Re: [PATCH tip/core/rcu 5/5] rcu: Maintain special bits at bottom of ->dynticks counter

From: Boqun Feng
Date: Mon Nov 14 2016 - 20:29:41 EST

Next message: Bart Van Assche: "Re: [PATCH V4 00/15] blk-throttle: add .high limit"
Previous message: Shaohua Li: "Re: [PATCH V4 00/15] blk-throttle: add .high limit"
In reply to: Paul E. McKenney: "[PATCH tip/core/rcu 5/5] rcu: Maintain special bits at bottom of ->dynticks counter"
Next in thread: Paul E. McKenney: "[PATCH tip/core/rcu 2/5] rcu: Abstract the dynticks snapshot operation"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

On Mon, Nov 14, 2016 at 10:30:21AM -0800, Paul E. McKenney wrote:
> Currently, IPIs are used to force other CPUs to invalidate their TLBs
> in response to a kernel virtual-memory mapping change. This works, but
> degrades both battery lifetime (for idle CPUs) and real-time response
> (for nohz_full CPUs), and in addition results in unnecessary IPIs due to
> the fact that CPUs executing in usermode are unaffected by stale kernel
> mappings. It would be better to cause a CPU executing in usermode to
> wait until it is entering kernel mode to do the flush, first to avoid
> interrupting usemode tasks and second to handle multiple flush requests
> with a single flush in the case of a long-running user task.
>
> This commit therefore reserves a bit at the bottom of the ->dynticks
> counter, which is checked upon exit from extended quiescent states.
> If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is
> invoked, which, if not supplied, is an empty single-pass do-while loop.
> If this bottom bit is set on -entry- to an extended quiescent state,
> then a WARN_ON_ONCE() triggers.
>
> This bottom bit may be set using a new rcu_eqs_special_set() function,
> which returns true if the bit was set, or false if the CPU turned
> out to not be in an extended quiescent state. Please note that this
> function refuses to set the bit for a non-nohz_full CPU when that CPU
> is executing in usermode because usermode execution is tracked by RCU
> as a dyntick-idle extended quiescent state only for nohz_full CPUs.
>
> Reported-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
> include/linux/rcutiny.h | 5 +++
> kernel/rcu/tree.c | 81 +++++++++++++++++++++++++++++++++++++------------
> kernel/rcu/tree.h | 1 +
> 3 files changed, 67 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> index 4f9b2fa2173d..7232d199a81c 100644
> --- a/include/linux/rcutiny.h
> +++ b/include/linux/rcutiny.h
> @@ -33,6 +33,11 @@ static inline int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
> return 0;
> }
>
> +static inline bool rcu_eqs_special_set(int cpu)
> +{
> + return false; /* Never flag non-existent other CPUs! */
> +}
> +
> static inline unsigned long get_state_synchronize_rcu(void)
> {
> return 0;
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c2b2f5b591b7..2c399db6df6e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -269,9 +269,19 @@ void rcu_bh_qs(void)
>
> static DEFINE_PER_CPU(int, rcu_sched_qs_mask);
>
> +/*
> + * Steal a bit from the bottom of ->dynticks for idle entry/exit
> + * control. Initially this is for TLB flushing.
> + */
> +#define RCU_DYNTICK_CTRL_MASK 0x1
> +#define RCU_DYNTICK_CTRL_CTR (RCU_DYNTICK_CTRL_MASK + 1)
> +#ifndef rcu_eqs_special_exit
> +#define rcu_eqs_special_exit() do { } while (0)
> +#endif
> +
> static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
> .dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
> - .dynticks = ATOMIC_INIT(1),
> + .dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR),
> #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> .dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
> .dynticks_idle = ATOMIC_INIT(1),
> @@ -285,17 +295,20 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
> static void rcu_dynticks_eqs_enter(void)
> {
> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> + int seq;
>
> /*
> - * CPUs seeing atomic_inc() must see prior RCU read-side critical
> - * sections, and we also must force ordering with the next idle
> - * sojourn.
> + * CPUs seeing atomic_inc_return() must see prior RCU read-side
> + * critical sections, and we also must force ordering with the
> + * next idle sojourn.
> */
> - smp_mb__before_atomic(); /* See above. */
> - atomic_inc(&rdtp->dynticks);
> - smp_mb__after_atomic(); /* See above. */
> + seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);
> + /* Better be in an extended quiescent state! */
> + WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> + (seq & RCU_DYNTICK_CTRL_CTR));
> + /* Better not have special action (TLB flush) pending! */

Ah.. you did the clean-up here ;-)

Never mind my previous comment on patch #3 ;-)

Regards,
Boqun

> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> - atomic_read(&rdtp->dynticks) & 0x1);
> + (seq & RCU_DYNTICK_CTRL_MASK));
> }
>
> /*
> @@ -305,17 +318,22 @@ static void rcu_dynticks_eqs_enter(void)
> static void rcu_dynticks_eqs_exit(void)
> {
> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> + int seq;
>
> /*
> - * CPUs seeing atomic_inc() must see prior idle sojourns,
> + * CPUs seeing atomic_inc_return() must see prior idle sojourns,
> * and we also must force ordering with the next RCU read-side
> * critical section.
> */
> - smp_mb__before_atomic(); /* See above. */
> - atomic_inc(&rdtp->dynticks);
> - smp_mb__after_atomic(); /* See above. */
> + seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> - !(atomic_read(&rdtp->dynticks) & 0x1));
> + !(seq & RCU_DYNTICK_CTRL_CTR));
> + if (seq & RCU_DYNTICK_CTRL_MASK) {
> + rcu_eqs_special_exit();
> + /* Prefer duplicate flushes to losing a flush. */
> + smp_mb__before_atomic(); /* NMI safety. */
> + atomic_and(~RCU_DYNTICK_CTRL_MASK, &rdtp->dynticks);
> + }
> }
>
> /*
> @@ -332,9 +350,9 @@ static void rcu_dynticks_eqs_online(void)
> {
> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
>
> - if (atomic_read(&rdtp->dynticks) & 0x1)
> + if (atomic_read(&rdtp->dynticks) & RCU_DYNTICK_CTRL_CTR)
> return;
> - atomic_add(0x1, &rdtp->dynticks);
> + atomic_add(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);
> }
>
> /*
> @@ -346,7 +364,7 @@ bool rcu_dynticks_curr_cpu_in_eqs(void)
> {
> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
>
> - return !(atomic_read(&rdtp->dynticks) & 0x1);
> + return !(atomic_read(&rdtp->dynticks) & RCU_DYNTICK_CTRL_CTR);
> }
>
> /*
> @@ -357,7 +375,7 @@ int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
> {
> int snap = atomic_add_return(0, &rdtp->dynticks);
>
> - return snap;
> + return snap & ~RCU_DYNTICK_CTRL_MASK;
> }
>
> /*
> @@ -366,7 +384,7 @@ int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
> */
> static bool rcu_dynticks_in_eqs(int snap)
> {
> - return !(snap & 0x1);
> + return !(snap & RCU_DYNTICK_CTRL_CTR);
> }
>
> /*
> @@ -386,10 +404,33 @@ static bool rcu_dynticks_in_eqs_since(struct rcu_dynticks *rdtp, int snap)
> static void rcu_dynticks_momentary_idle(void)
> {
> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> - int special = atomic_add_return(2, &rdtp->dynticks);
> + int special = atomic_add_return(2 * RCU_DYNTICK_CTRL_CTR,
> + &rdtp->dynticks);
>
> /* It is illegal to call this from idle state. */
> - WARN_ON_ONCE(!(special & 0x1));
> + WARN_ON_ONCE(!(special & RCU_DYNTICK_CTRL_CTR));
> +}
> +
> +/*
> + * Set the special (bottom) bit of the specified CPU so that it
> + * will take special action (such as flushing its TLB) on the
> + * next exit from an extended quiescent state. Returns true if
> + * the bit was successfully set, or false if the CPU was not in
> + * an extended quiescent state.
> + */
> +bool rcu_eqs_special_set(int cpu)
> +{
> + int old;
> + int new;
> + struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> +
> + do {
> + old = atomic_read(&rdtp->dynticks);
> + if (old & RCU_DYNTICK_CTRL_CTR)
> + return false;
> + new = old | RCU_DYNTICK_CTRL_MASK;
> + } while (atomic_cmpxchg(&rdtp->dynticks, old, new) != old);
> + return true;
> }
>
> DEFINE_PER_CPU_SHARED_ALIGNED(unsigned long, rcu_qs_ctr);
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 3b953dcf6afc..7dcdd59d894c 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -596,6 +596,7 @@ extern struct rcu_state rcu_preempt_state;
> #endif /* #ifdef CONFIG_PREEMPT_RCU */
>
> int rcu_dynticks_snap(struct rcu_dynticks *rdtp);
> +bool rcu_eqs_special_set(int cpu);
>
> #ifdef CONFIG_RCU_BOOST
> DECLARE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
> --
> 2.5.2
>

Attachment: signature.asc
Description: PGP signature

Next message: Bart Van Assche: "Re: [PATCH V4 00/15] blk-throttle: add .high limit"
Previous message: Shaohua Li: "Re: [PATCH V4 00/15] blk-throttle: add .high limit"
In reply to: Paul E. McKenney: "[PATCH tip/core/rcu 5/5] rcu: Maintain special bits at bottom of ->dynticks counter"
Next in thread: Paul E. McKenney: "[PATCH tip/core/rcu 2/5] rcu: Abstract the dynticks snapshot operation"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]