linux-kernel - Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

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Date:	Fri, 8 May 2015 15:27:00 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	fweisbec@...hat.com, Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>, williams@...hat.com
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry

* Andy Lutomirski <luto@...capital.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
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