linux-kernel - Re: [PATCH 2/2] smp: introduce kick_active_cpus

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Date:   Wed, 28 Mar 2018 13:58:13 +0300
From:   Yury Norov <ynorov@...iumnetworks.com>
To:     Will Deacon <will.deacon@....com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Christopher Lameter <cl@...ux.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Mark Rutland <mark.rutland@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-ppc@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] smp: introduce kick_active_cpus_sync()

On Tue, Mar 27, 2018 at 11:21:17AM +0100, Will Deacon wrote:
> On Sun, Mar 25, 2018 at 08:50:04PM +0300, Yury Norov wrote:
> > kick_all_cpus_sync() forces all CPUs to sync caches by sending broadcast IPI.
> > If CPU is in extended quiescent state (idle task or nohz_full userspace), this
> > work may be done at the exit of this state. Delaying synchronization helps to
> > save power if CPU is in idle state and decrease latency for real-time tasks.
> > 
> > This patch introduces kick_active_cpus_sync() and uses it in mm/slab and arm64
> > code to delay syncronization.
> > 
> > For task isolation (https://lkml.org/lkml/2017/11/3/589), IPI to the CPU running
> > isolated task would be fatal, as it breaks isolation. The approach with delaying
> > of synchronization work helps to maintain isolated state.
> > 
> > I've tested it with test from task isolation series on ThunderX2 for more than
> > 10 hours (10k giga-ticks) without breaking isolation.
> > 
> > Signed-off-by: Yury Norov <ynorov@...iumnetworks.com>
> > ---
> >  arch/arm64/kernel/insn.c |  2 +-
> >  include/linux/smp.h      |  2 ++
> >  kernel/smp.c             | 24 ++++++++++++++++++++++++
> >  mm/slab.c                |  2 +-
> >  4 files changed, 28 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
> > index 2718a77da165..9d7c492e920e 100644
> > --- a/arch/arm64/kernel/insn.c
> > +++ b/arch/arm64/kernel/insn.c
> > @@ -291,7 +291,7 @@ int __kprobes aarch64_insn_patch_text(void *addrs[], u32 insns[], int cnt)
> >  			 * synchronization.
> >  			 */
> >  			ret = aarch64_insn_patch_text_nosync(addrs[0], insns[0]);
> > -			kick_all_cpus_sync();
> > +			kick_active_cpus_sync();
> >  			return ret;
> >  		}
> >  	}
> 
> I think this means that runtime modifications to the kernel text might not
> be picked up by CPUs coming out of idle. Shouldn't we add an ISB on that
> path to avoid executing stale instructions?

Thanks, Will, for the hint. I'll do that.

Yury