Re: [PATCH 18/19] rcu/context_tracking: Merge dynticks counter and context tracking states

From: Paul E. McKenney
Date: Thu Mar 10 2022 - 15:32:33 EST

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On Wed, Mar 02, 2022 at 04:48:09PM +0100, Frederic Weisbecker wrote:
> Updating the context tracking state and the RCU dynticks counter
> atomically in a single operation is a first step towards improving CPU
> isolation. This makes the context tracking state updates fully ordered
> and therefore allow for later enhancements such as postponing some work
> while a task is running isolated in userspace until it ever comes back
> to the kernel.
>
> The state field becomes divided in two parts:
>
> 1) Lower bits for context tracking state:
>
> CONTEXT_IDLE = 1,
> CONTEXT_USER = 2,
> CONTEXT_GUEST = 4,

And the CONTEXT_DISABLED value of -1 works because you can have only
one of the above three bits set at a time?

Except that RCU needs this to unconditionally at least distinguish
between kernel and idle, given the prevalence of CONFIG_NO_HZ_IDLE=y.
So does the CONTEXT_DISABLED really happen anymore?

A few more questions interspersed below.

Thanx, Paul

> A value of 0 means we are in CONTEXT_KERNEL.
>
> 2) Higher bits for RCU eqs dynticks counting:
>
> RCU_DYNTICKS_IDX = 8
>
> The dynticks counting is always incremented by this value.
> (state & RCU_DYNTICKS_IDX) means we are NOT in an extended quiescent
> state. This makes the chance for a collision more likely between two
> RCU dynticks snapshots but wrapping up 24 bits of eqs dynticks
> increments still takes some bad luck (also rdp.dynticks_snap could be
> converted from int to long?)
>
> Some RCU eqs functions have been renamed to better reflect their broader
> scope that now include context tracking state.
>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> Cc: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Nicolas Saenz Julienne <nsaenz@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
> Cc: Yu Liao<liaoyu15@xxxxxxxxxx>
> Cc: Phil Auld <pauld@xxxxxxxxxx>
> Cc: Paul Gortmaker<paul.gortmaker@xxxxxxxxxxxxx>
> Cc: Alex Belits <abelits@xxxxxxxxxxx>
> ---
> include/linux/context_tracking.h | 4 +-
> include/linux/context_tracking_state.h | 33 ++++++---
> kernel/context_tracking.c | 92 +++++++++++++-------------
> kernel/rcu/tree.c | 13 ++--
> kernel/rcu/tree_stall.h | 2 +-
> 5 files changed, 81 insertions(+), 63 deletions(-)
>
> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> index 63343c34ab4e..aa0e6fbf6946 100644
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -110,7 +110,7 @@ static inline void context_tracking_init(void) { }
> #ifdef CONFIG_CONTEXT_TRACKING
> extern void ct_idle_enter(void);
> extern void ct_idle_exit(void);
> -extern unsigned long rcu_dynticks_inc(int incby);
> +extern unsigned long ct_state_inc(int incby);
>
> /*
> * Is the current CPU in an extended quiescent state?
> @@ -119,7 +119,7 @@ extern unsigned long rcu_dynticks_inc(int incby);
> */
> static __always_inline bool rcu_dynticks_curr_cpu_in_eqs(void)
> {
> - return !(arch_atomic_read(this_cpu_ptr(&context_tracking.dynticks)) & 0x1);
> + return !(arch_atomic_read(this_cpu_ptr(&context_tracking.state)) & RCU_DYNTICKS_IDX);
> }
>
> #else
> diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
> index bca0d3e0bd3d..b8a309532c18 100644
> --- a/include/linux/context_tracking_state.h
> +++ b/include/linux/context_tracking_state.h
> @@ -9,13 +9,29 @@
> /* Offset to allow distinguishing irq vs. task-based idle entry/exit. */
> #define DYNTICK_IRQ_NONIDLE ((LONG_MAX / 2) + 1)
>
> +enum {
> + CONTEXT_IDLE_OFFSET = 0,
> + CONTEXT_USER_OFFSET,
> + CONTEXT_GUEST_OFFSET,
> + CONTEXT_MAX_OFFSET,
> +};
> +
> enum ctx_state {
> CONTEXT_> DISABLED = -1, /* returned by ct_state() if unknown */
> CONTEXT_KERNEL = 0,
> - CONTEXT_USER,
> - CONTEXT_GUEST,
> + CONTEXT_IDLE = BIT(CONTEXT_IDLE_OFFSET),
> + CONTEXT_USER = BIT(CONTEXT_USER_OFFSET),
> + CONTEXT_GUEST = BIT(CONTEXT_GUEST_OFFSET),
> + CONTEXT_MAX = BIT(CONTEXT_MAX_OFFSET),
> };
>
> +/* Even value for idle, else odd. */
> +#define RCU_DYNTICKS_SHIFT CONTEXT_MAX_OFFSET
> +#define RCU_DYNTICKS_IDX CONTEXT_MAX
> +
> +#define CT_STATE_MASK (CONTEXT_MAX - 1)
> +#define CT_DYNTICKS_MASK (~CT_STATE_MASK)
> +
> struct context_tracking {
> #ifdef CONFIG_CONTEXT_TRACKING_USER
> /*
> @@ -26,9 +42,8 @@ struct context_tracking {
> */
> bool active;
> int recursion;
> +#endif
> atomic_t state;
> -#endif
> - atomic_t dynticks; /* Even value for idle, else odd. */
> long dynticks_nesting; /* Track process nesting level. */
> long dynticks_nmi_nesting; /* Track irq/NMI nesting level. */
> };
> @@ -38,24 +53,26 @@ DECLARE_PER_CPU(struct context_tracking, context_tracking);
>
> static __always_inline int __ct_state(void)
> {
> - return atomic_read(this_cpu_ptr(&context_tracking.state));
> + return atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_STATE_MASK;
> }
>
> static __always_inline int ct_dynticks(void)
> {
> - return atomic_read(this_cpu_ptr(&context_tracking.dynticks));
> + return atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_DYNTICKS_MASK;
> }
>
> static __always_inline int ct_dynticks_cpu(int cpu)
> {
> struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> - return atomic_read(&ct->dynticks);
> +
> + return atomic_read(&ct->state) & CT_DYNTICKS_MASK;
> }
>
> static __always_inline int ct_dynticks_cpu_acquire(int cpu)
> {
> struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> - return atomic_read_acquire(&ct->state);
> +
> + return atomic_read_acquire(&ct->state) & CT_DYNTICKS_MASK;
> }
> #endif
>
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index fe9066fdfaab..87e7b748791c 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -63,9 +63,9 @@ static __always_inline void rcu_dynticks_task_trace_exit(void)
> * Increment the current CPU's context_tracking structure's ->dynticks field
> * with ordering. Return the new value.
> */
> -noinstr unsigned long rcu_dynticks_inc(int incby)
> +noinstr unsigned long ct_state_inc(int incby)
> {
> - return arch_atomic_add_return(incby, this_cpu_ptr(&context_tracking.dynticks));
> + return arch_atomic_add_return(incby, this_cpu_ptr(&context_tracking.state));
> }
>
> /*
> @@ -74,7 +74,7 @@ noinstr unsigned long rcu_dynticks_inc(int incby)
> * RCU is watching prior to the call to this function and is no longer
> * watching upon return.
> */
> -static noinstr void rcu_dynticks_eqs_enter(void)
> +static noinstr void ct_kernel_exit_state(int offset)
> {
> int seq;
>
> @@ -84,9 +84,9 @@ static noinstr void rcu_dynticks_eqs_enter(void)
> * next idle sojourn.
> */
> rcu_dynticks_task_trace_enter(); // Before ->dynticks update!
> - seq = rcu_dynticks_inc(1);
> + seq = ct_state_inc(offset);
> // RCU is no longer watching. Better be in extended quiescent state!
> - WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & 0x1));
> + WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & RCU_DYNTICKS_IDX));
> }
>
> /*
> @@ -94,7 +94,7 @@ static noinstr void rcu_dynticks_eqs_enter(void)
> * called from an extended quiescent state, that is, RCU is not watching
> * prior to the call to this function and is watching upon return.
> */
> -static noinstr void rcu_dynticks_eqs_exit(void)
> +static noinstr void ct_kernel_enter_state(int offset)
> {
> int seq;
>
> @@ -103,10 +103,10 @@ static noinstr void rcu_dynticks_eqs_exit(void)
> * and we also must force ordering with the next RCU read-side
> * critical section.
> */
> - seq = rcu_dynticks_inc(1);
> + seq = ct_state_inc(offset);
> // RCU is now watching. Better not be in an extended quiescent state!
> rcu_dynticks_task_trace_exit(); // After ->dynticks update!
> - WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & 0x1));
> + WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & RCU_DYNTICKS_IDX));
> }
>
> /*
> @@ -117,7 +117,7 @@ static noinstr void rcu_dynticks_eqs_exit(void)
> * the possibility of usermode upcalls having messed up our count
> * of interrupt nesting level during the prior busy period.
> */
> -static noinstr void rcu_eqs_enter(bool user)
> +static noinstr void ct_kernel_exit(bool user, int offset)
> {
> struct context_tracking *ct = this_cpu_ptr(&context_tracking);
>
> @@ -137,13 +137,13 @@ static noinstr void rcu_eqs_enter(bool user)
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> rcu_preempt_deferred_qs(current);
>
> - // instrumentation for the noinstr rcu_dynticks_eqs_enter()
> - instrument_atomic_write(&ct->dynticks, sizeof(ct->dynticks));
> + // instrumentation for the noinstr ct_kernel_exit_state()
> + instrument_atomic_write(&ct->state, sizeof(ct->state));
>
> instrumentation_end();
> WRITE_ONCE(ct->dynticks_nesting, 0); /* Avoid irq-access tearing. */
> // RCU is watching here ...
> - rcu_dynticks_eqs_enter();
> + ct_kernel_exit_state(offset);
> // ... but is no longer watching here.
> rcu_dynticks_task_enter();
> }
> @@ -152,7 +152,7 @@ static noinstr void rcu_eqs_enter(bool user)
> * ct_nmi_exit - inform RCU of exit from NMI context
> *
> * If we are returning from the outermost NMI handler that interrupted an
> - * RCU-idle period, update ct->dynticks and ct->dynticks_nmi_nesting
> + * RCU-idle period, update ct->state and ct->dynticks_nmi_nesting
> * to let the RCU grace-period handling know that the CPU is back to
> * being RCU-idle.
> *
> @@ -189,12 +189,12 @@ noinstr void ct_nmi_exit(void)
> trace_rcu_dyntick(TPS("Startirq"), ct->dynticks_nmi_nesting, 0, ct_dynticks());
> WRITE_ONCE(ct->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
>
> - // instrumentation for the noinstr rcu_dynticks_eqs_enter()
> - instrument_atomic_write(&ct->dynticks, sizeof(ct->dynticks));
> + // instrumentation for the noinstr ct_kernel_exit_state()
> + instrument_atomic_write(&ct->state, sizeof(ct->state));
> instrumentation_end();
>
> // RCU is watching here ...
> - rcu_dynticks_eqs_enter();
> + ct_kernel_exit_state(RCU_DYNTICKS_IDX);
> // ... but is no longer watching here.
>
> if (!in_nmi())
> @@ -209,7 +209,7 @@ noinstr void ct_nmi_exit(void)
> * allow for the possibility of usermode upcalls messing up our count of
> * interrupt nesting level during the busy period that is just now starting.
> */
> -static void noinstr rcu_eqs_exit(bool user)
> +static void noinstr ct_kernel_enter(bool user, int offset)
> {
> struct context_tracking *ct = this_cpu_ptr(&context_tracking);
> long oldval;
> @@ -224,12 +224,12 @@ static void noinstr rcu_eqs_exit(bool user)
> }
> rcu_dynticks_task_exit();
> // RCU is not watching here ...
> - rcu_dynticks_eqs_exit();
> + ct_kernel_enter_state(offset);
> // ... but is watching here.
> instrumentation_begin();
>
> - // instrumentation for the noinstr rcu_dynticks_eqs_exit()
> - instrument_atomic_write(&ct->dynticks, sizeof(ct->dynticks));
> + // instrumentation for the noinstr ct_kernel_enter_state()
> + instrument_atomic_write(&ct->state, sizeof(ct->state));
>
> trace_rcu_dyntick(TPS("End"), ct->dynticks_nesting, 1, ct_dynticks());
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> @@ -242,7 +242,7 @@ static void noinstr rcu_eqs_exit(bool user)
> /**
> * ct_nmi_enter - inform RCU of entry to NMI context
> *
> - * If the CPU was idle from RCU's viewpoint, update ct->dynticks and
> + * If the CPU was idle from RCU's viewpoint, update ct->state and
> * ct->dynticks_nmi_nesting to let the RCU grace-period handling know
> * that the CPU is active. This implementation permits nested NMIs, as
> * long as the nesting level does not overflow an int. (You will probably
> @@ -273,14 +273,14 @@ noinstr void ct_nmi_enter(void)
> rcu_dynticks_task_exit();
>
> // RCU is not watching here ...
> - rcu_dynticks_eqs_exit();
> + ct_kernel_enter_state(RCU_DYNTICKS_IDX);
> // ... but is watching here.
>
> instrumentation_begin();
> // instrumentation for the noinstr rcu_dynticks_curr_cpu_in_eqs()
> - instrument_atomic_read(&ct->dynticks, sizeof(ct->dynticks));
> - // instrumentation for the noinstr rcu_dynticks_eqs_exit()
> - instrument_atomic_write(&ct->dynticks, sizeof(ct->dynticks));
> + instrument_atomic_read(&ct->state, sizeof(ct->state));
> + // instrumentation for the noinstr ct_kernel_enter_state()
> + instrument_atomic_write(&ct->state, sizeof(ct->state));
>
> incby = 1;
> } else if (!in_nmi()) {
> @@ -373,22 +373,23 @@ void noinstr __ct_user_enter(enum ctx_state state)
> * CPU doesn't need to maintain the tick for RCU maintenance purposes
> * when the CPU runs in userspace.
> */
> - rcu_eqs_enter(true);
> + ct_kernel_exit(true, RCU_DYNTICKS_IDX + state);
> + } else {
> + /*
> + * Even if context tracking is disabled on this CPU, because it's outside
> + * the full dynticks mask for example, we still have to keep track of the
> + * context transitions and states to prevent inconsistency on those of
> + * other CPUs.
> + * If a task triggers an exception in userspace, sleep on the exception
> + * handler and then migrate to another CPU, that new CPU must know where
> + * the exception returns by the time we call exception_exit().
> + * This information can only be provided by the previous CPU when it called
> + * exception_enter().
> + * OTOH we can spare the calls to vtime and RCU when context_tracking.active
> + * is false because we know that CPU is not tickless.
> + */
> + atomic_add(state, &ct->state);
> }
> - /*
> - * Even if context tracking is disabled on this CPU, because it's outside
> - * the full dynticks mask for example, we still have to keep track of the
> - * context transitions and states to prevent inconsistency on those of
> - * other CPUs.
> - * If a task triggers an exception in userspace, sleep on the exception
> - * handler and then migrate to another CPU, that new CPU must know where
> - * the exception returns by the time we call exception_exit().
> - * This information can only be provided by the previous CPU when it called
> - * exception_enter().
> - * OTOH we can spare the calls to vtime and RCU when context_tracking.active
> - * is false because we know that CPU is not tickless.
> - */
> - atomic_set(&ct->state, state);
> }
> context_tracking_recursion_exit();
> }
> @@ -452,15 +453,16 @@ void noinstr __ct_user_exit(enum ctx_state state)
> * Exit RCU idle mode while entering the kernel because it can
> * run a RCU read side critical section anytime.
> */
> - rcu_eqs_exit(true);
> + ct_kernel_enter(true, RCU_DYNTICKS_IDX - state);
> if (state == CONTEXT_USER) {
> instrumentation_begin();
> vtime_user_exit(current);
> trace_user_exit(0);
> instrumentation_end();
> }
> + } else {
> + atomic_sub(state, &ct->state);

OK, atomic_sub() got my attention. What is going on here? ;-)

> }
> - atomic_set(&ct->state, CONTEXT_KERNEL);
> }
> context_tracking_recursion_exit();
> }
> @@ -530,7 +532,7 @@ void __init context_tracking_init(void)
> DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
> .dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
> .dynticks_nesting = 1,
> - .dynticks = ATOMIC_INIT(1),
> + .state = ATOMIC_INIT(RCU_DYNTICKS_IDX),
> };
> EXPORT_SYMBOL_GPL(context_tracking);
>
> @@ -548,7 +550,7 @@ EXPORT_SYMBOL_GPL(context_tracking);
> void ct_idle_enter(void)
> {
> lockdep_assert_irqs_disabled();
> - rcu_eqs_enter(false);
> + ct_kernel_exit(false, RCU_DYNTICKS_IDX + CONTEXT_IDLE);
> }
> EXPORT_SYMBOL_GPL(ct_idle_enter);
>
> @@ -566,7 +568,7 @@ void ct_idle_exit(void)
> unsigned long flags;
>
> local_irq_save(flags);
> - rcu_eqs_exit(false);
> + ct_kernel_enter(false, RCU_DYNTICKS_IDX - CONTEXT_IDLE);

Nice! This works because all transitions must be either from or
to kernel context, correct?

> local_irq_restore(flags);
>
> }
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 90a22dd2189d..98fac3d327c9 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -272,9 +272,9 @@ void rcu_softirq_qs(void)
> */
> static void rcu_dynticks_eqs_online(void)
> {
> - if (ct_dynticks() & 0x1)
> + if (ct_dynticks() & RCU_DYNTICKS_IDX)
> return;
> - rcu_dynticks_inc(1);
> + ct_state_inc(RCU_DYNTICKS_IDX);
> }
>
> /*
> @@ -293,7 +293,7 @@ static int rcu_dynticks_snap(int cpu)
> */
> static bool rcu_dynticks_in_eqs(int snap)
> {
> - return !(snap & 0x1);
> + return !(snap & RCU_DYNTICKS_IDX);
> }
>
> /* Return true if the specified CPU is currently idle from an RCU viewpoint. */
> @@ -321,8 +321,7 @@ bool rcu_dynticks_zero_in_eqs(int cpu, int *vp)
> int snap;
>
> // If not quiescent, force back to earlier extended quiescent state.
> - snap = ct_dynticks_cpu(cpu) & ~0x1;
> -
> + snap = ct_dynticks_cpu(cpu) & ~RCU_DYNTICKS_IDX;

Do we also need to get rid of the low-order bits? Or is that happening
elsewhere? Or is there some reason that they can stick around?

> smp_rmb(); // Order ->dynticks and *vp reads.
> if (READ_ONCE(*vp))
> return false; // Non-zero, so report failure;
> @@ -348,9 +347,9 @@ notrace void rcu_momentary_dyntick_idle(void)
> int seq;
>
> raw_cpu_write(rcu_data.rcu_need_heavy_qs, false);
> - seq = rcu_dynticks_inc(2);
> + seq = ct_state_inc(2 * RCU_DYNTICKS_IDX);
> /* It is illegal to call this from idle state. */
> - WARN_ON_ONCE(!(seq & 0x1));
> + WARN_ON_ONCE(!(seq & RCU_DYNTICKS_IDX));
> rcu_preempt_deferred_qs(current);
> }
> EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 9bf5cc79d5eb..1ac48c804006 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -459,7 +459,7 @@ static void print_cpu_stall_info(int cpu)
> rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
> "!."[!delta],
> ticks_value, ticks_title,
> - rcu_dynticks_snap(cpu) & 0xfff,
> + (rcu_dynticks_snap(cpu) >> RCU_DYNTICKS_SHIFT) & 0xfff ,

Actually, the low-ordder several bits are useful when debugging, so
could you please not shift them away? Maybe also go to 0xffff to allow
for more bits taken?

> ct->dynticks_nesting, ct->dynticks_nmi_nesting,
> rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
> data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
> --
> 2.25.1
>

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