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

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Date:   Mon, 11 Sep 2023 17:29:43 +0100
From:   andrew.cooper3@...rix.com
To:     Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        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, jgross@...e.com
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

On 11/09/2023 4:04 pm, Peter Zijlstra wrote:
> On Sun, Sep 10, 2023 at 11:32:32AM -0700, Linus Torvalds wrote:
>
>> I was hoping that we'd have some generic way to deal with this where
>> we could just say "this thing is reschedulable", and get rid of - or
>> at least not increasingly add to - the cond_resched() mess.
> Isn't that called PREEMPT=y ? That tracks precisely all the constraints
> required to know when/if we can preempt.
>
> The whole voluntary preempt model is basically the traditional
> co-operative preemption model and that fully relies on manual yields.
>
> The problem with the REP prefix (and Xen hypercalls) is that
> they're long running instructions and it becomes fundamentally
> impossible to put a cond_resched() in.

Any VMM - Xen isn't special here.

And if we're talking about instructions, then CPUID, GETSEC and
ENCL{S,U} and plenty of {RD,WR}MSRs in in a similar category, being
effectively blocking RPC operations to something else in the platform.

The Xen evtchn upcall logic in Linux does cond_resched() when possible. 
i.e. long-running hypercalls issued with interrupts enabled can
reschedule if an interrupt occurs, which is pretty close to how REP
works too.

~Andrew