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From:   Ankur Arora <ankur.a.arora@oracle.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        willy@infradead.org, mgorman@suse.de, rostedt@goodmis.org,
        jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com,
        boris.ostrovsky@oracle.com, konrad.wilk@oracle.com,
        jgross@suse.com, andrew.cooper3@citrix.com
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED
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Thomas Gleixner <tglx@linutronix.de> writes:

> On Wed, Sep 20 2023 at 07:22, Ankur Arora wrote:
>> Thomas Gleixner <tglx@linutronix.de> writes:
>>
>>> So the decision matrix would be:
>>>
>>>                 Ret2user        Ret2kernel      PreemptCnt=0
>>>
>>> NEED_RESCHED       Y                Y               Y
>>> LAZY_RESCHED       Y                N               N
>>>
>>> That is completely independent of the preemption model and the
>>> differentiation of the preemption models happens solely at the scheduler
>>> level:
>>
>> This is relatively minor, but do we need two flags? Seems to me we
>> can get to the same decision matrix by letting the scheduler fold
>> into the preempt-count based on current preemption model.
>
> You still need the TIF flags because there is no way to do remote
> modification of preempt count.

Yes, agreed. In my version, I was envisaging that the remote cpu always
only sets up TIF_NEED_RESCHED and then we decide which one we want at
the preemption point.

Anyway, I see what you meant in your PoC.

>>> But they support PREEMPT_COUNT, so we might get away with a reduced
>>> preemption point coverage:
>>>
>>>                 Ret2user        Ret2kernel      PreemptCnt=0
>>>
>>> NEED_RESCHED       Y                N               Y
>>> LAZY_RESCHED       Y                N               N
>>
>> So from the discussion in the other thread, for the ARCH_NO_PREEMPT
>> configs that don't support preemption, we probably need a fourth
>> preemption model, say PREEMPT_UNSAFE.
>
> As discussed they wont really notice the latency issues because the
> museum pieces are not used for anything crucial and for UM that's the
> least of the correctness worries.
>
> So no, we don't need yet another knob. We keep them chucking along and
> if they really want they can adopt to the new world order. :)

Will they chuckle along, or die trying ;)?

I grepped for "preempt_enable|preempt_disable" for all the archs and
hexagon and m68k don't seem to do any explicit accounting at all.
(Though, neither do nios2 and openrisc, and both csky and microblaze
only do it in the tlbflush path.)

        arch/hexagon      0
        arch/m68k         0
        arch/nios2        0
        arch/openrisc     0
        arch/csky         3
        arch/microblaze   3
        arch/um           4
        arch/riscv        8
        arch/arc         14
        arch/parisc      15
        arch/arm         16
        arch/sparc       16
        arch/xtensa      19
        arch/sh          21
        arch/alpha       23
        arch/ia64        27
        arch/loongarch   53
        arch/arm64       54
        arch/s390        91
        arch/mips       115
        arch/x86        146
        arch/powerpc    201

My concern is given that we preempt on timeslice expiration for all
three preemption models, we could end up preempting at an unsafe
location.

Still, not the most pressing of problems.


Thanks
--
ankur