linux-kernel - Re: [PATCH v2 7/9] sched: define TIF_ALLOW

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Date:   Sat, 09 Sep 2023 20:48:50 -0700
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, rostedt@...dmis.org,
        tglx@...utronix.de, jon.grimm@....com, bharata@....com,
        raghavendra.kt@....com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED


Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Sat, 9 Sept 2023 at 13:16, Ankur Arora <ankur.a.arora@...cle.com> wrote:
>>
>> > +     if (WARN_ON(resched_allowed()))
>> > +             return;
>>
>> And, maybe something like this to guard against __this_cpu_read()
>> etc:
>>
>> +++ b/lib/smp_processor_id.c
>> @@ -13,6 +13,9 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
>>  {
>>         int this_cpu = raw_smp_processor_id();
>>
>> +       if (unlikely(resched_allowed()))
>> +               goto out_error;
>
> Again, both of those checks are WRONG.
>
> They'll error out even in exceptions / interrupts, when we have a
> preempt count already from the exception itself.
>
> So testing "resched_allowed()" that only tests the TIF_RESCHED_ALLOW
> bit is wrong, wrong, wrong.

Yeah, you are right.

I think we can keep these checks, but with this fixed definition of
resched_allowed(). This might be better:

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2260,7 +2260,8 @@ static inline void disallow_resched(void)

 static __always_inline bool resched_allowed(void)
 {
-       return unlikely(test_tsk_thread_flag(current, TIF_RESCHED_ALLOW));
+       return unlikely(!preempt_count() &&
+                        test_tsk_thread_flag(current, TIF_RESCHED_ALLOW));
 }

Ankur

> These situations aren't errors if we already had a preemption count
> for other reasons. Only trying to disable preemption when in process
> context (while TIF_RESCHED_ALLOW) is a problem. Your patch is missing
> the check for "are we in a process context" part.
>
>                 Linus