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Date: Fri, 8 May 2015 15:27:00 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: fweisbec@redhat.com, Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Rik van Riel <riel@redhat.com>, williams@redhat.com
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry
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* Andy Lutomirski <luto@amacapital.net> wrote:

> > on return from idle we'd do:
> >
> >         rcu_qs_ctr += 4-2+1; /* Register QS, clear bit 1, set bit 0 */
> >
> > etc. On all boundary transitions we can use a constant ADD on a 
> > suitable percpu variable.
> 
> Sounds good to me, except that we need to be careful to distinguish 
> between non-syscall entries from quiescent states
>     and non-syscall entries from quiescent states.

It might be hard to make that distinction! ;-)

I suspect you wanted to raise the issue of various contexts nesting on 
each other, such as syscall triggering a page fault, which gets an irq 
nested, etc. - versus non-nested contexts such as user-space 
triggering a page fault or user-space getting an irq?

> [...]  We could save the old state (as the current exception_enter 
> code does) or we could allocate enough low bits for the state that 
> the problem goes away.

So I think, if it all works in practice just as well as it does in 
email, we might be better off with more state bits: that would tell 
any remote statistics/sampling code more as well.

It might also be easier to patch in/out, because this kind of state 
tracking will affect non-RT CPUs as well. (Later on we could do a 
separate IDT for RT CPUs as well, with a patched version of the entry 
code.)

> I don't think the TIF_RCU_QS variant is worthwhile -- merging the 
> counter and state is probably both easier and faster.

Yeah.

Thanks,

	Ingo