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SubjectRe: [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@caviumnetworks.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

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