
>>> 635         *  utime_i+1 = rtime_i+1 - stime_i
>>> 636         *            = rtime_i+1 - (rtime_i - utime_i)
>>> 637         *            = (rtime_i+1 - rtime_i) + utime_i
>>> 638         *            >= utime_i
>>> 639         */
>>> 640        if (stime < prev->stime)
>>> 641                stime = prev->stime;
>>> 642        utime = rtime - stime;
>>> 643
>>> 644        /*
>>> 645         * Make sure utime doesn't go backwards; this still preserves
>>> 646         * monotonicity for stime, analogous argument to above.
>>> 647         */
>>> 648        if (utime < prev->utime) {
>>> 649                utime = prev->utime;
>>> 650                stime = rtime - utime;
>>> 651        }
>>> 652
>>> 653        prev->stime = stime;
>>> 654        prev->utime = utime;
>>> 655out:
>>> 656        *ut = prev->utime;
>>> 657        *st = prev->stime;
>>> 658        raw_spin_unlock_irqrestore(&prev->lock, flags);
>>> 659}
>>>
>>>
>>> The issue here is that the value assigned to variable utime at line 619 is
>>> overwritten at line 642, which would make such variable assignment useless.
>>
>> It isn't completely useless, since utime is used in line 628 to  
>> calculate stime.
>
> Oh, I missed 'goto update'. Never mind about this one.
>
>
>>> But I'm suspicious that such assignment is actually correct and that line
>>> 642 should be included into the IF block at line 640. Something similar to
>>> the following patch:
>>>
>>> --- a/kernel/sched/cputime.c
>>> +++ b/kernel/sched/cputime.c
>>> @@ -637,9 +637,10 @@ static void cputime_adjust(struct task_cputime *curr,
>>>          *            = (rtime_i+1 - rtime_i) + utime_i
>>>          *            >= utime_i
>>>          */
>>> -       if (stime < prev->stime)
>>> +       if (stime < prev->stime) {
>>>                 stime = prev->stime;
>>> -       utime = rtime - stime;
>>> +               utime = rtime - stime;
>>> +       }
>>>
>>>
>>> If you confirm this, I will send a patch in a full and proper form.
>>>
>>> I'd really appreciate your comments.
>>
>> If you do that, how would you meet the guarantee made in line 583?
>>

You are right, I see now.

Then in this case the following patch would be the way to go:

--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -615,10 +615,8 @@ static void cputime_adjust(struct task_cputime *curr,
          * userspace. Once a task gets some ticks, the monotonicy code at
          * 'update' will ensure things converge to the observed ratio.
          */
-       if (stime == 0) {
-               utime = rtime;
+       if (stime == 0)
                 goto update;
-       }

         if (utime == 0) {
                 stime = rtime;


but I think this one is even better:


--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -615,19 +615,11 @@ static void cputime_adjust(struct task_cputime *curr,
          * userspace. Once a task gets some ticks, the monotonicy code at
          * 'update' will ensure things converge to the observed ratio.
          */
-       if (stime == 0) {
-               utime = rtime;
-               goto update;
-       }
-
-       if (utime == 0) {
+       if (stime != 0 && utime == 0)
                 stime = rtime;
-               goto update;
-       }
-
-       stime = scale_stime(stime, rtime, stime + utime);
+       else
+               stime = scale_stime(stime, rtime, stime + utime);

-update:
         /*
          * Make sure stime doesn't go backwards; this preserves monotonicity
          * for utime because rtime is monotonic.



What do you think?

Thank you!
--
Gustavo A. R. Silva

Re: [kernel-sched-cputime] question about probable bug in cputime_adjust()

[Frans Klaver]

On 29 June 2017 01:57:19 CEST, "Gustavo A. R. Silva" <garsilva@embeddedor.com> wrote:
>>>> --- a/kernel/sched/cputime.c
>>>> +++ b/kernel/sched/cputime.c
>>>> @@ -637,9 +637,10 @@ static void cputime_adjust(struct task_cputime
>*curr,
>>>>          *            = (rtime_i+1 - rtime_i) + utime_i
>>>>          *            >= utime_i
>>>>          */
>>>> -       if (stime < prev->stime)
>>>> +       if (stime < prev->stime) {
>>>>                 stime = prev->stime;
>>>> -       utime = rtime - stime;
>>>> +               utime = rtime - stime;
>>>> +       }
>>>>
>>>>
>>>> If you confirm this, I will send a patch in a full and proper form.
>>>>
>>>> I'd really appreciate your comments.
>>>
>>> If you do that, how would you meet the guarantee made in line 583?
>>>
>
>You are right, I see now.
>
>Then in this case the following patch would be the way to go:
>
>--- a/kernel/sched/cputime.c
>+++ b/kernel/sched/cputime.c
>@@ -615,10 +615,8 @@ static void cputime_adjust(struct task_cputime
>*curr,
>       * userspace. Once a task gets some ticks, the monotonicy code at
>          * 'update' will ensure things converge to the observed ratio.
>          */
>-       if (stime == 0) {
>-               utime = rtime;
>+       if (stime == 0)
>                 goto update;
>-       }
>
>         if (utime == 0) {
>                 stime = rtime;
>
>
>but I think this one is even better:
>
>
>--- a/kernel/sched/cputime.c
>+++ b/kernel/sched/cputime.c
>@@ -615,19 +615,11 @@ static void cputime_adjust(struct task_cputime
>*curr,
>       * userspace. Once a task gets some ticks, the monotonicy code at
>          * 'update' will ensure things converge to the observed ratio.
>          */
>-       if (stime == 0) {
>-               utime = rtime;
>-               goto update;
>-       }
>-
>-       if (utime == 0) {
>+       if (stime != 0 && utime == 0)
>                 stime = rtime;
>-               goto update;
>-       }
>-
>-       stime = scale_stime(stime, rtime, stime + utime);
>+       else
>+               stime = scale_stime(stime, rtime, stime + utime);

I don't think it is better. The stime == 0 case is gone now. So scale_time() will be called in that case. This whole if/else block should only be executed if stime != 0. 

Re: [kernel-sched-cputime] question about probable bug in cputime_adjust()

[Gustavo A. R. Silva]

Quoting Frans Klaver <fransklaver@gmail.com>:

> On 29 June 2017 01:57:19 CEST, "Gustavo A. R. Silva"  
> <garsilva@embeddedor.com> wrote:
>>>>> --- a/kernel/sched/cputime.c
>>>>> +++ b/kernel/sched/cputime.c
>>>>> @@ -637,9 +637,10 @@ static void cputime_adjust(struct task_cputime
>> *curr,
>>>>>          *            = (rtime_i+1 - rtime_i) + utime_i
>>>>>          *            >= utime_i
>>>>>          */
>>>>> -       if (stime < prev->stime)
>>>>> +       if (stime < prev->stime) {
>>>>>                 stime = prev->stime;
>>>>> -       utime = rtime - stime;
>>>>> +               utime = rtime - stime;
>>>>> +       }
>>>>>
>>>>>
>>>>> If you confirm this, I will send a patch in a full and proper form.
>>>>>
>>>>> I'd really appreciate your comments.
>>>>
>>>> If you do that, how would you meet the guarantee made in line 583?
>>>>
>>
>> You are right, I see now.
>>
>> Then in this case the following patch would be the way to go:
>>
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -615,10 +615,8 @@ static void cputime_adjust(struct task_cputime
>> *curr,
>>       * userspace. Once a task gets some ticks, the monotonicy code at
>>          * 'update' will ensure things converge to the observed ratio.
>>          */
>> -       if (stime == 0) {
>> -               utime = rtime;
>> +       if (stime == 0)
>>                 goto update;
>> -       }
>>
>>         if (utime == 0) {
>>                 stime = rtime;
>>
>>
>> but I think this one is even better:
>>
>>
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -615,19 +615,11 @@ static void cputime_adjust(struct task_cputime
>> *curr,
>>       * userspace. Once a task gets some ticks, the monotonicy code at
>>          * 'update' will ensure things converge to the observed ratio.
>>          */
>> -       if (stime == 0) {
>> -               utime = rtime;
>> -               goto update;
>> -       }
>> -
>> -       if (utime == 0) {
>> +       if (stime != 0 && utime == 0)
>>                 stime = rtime;
>> -               goto update;
>> -       }
>> -
>> -       stime = scale_stime(stime, rtime, stime + utime);
>> +       else
>> +               stime = scale_stime(stime, rtime, stime + utime);
>
> I don't think it is better. The stime == 0 case is gone now. So  
> scale_time() will be called in that case. This whole if/else block  
> should only be executed if stime != 0.

Oh yeah! something like:

if (stime != 0) {
         if (stime != 0 && utime == 0)
                 stime = rtime;
         else
                 stime = scale_stime(stime, rtime, stime + utime);
}

I'll be right back with the final patch.

Thanks for your time, Frans.
Much appreciated :)
--
Gustavo A. R. Silva

[PATCH] sched/cputime: code refactoring in cputime_adjust()

[Gustavo A. R. Silva]

Value assigned to variable utime at line 619:utime = rtime;
is overwritten at line 642:utime = rtime - stime; before it
can be used. This makes such variable assignment useless.

Remove this variable assignment and refactor the code related.

Addresses-Coverity-ID: 1371643
Cc: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
---
 kernel/sched/cputime.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index aea3135..a83fd9a 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -615,19 +615,13 @@ static void cputime_adjust(struct task_cputime *curr,
 	 * userspace. Once a task gets some ticks, the monotonicy code at
 	 * 'update' will ensure things converge to the observed ratio.
 	 */
-	if (stime == 0) {
-		utime = rtime;
-		goto update;
+	if (stime != 0) {
+		if (utime == 0)
+			stime = rtime;
+		else
+			stime = scale_stime(stime, rtime, stime + utime);
 	}
 
-	if (utime == 0) {
-		stime = rtime;
-		goto update;
-	}
-
-	stime = scale_stime(stime, rtime, stime + utime);
-
-update:
 	/*
 	 * Make sure stime doesn't go backwards; this preserves monotonicity
 	 * for utime because rtime is monotonic.
-- 
2.5.0

[tip:sched/core] sched/cputime: Refactor the cputime_adjust() code

[tip-bot for Gustavo A. R. Silva]

Commit-ID:  72298e5c92c50edd8cb7cfda4519483ce65fa166
Gitweb:     http://git.kernel.org/tip/72298e5c92c50edd8cb7cfda4519483ce65fa166
Author:     Gustavo A. R. Silva <garsilva@embeddedor.com>
AuthorDate: Thu, 29 Jun 2017 13:41:28 -0500
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 30 Jun 2017 09:37:59 +0200

sched/cputime: Refactor the cputime_adjust() code

Address a Coverity false positive, which is caused by overly
convoluted code:

Value assigned to variable 'utime' at line 619:utime = rtime;
is overwritten at line 642:utime = rtime - stime; before it
can be used. This makes such variable assignment useless.

Remove this variable assignment and refactor the code related.

Addresses-Coverity-ID: 1371643
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Cc: Frans Klaver <fransklaver@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Link: http://lkml.kernel.org/r/20170629184128.GA5271@embeddedgus
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/cputime.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index aea3135..67c70e2 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -615,19 +615,13 @@ static void cputime_adjust(struct task_cputime *curr,
 	 * userspace. Once a task gets some ticks, the monotonicy code at
 	 * 'update' will ensure things converge to the observed ratio.
 	 */
-	if (stime == 0) {
-		utime = rtime;
-		goto update;
+	if (stime != 0) {
+		if (utime == 0)
+			stime = rtime;
+		else
+			stime = scale_stime(stime, rtime, stime + utime);
 	}
 
-	if (utime == 0) {
-		stime = rtime;
-		goto update;
-	}
-
-	stime = scale_stime(stime, rtime, stime + utime);
-
-update:
 	/*
 	 * Make sure stime doesn't go backwards; this preserves monotonicity
 	 * for utime because rtime is monotonic.

Re: [tip:sched/core] sched/cputime: Refactor the cputime_adjust() code

[Rik van Riel]

On Fri, 2017-06-30 at 06:10 -0700, tip-bot for Gustavo A. R. Silva
wrote:

> +++ b/kernel/sched/cputime.c
> @@ -615,19 +615,13 @@ static void cputime_adjust(struct task_cputime
> *curr,
>  	 * userspace. Once a task gets some ticks, the monotonicy
> code at
>  	 * 'update' will ensure things converge to the observed
> ratio.
>  	 */
> -	if (stime == 0) {
> -		utime = rtime;
> -		goto update;
> +	if (stime != 0) {
> +		if (utime == 0)
> +			stime = rtime;
> +		else
> +			stime = scale_stime(stime, rtime, stime +
> utime);
>  	}
>  
> -	if (utime == 0) {
> -		stime = rtime;
> -		goto update;
> -	}
> -
> -	stime = scale_stime(stime, rtime, stime + utime);
> -
> -update:

Wait, what?

This get rid of the utime = rtime assignment, when
stime == 0.  That could be a correctness issue.

>  	/*
>  	 * Make sure stime doesn't go backwards; this preserves
> monotonicity
>  	 * for utime because rtime is monotonic.

Re: [tip:sched/core] sched/cputime: Refactor the cputime_adjust() code

[Frans Klaver]

On Fri, Jun 30, 2017 at 4:00 PM, Rik van Riel <riel@redhat.com> wrote:
> On Fri, 2017-06-30 at 06:10 -0700, tip-bot for Gustavo A. R. Silva
> wrote:
>
>> +++ b/kernel/sched/cputime.c
>> @@ -615,19 +615,13 @@ static void cputime_adjust(struct task_cputime
>> *curr,
>>        * userspace. Once a task gets some ticks, the monotonicy
>> code at
>>        * 'update' will ensure things converge to the observed
>> ratio.
>>        */
>> -     if (stime == 0) {
>> -             utime = rtime;
>> -             goto update;
>> +     if (stime != 0) {
>> +             if (utime == 0)
>> +                     stime = rtime;
>> +             else
>> +                     stime = scale_stime(stime, rtime, stime +
>> utime);
>>       }
>>
>> -     if (utime == 0) {
>> -             stime = rtime;
>> -             goto update;
>> -     }
>> -
>> -     stime = scale_stime(stime, rtime, stime + utime);
>> -
>> -update:
>
> Wait, what?
>
> This get rid of the utime = rtime assignment, when
> stime == 0.  That could be a correctness issue.

The first time utime is used after that assignment, it is overwritten
with rtime - stime. The utime = rtime assignment is then pointless.

Re: [tip:sched/core] sched/cputime: Refactor the cputime_adjust() code

[Frederic Weisbecker]

On Fri, Jun 30, 2017 at 04:41:50PM +0200, Frans Klaver wrote:
> On Fri, Jun 30, 2017 at 4:00 PM, Rik van Riel <riel@redhat.com> wrote:
> > On Fri, 2017-06-30 at 06:10 -0700, tip-bot for Gustavo A. R. Silva
> > wrote:
> >
> >> +++ b/kernel/sched/cputime.c
> >> @@ -615,19 +615,13 @@ static void cputime_adjust(struct task_cputime
> >> *curr,
> >>        * userspace. Once a task gets some ticks, the monotonicy
> >> code at
> >>        * 'update' will ensure things converge to the observed
> >> ratio.
> >>        */
> >> -     if (stime == 0) {
> >> -             utime = rtime;
> >> -             goto update;
> >> +     if (stime != 0) {
> >> +             if (utime == 0)
> >> +                     stime = rtime;
> >> +             else
> >> +                     stime = scale_stime(stime, rtime, stime +
> >> utime);
> >>       }
> >>
> >> -     if (utime == 0) {
> >> -             stime = rtime;
> >> -             goto update;
> >> -     }
> >> -
> >> -     stime = scale_stime(stime, rtime, stime + utime);
> >> -
> >> -update:
> >
> > Wait, what?
> >
> > This get rid of the utime = rtime assignment, when
> > stime == 0.  That could be a correctness issue.
> 
> The first time utime is used after that assignment, it is overwritten
> with rtime - stime. The utime = rtime assignment is then pointless.

Right, I also got confused first but after starring at the code, the patch looks right.

Thanks.

Re: [tip:sched/core] sched/cputime: Refactor the cputime_adjust() code

[Stanislaw Gruszka]

On Fri, Jun 30, 2017 at 06:10:35AM -0700, tip-bot for Gustavo A. R. Silva wrote:
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index aea3135..67c70e2 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -615,19 +615,13 @@ static void cputime_adjust(struct task_cputime *curr,
>  	 * userspace. Once a task gets some ticks, the monotonicy code at
>  	 * 'update' will ensure things converge to the observed ratio.
            ^^^^^^

>  	 */
> -	if (stime == 0) {
> -		utime = rtime;
> -		goto update;
> +	if (stime != 0) {
> +		if (utime == 0)
> +			stime = rtime;
> +		else
> +			stime = scale_stime(stime, rtime, stime + utime);
>  	}
>  
> -	if (utime == 0) {
> -		stime = rtime;
> -		goto update;
> -	}
> -
> -	stime = scale_stime(stime, rtime, stime + utime);
> -
> -update:
Since 'update' label is removed, I think above comment should be
corrected too. Eventually patch could just remove 'utime = rtime;'
line to shut up coverity.

Stanislaw

Re: [tip:sched/core] sched/cputime: Refactor the cputime_adjust() code

[Peter Zijlstra]

On Fri, Jun 30, 2017 at 06:10:35AM -0700, tip-bot for Gustavo A. R. Silva wrote:
> Commit-ID:  72298e5c92c50edd8cb7cfda4519483ce65fa166
> Gitweb:     http://git.kernel.org/tip/72298e5c92c50edd8cb7cfda4519483ce65fa166
> Author:     Gustavo A. R. Silva <garsilva@embeddedor.com>
> AuthorDate: Thu, 29 Jun 2017 13:41:28 -0500
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Fri, 30 Jun 2017 09:37:59 +0200
> 
> sched/cputime: Refactor the cputime_adjust() code
> 
> Address a Coverity false positive, which is caused by overly
> convoluted code:
> 
> Value assigned to variable 'utime' at line 619:utime = rtime;
> is overwritten at line 642:utime = rtime - stime; before it
> can be used. This makes such variable assignment useless.
> 
> Remove this variable assignment and refactor the code related.
> 
> Addresses-Coverity-ID: 1371643
> Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
> Cc: Frans Klaver <fransklaver@gmail.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Stanislaw Gruszka <sgruszka@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Wanpeng Li <wanpeng.li@hotmail.com>
> Link: http://lkml.kernel.org/r/20170629184128.GA5271@embeddedgus
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
>  kernel/sched/cputime.c | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index aea3135..67c70e2 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -615,19 +615,13 @@ static void cputime_adjust(struct task_cputime *curr,
>  	 * userspace. Once a task gets some ticks, the monotonicy code at
>  	 * 'update' will ensure things converge to the observed ratio.
>  	 */
> -	if (stime == 0) {
> -		utime = rtime;
> -		goto update;
> +	if (stime != 0) {
> +		if (utime == 0)
> +			stime = rtime;
> +		else
> +			stime = scale_stime(stime, rtime, stime + utime);
>  	}
>  
> -	if (utime == 0) {
> -		stime = rtime;
> -		goto update;
> -	}
> -
> -	stime = scale_stime(stime, rtime, stime + utime);
> -
> -update:
>  	/*
>  	 * Make sure stime doesn't go backwards; this preserves monotonicity
>  	 * for utime because rtime is monotonic.


Argh, no... That code was perfectly fine. The new code otoh is
convoluted crap.

It had the form:

	if (exception1)
	  deal with exception1

	if (execption2)
	  deal with exception2

	do normal stuff

Which is as simple and straight forward as it gets.

The new code otoh reads like:

	if (!exception1) {
		if (exception2)
		  deal with exception 2
		else
		  do normal stuff
	}

which is absolute shit.

So NAK on this.

Re: [tip:sched/core] sched/cputime: Refactor the cputime_adjust() code

[Ingo Molnar]

* Peter Zijlstra <peterz@infradead.org> wrote:

> Argh, no... That code was perfectly fine. The new code otoh is
> convoluted crap.
> 
> It had the form:
> 
> 	if (exception1)
> 	  deal with exception1
> 
> 	if (execption2)
> 	  deal with exception2
> 
> 	do normal stuff
> 
> Which is as simple and straight forward as it gets.
> 
> The new code otoh reads like:
> 
> 	if (!exception1) {
> 		if (exception2)
> 		  deal with exception 2
> 		else
> 		  do normal stuff
> 	}
> 
> which is absolute shit.
> 
> So NAK on this.

Agreed - I've queued up a revert.

Note that I fixed the old comment, which was arguably wrong:

	/*
	 * If either stime or both stime and utime are 0, assume all runtime is
	 * userspace. Once a task gets some ticks, the monotonicy code at
	 * 'update' will ensure things converge to the observed ratio.
	 */

The correct comment is something like:

	/*
	 * If either stime or utime are 0, assume all runtime is userspace.
	 * Once a task gets some ticks, the monotonicy code at 'update:'
	 * will ensure things converge to the observed ratio.
	 */

Thanks,

	Ingo