Re: [PATCH 2/2] smp: introduce kick_active_cpus_sync()

From: Paul E. McKenney
Date: Sun Apr 01 2018 - 10:10:12 EST

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On Sun, Apr 01, 2018 at 02:11:08PM +0300, Yury Norov wrote:
> 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@xxxxxxxxxxxxxxxxxx>
> > > ---
> > > 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?
> >
> > Will
>
> commit 153ae9d5667e7baab4d48c48e8ec30fbcbd86f1e
> Author: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx>
> Date: Sat Mar 31 15:05:23 2018 +0300
>
> Hi Will, Paul,
>
> On my system there are 3 paths that go thru rcu_dynticks_eqs_exit(),
> and so require isb().
>
> First path starts at gic_handle_irq() on secondary_start_kernel stack.
> gic_handle_irq() already issues isb(), and so we can do nothing.
>
> Second path starts at el0_svc entry; and third path is the exit from
> do_idle() on secondary_start_kernel stack.
>
> For do_idle() path there is arch_cpu_idle_exit() hook that is not used by
> arm64 at now, so I picked it. And for el0_svc, I've introduced isb_if_eqs
> macro and call it at the beginning of el0_svc_naked.
>
> I've tested it on ThunderX2 machine, and it works for me.
>
> Below is my call traces and patch for them. If you OK with it, I think I'm
> ready to submit v2 (but maybe split this patch for better readability).

I must defer to Will on this one.

Thanx, Paul

> Yury
>
> [ 585.412095] Call trace:
> [ 585.412097] [<fffffc00080878d8>] dump_backtrace+0x0/0x380
> [ 585.412099] [<fffffc0008087c6c>] show_stack+0x14/0x20
> [ 585.412101] [<fffffc0008a091ec>] dump_stack+0x98/0xbc
> [ 585.412104] [<fffffc0008122080>] rcu_dynticks_eqs_exit+0x68/0x70
> [ 585.412105] [<fffffc00081260f0>] rcu_irq_enter+0x48/0x50
> [ 585.412106] [<fffffc00080c92c4>] irq_enter+0xc/0x70
> [ 585.412108] [<fffffc0008113a64>] __handle_domain_irq+0x3c/0x120
> [ 585.412109] [<fffffc00080816c4>] gic_handle_irq+0xc4/0x180
> [ 585.412110] Exception stack(0xfffffc001130fe20 to 0xfffffc001130ff60)
> [ 585.412112] fe20: 00000000000000a0 0000000000000000 0000000000000001 0000000000000000
> [ 585.412113] fe40: 0000028f6f0b0000 0000000000000020 0013cd6f53963b31 0000000000000000
> [ 585.412144] fe60: 0000000000000002 fffffc001130fed0 0000000000000b80 0000000000003400
> [ 585.412146] fe80: 0000000000000000 0000000000000001 0000000000000000 00000000000001db
> [ 585.412147] fea0: fffffc0008247a78 000003ff86dc61f8 0000000000000014 fffffc0008fc0000
> [ 585.412149] fec0: fffffc00090143e8 fffffc0009014000 fffffc0008fc94a0 0000000000000000
> [ 585.412150] fee0: 0000000000000000 fffffe8f46bb1700 0000000000000000 0000000000000000
> [ 585.412152] ff00: 0000000000000000 fffffc001130ff60 fffffc0008085034 fffffc001130ff60
> [ 585.412153] ff20: fffffc0008085038 0000000000400149 fffffc0009014000 fffffc0008fc94a0
> [ 585.412155] ff40: ffffffffffffffff 0000000000000000 fffffc001130ff60 fffffc0008085038
> [ 585.412156] [<fffffc0008082fb0>] el1_irq+0xb0/0x124
> [ 585.412158] [<fffffc0008085038>] arch_cpu_idle+0x10/0x18
> [ 585.412159] [<fffffc00080ff38c>] do_idle+0x10c/0x1d8
> [ 585.412160] [<fffffc00080ff5ec>] cpu_startup_entry+0x24/0x28
> [ 585.412162] [<fffffc000808db04>] secondary_start_kernel+0x15c/0x1a0
> [ 585.412164] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.14.0-isolation-160735-g59b71c1-dirty #18
>
> [ 585.412058] Call trace:
> [ 585.412060] [<fffffc00080878d8>] dump_backtrace+0x0/0x380
> [ 585.412062] [<fffffc0008087c6c>] show_stack+0x14/0x20
> [ 585.412064] [<fffffc0008a091ec>] dump_stack+0x98/0xbc
> [ 585.412066] [<fffffc0008122080>] rcu_dynticks_eqs_exit+0x68/0x70
> [ 585.412068] [<fffffc00081232bc>] rcu_eqs_exit.isra.23+0x64/0x80
> [ 585.412069] [<fffffc000812609c>] rcu_user_exit+0xc/0x18
> [ 585.412071] [<fffffc000817c34c>] __context_tracking_exit.part.2+0x74/0x98
> [ 585.412072] [<fffffc000817c3e0>] context_tracking_exit.part.3+0x40/0x50
> [ 585.412074] [<fffffc000817c4e0>] context_tracking_user_exit+0x30/0x38
> [ 585.412075] Exception stack(0xfffffc00385afec0 to 0xfffffc00385b0000)
> [ 585.412076] fec0: 00000000000000b1 000002aacd702420 0000000000000200 00000000000001f4
> [ 585.412078] fee0: 0000000000000000 0000000000000008 000002aabec9af17 ffffffffffffffff
> [ 585.412079] ff00: 0000000000000016 ffffffffffffffff 000003ffe7619470 0000000000000057
> [ 585.412081] ff20: a3d70a3d70a3d70b 000000000000016d 2ce33e6c02ce33e7 00000000000001db
> [ 585.412082] ff40: 000002aabec7d260 000003ff86dc61f8 0000000000000014 00000000000001f4
> [ 585.412083] ff60: 0000000000000000 000002aabecab000 000002aacd6e2830 0000000000000001
> [ 585.412085] ff80: 000002aacd6e2830 000002aabec58380 0000000000000054 000002aabebccf50
> [ 585.412086] ffa0: 0000000000000054 000003ffe7619540 000002aabebcf538 000003ffe7619540
> [ 585.412088] ffc0: 000003ff86dc6410 0000000060000000 00000000000000b1 0000000000000016
> [ 585.412089] ffe0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
> [ 585.412091] [<fffffc0008083498>] el0_svc_naked+0xc/0x3c
> [ 585.446067] CPU: 68 PID: 0 Comm: swapper/68 Not tainted 4.14.0-isolation-160735-g59b71c1-dirty #18
>
> [ 585.412038] Call trace:
> [ 585.412041] [<fffffc00080878d8>] dump_backtrace+0x0/0x380
> [ 585.412042] [<fffffc0008087c6c>] show_stack+0x14/0x20
> [ 585.412045] [<fffffc0008a091ec>] dump_stack+0x98/0xbc
> [ 585.412047] [<fffffc0008122080>] rcu_dynticks_eqs_exit+0x68/0x70
> [ 585.412049] [<fffffc00081232bc>] rcu_eqs_exit.isra.23+0x64/0x80
> [ 585.412050] [<fffffc0008126080>] rcu_idle_exit+0x18/0x28
> [ 585.412052] [<fffffc00080ff398>] do_idle+0x118/0x1d8
> [ 585.412053] [<fffffc00080ff5ec>] cpu_startup_entry+0x24/0x28
> [ 585.412055] [<fffffc000808db04>] secondary_start_kernel+0x15c/0x1a0
> [ 585.412057] CPU: 22 PID: 4315 Comm: nginx Not tainted 4.14.0-isolation-160735-g59b71c1-dirty #18
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index e1c59d4008a8..efa5060a2f1c 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -35,22 +35,29 @@
> #include <asm/unistd.h>
>
> /*
> - * Context tracking subsystem. Used to instrument transitions
> - * between user and kernel mode.
> + * Save/restore needed during syscalls. Restore syscall arguments from
> + * the values already saved on stack during kernel_entry.
> */
> - .macro ct_user_exit, syscall = 0
> -#ifdef CONFIG_CONTEXT_TRACKING
> - bl context_tracking_user_exit
> - .if \syscall == 1
> - /*
> - * Save/restore needed during syscalls. Restore syscall arguments from
> - * the values already saved on stack during kernel_entry.
> - */
> + .macro __restore_syscall_args
> ldp x0, x1, [sp]
> ldp x2, x3, [sp, #S_X2]
> ldp x4, x5, [sp, #S_X4]
> ldp x6, x7, [sp, #S_X6]
> - .endif
> + .endm
> +
> + .macro el0_svc_restore_syscall_args
> +#if !defined(CONFIG_TINY_RCU) || defined(CONFIG_CONTEXT_TRACKING)
> + __restore_syscall_args
> +#endif
> + .endm
> +
> +/*
> + * Context tracking subsystem. Used to instrument transitions
> + * between user and kernel mode.
> + */
> + .macro ct_user_exit
> +#ifdef CONFIG_CONTEXT_TRACKING
> + bl context_tracking_user_exit
> #endif
> .endm
>
> @@ -433,6 +440,20 @@ __bad_stack:
> ASM_BUG()
> .endm
>
> +/*
> + * Flush I-cache if CPU is in extended quiescent state
> + */
> + .macro isb_if_eqs
> +#ifndef CONFIG_TINY_RCU
> + bl rcu_is_watching
> + tst w0, #0xff
> + b.ne 1f
> + /* Pairs with aarch64_insn_patch_text for EQS CPUs. */
> + isb
> +1:
> +#endif
> + .endm
> +
> el0_sync_invalid:
> inv_entry 0, BAD_SYNC
> ENDPROC(el0_sync_invalid)
> @@ -840,8 +861,10 @@ el0_svc:
> mov wsc_nr, #__NR_syscalls
> el0_svc_naked: // compat entry point
> stp x0, xscno, [sp, #S_ORIG_X0] // save the original x0 and syscall number
> + isb_if_eqs
> enable_dbg_and_irq
> - ct_user_exit 1
> + ct_user_exit
> + el0_svc_restore_syscall_args
>
> ldr x16, [tsk, #TSK_TI_FLAGS] // check for syscall hooks
> tst x16, #_TIF_SYSCALL_WORK
> @@ -874,10 +897,7 @@ __sys_trace:
> mov x1, sp // pointer to regs
> cmp wscno, wsc_nr // check upper syscall limit
> b.hs __ni_sys_trace
> - ldp x0, x1, [sp] // restore the syscall args
> - ldp x2, x3, [sp, #S_X2]
> - ldp x4, x5, [sp, #S_X4]
> - ldp x6, x7, [sp, #S_X6]
> + __restore_syscall_args
> ldr x16, [stbl, xscno, lsl #3] // address in the syscall table
> blr x16 // call sys_* routine
>
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 2dc0f8482210..f11afd2aa33a 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -88,6 +88,12 @@ void arch_cpu_idle(void)
> trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
> }
>
> +void arch_cpu_idle_exit(void)
> +{
> + if (!rcu_is_watching())
> + isb();
> +}
> +
> #ifdef CONFIG_HOTPLUG_CPU
> void arch_cpu_idle_dead(void)
> {
>

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