linux-kernel - Re: [patch 11/12] hrtimer: Prepare support for PREEMPT

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Message-ID: <0910bd2b-b5b1-aa41-7f5e-b44f51a516d3@suse.com>
Date:   Wed, 31 Jul 2019 10:45:56 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>, x86@...nel.org,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch 11/12] hrtimer: Prepare support for PREEMPT_RT

On 29.07.19 19:30, Paolo Bonzini wrote:
> On 29/07/19 17:08, Steven Rostedt wrote:
>> On Sun, 28 Jul 2019 11:06:50 +0200
>> Juergen Gross <jgross@...e.com> wrote:
>>
>>> In case we'd want to change that I'd rather not special case timers, but
>>> apply a more general solution to the quite large amount of similar
>>> cases: I assume the majority of cpu_relax() uses are affected, so adding
>>> a paravirt op cpu_relax() might be appropriate.
>>>
>>> That could be put under CONFIG_PARAVIRT_SPINLOCK. If called in a guest
>>> it could ask the hypervisor to give up the physical cpu voluntarily
>>> (in Xen this would be a "yield" hypercall).
>>
>> Seems paravirt wants our cpu_chill() ;-)
> 
> Actually that is not really a joke! :)

As CONFIG_PARAVIRT is no longer so massive intrusive as some time ago
it might really be worth thinking of.

 From Xen perspective I'd really like a way to give up the vcpu instead
of doing a busy wait. And having another user and (even better) already
some patches addressing some (or all?) callsites sounds like a win-win
situation for me.

So +1 from me for cpu_chill() via a new paravirt op.


Juergen