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References: <20231214024727.3503870-1-vineeth@bitbyteword.org>
 <20231214024727.3503870-9-vineeth@bitbyteword.org> <87zfybml5w.ffs@tglx>
In-Reply-To: <87zfybml5w.ffs@tglx>
From: Vineeth Remanan Pillai <vineeth@bitbyteword.org>
Date: Fri, 15 Dec 2023 13:52:38 -0500
Message-ID: <CAO7JXPiFyLybt3F3bQca1iQqssYTwGn=ZyhXmutH84jUa_QVUg@mail.gmail.com>
Subject: Re: [RFC PATCH 8/8] irq: boost/unboost in irq/nmi entry/exit and softirq
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Ben Segall <bsegall@google.com>, Borislav Petkov <bp@alien8.de>, 
	Daniel Bristot de Oliveira <bristot@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>, 
	Dietmar Eggemann <dietmar.eggemann@arm.com>, "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>, 
	Juri Lelli <juri.lelli@redhat.com>, Mel Gorman <mgorman@suse.de>, 
	Paolo Bonzini <pbonzini@redhat.com>, Andy Lutomirski <luto@kernel.org>, 
	Peter Zijlstra <peterz@infradead.org>, Sean Christopherson <seanjc@google.com>, 
	Steven Rostedt <rostedt@goodmis.org>, Valentin Schneider <vschneid@redhat.com>, 
	Vincent Guittot <vincent.guittot@linaro.org>, Vitaly Kuznetsov <vkuznets@redhat.com>, 
	Wanpeng Li <wanpengli@tencent.com>, Suleiman Souhlal <suleiman@google.com>, 
	Masami Hiramatsu <mhiramat@google.com>, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, 
	x86@kernel.org, Joel Fernandes <joel@joelfernandes.org>
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[...snip..]
> > +#ifdef CONFIG_PARAVIRT_SCHED
> > +     if (pv_sched_enabled() && !in_hardirq() && !local_softirq_pending() &&
> > +                     !need_resched() && !task_is_realtime(current))
> > +             pv_sched_unboost_vcpu();
> > +#endif
>
> Symmetry is overrated. Just pick a spot and slap your hackery in.
>
> Aside of that this whole #ifdeffery is tasteless at best.
>
> >       instrumentation_end();
>
> > +#ifdef CONFIG_PARAVIRT_SCHED
> > +     if (pv_sched_enabled())
> > +             pv_sched_boost_vcpu_lazy();
> > +#endif
>
> But what's worse is that this is just another approach of sprinkling
> random hints all over the place and see what sticks.
>
Understood. WIll have a full re-write of guest side logic for v2.

> Abusing KVM as the man in the middle to communicate between guest and
> host scheduler is creative, but ill defined.
>
> From the host scheduler POV the vCPU is just a user space thread and
> making the guest special is just the wrong approach.
>
> The kernel has no proper general interface to convey information from a
> user space task to the scheduler.
>
> So instead of adding some ad-hoc KVM hackery the right thing is to solve
> this problem from ground up in a generic way and KVM can just utilize
> that instead of having the special snow-flake hackery which is just a
> maintainability nightmare.
>
We had a constructive and productive discussion on the cover letter
thread and reaching a consensus on the kvm side of the design. Will
work towards that and post iterative revisions.

Thanks,
Vineeth