On Wed, Sep 29, 2021 at 05:17:31PM +0200, Peter Zijlstra wrote:
> XXX I'm pretty sure I broke task-trace-rcu.
> XXX trace_rcu_*() now gets an unconditional 0
>
> Other than that, it seems like a fairly straight-forward replacement
> of the RCU count with the context_tracking count.
>
> Using context-tracking for this avoids having two (expensive) atomic
> ops on the entry paths where one will do.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
A few questions and exclamations interspersed.
> ---
> include/linux/context_tracking.h | 6 ++
> kernel/context_tracking.c | 14 +++++
> kernel/rcu/tree.c | 101 +++++----------------------------------
> kernel/rcu/tree.h | 1
> 4 files changed, 33 insertions(+), 89 deletions(-)
>
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -50,6 +50,9 @@ extern void context_tracking_user_exit(v
>
> extern bool context_tracking_set_cpu_work(unsigned int cpu, unsigned int work);
>
> +extern void context_tracking_idle(void);
> +extern void context_tracking_online(void);
> +
> static inline void ct_user_enter(void)
> {
> if (context_tracking_enabled())
> @@ -162,6 +165,9 @@ static __always_inline unsigned int __co
> return 0;
> }
>
> +static inline void context_tracking_idle(void) { }
> +static inline void context_tracking_online(void) { }
> +
> #endif /* !CONFIG_CONTEXT_TRACKING */
>
> static __always_inline bool context_tracking_cpu_in_user(unsigned int cpu)
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -281,6 +281,20 @@ void noinstr __context_tracking_nmi_exit
> ct_seq_nmi_exit(raw_cpu_ptr(&context_tracking));
> }
>
> +void context_tracking_online(void)
> +{
> + struct context_tracking *ct = raw_cpu_ptr(&context_tracking);
> + unsigned int seq = atomic_read(&ct->seq);
> +
> + if (__context_tracking_seq_in_user(seq))
> + atomic_add_return(CT_SEQ - CT_SEQ_USER, &ct->seq);
What if the CPU went offline marked idle instead of user? (Yes, that
no longer happens, but checking my understanding.)
The CT_SEQ_WORK bit means neither idle nor nohz_full user, correct?
So let's see if I intuited the decoder ring, where "kernel" means that
portion of the kernel that is non-noinstr...
CT_SEQ_WORK CT_SEQ_NMI CT_SEQ_USER Description?
0 0 0 Idle or non-nohz_full user
0 0 1 nohz_full user
0 1 0 NMI from 0,0,0
0 1 1 NMI from 0,0,1
1 0 0 Non-idle kernel
1 0 1 Cannot happen?
1 1 0 NMI from 1,0,1
1 1 1 NMI from cannot happen
And of course if a state cannot happen, Murphy says that you will take
an NMI in that state.
> +}
> +
> +void context_tracking_idle(void)
> +{
> + atomic_add_return(CT_SEQ, &raw_cpu_ptr(&context_tracking)->seq);
This is presumably a momentary idle.
And what happens to all of this in !CONFIG_CONTEXT_TRACKING kernels?
Of course, RCU needs it unconditionally. (There appear to be at least
parts of it that are unconditionally available, but I figured that I
should ask. Especially given the !CONFIG_CONTEXT_TRACKING definition
of the __context_tracking_cpu_seq() function.)
> +}
> +
> void __init context_tracking_cpu_set(int cpu)
> {
> static __initdata bool initialized = false;
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -62,6 +62,7 @@
> #include <linux/vmalloc.h>
> #include <linux/mm.h>
> #include <linux/kasan.h>
> +#include <linux/context_tracking.h>
> #include "../time/tick-internal.h"
>
> #include "tree.h"
> @@ -77,7 +78,6 @@
> static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
> .dynticks_nesting = 1,
> .dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
> - .dynticks = ATOMIC_INIT(1),
> #ifdef CONFIG_RCU_NOCB_CPU
> .cblist.flags = SEGCBLIST_SOFTIRQ_ONLY,
> #endif
> @@ -252,56 +252,6 @@ void rcu_softirq_qs(void)
> }
>
> /*
> - * Increment the current CPU's rcu_data structure's ->dynticks field
> - * with ordering. Return the new value.
> - */
> -static noinline noinstr unsigned long rcu_dynticks_inc(int incby)
> -{
> - return arch_atomic_add_return(incby, this_cpu_ptr(&rcu_data.dynticks));
> -}
> -
> -/*
> - * Record entry into an extended quiescent state. This is only to be
> - * called when not already in an extended quiescent state, that is,
> - * 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)
> -{
> - int seq;
> -
> - /*
> - * CPUs seeing atomic_add_return() must see prior RCU read-side
> - * critical sections, and we also must force ordering with the
> - * next idle sojourn.
> - */
> - rcu_dynticks_task_trace_enter(); // Before ->dynticks update!
> - seq = rcu_dynticks_inc(1);
> - // RCU is no longer watching. Better be in extended quiescent state!
> - WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & 0x1));
> -}
> -
> -/*
> - * Record exit from an extended quiescent state. This is only to be
> - * 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)
> -{
> - int seq;
> -
> - /*
> - * CPUs seeing atomic_add_return() must see prior idle sojourns,
> - * and we also must force ordering with the next RCU read-side
> - * critical section.
> - */
> - seq = rcu_dynticks_inc(1);
> - // 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));
> -}
> -
> -/*
> * Reset the current CPU's ->dynticks counter to indicate that the
> * newly onlined CPU is no longer in an extended quiescent state.
> * This will either leave the counter unchanged, or increment it
> @@ -313,11 +263,7 @@ static noinstr void rcu_dynticks_eqs_exi
> */
> static void rcu_dynticks_eqs_online(void)
> {
> - struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> -
> - if (atomic_read(&rdp->dynticks) & 0x1)
> - return;
> - rcu_dynticks_inc(1);
> + context_tracking_online();
> }
>
> /*
> @@ -327,7 +273,7 @@ static void rcu_dynticks_eqs_online(void
> */
> static __always_inline bool rcu_dynticks_curr_cpu_in_eqs(void)
> {
> - return !(atomic_read(this_cpu_ptr(&rcu_data.dynticks)) & 0x1);
> + return context_tracking_cpu_in_user(smp_processor_id());
This needs to return true in any type of extended quiescent state,
not just nohz_full user execution.
> }
>
> /*
> @@ -337,7 +283,7 @@ static __always_inline bool rcu_dynticks
> static int rcu_dynticks_snap(struct rcu_data *rdp)
> {
> smp_mb(); // Fundamental RCU ordering guarantee.
> - return atomic_read_acquire(&rdp->dynticks);
> + return __context_tracking_cpu_seq(rdp->cpu);
This will most definitely break RCU in !CONFIG_CONTEXT_TRACKING kernels.
It will always return zero. Albeit fully ordered. Might this be
why RCU Tasks Trace is unhappy?
Similar issues for later uses of __context_tracking_cpu_seq().
> }
>
> /*
> @@ -346,7 +292,7 @@ static int rcu_dynticks_snap(struct rcu_
> */
> static bool rcu_dynticks_in_eqs(int snap)
> {
> - return !(snap & 0x1);
> + return __context_tracking_seq_in_user(snap);
> }
>
> /* Return true if the specified CPU is currently idle from an RCU viewpoint. */
> @@ -377,7 +323,7 @@ bool rcu_dynticks_zero_in_eqs(int cpu, i
> int snap;
>
> // If not quiescent, force back to earlier extended quiescent state.
> - snap = atomic_read(&rdp->dynticks) & ~0x1;
> + snap = __context_tracking_cpu_seq(rdp->cpu) & ~0x7;
>
> smp_rmb(); // Order ->dynticks and *vp reads.
> if (READ_ONCE(*vp))
> @@ -385,7 +331,7 @@ bool rcu_dynticks_zero_in_eqs(int cpu, i
> smp_rmb(); // Order *vp read and ->dynticks re-read.
>
> // If still in the same extended quiescent state, we are good!
> - return snap == atomic_read(&rdp->dynticks);
> + return snap == __context_tracking_cpu_seq(rdp->cpu);
> }
>
> /*
> @@ -401,12 +347,8 @@ bool rcu_dynticks_zero_in_eqs(int cpu, i
> */
> notrace void rcu_momentary_dyntick_idle(void)
> {
> - int seq;
> -
> raw_cpu_write(rcu_data.rcu_need_heavy_qs, false);
> - seq = rcu_dynticks_inc(2);
> - /* It is illegal to call this from idle state. */
> - WARN_ON_ONCE(!(seq & 0x1));
> + context_tracking_idle();
> rcu_preempt_deferred_qs(current);
> }
> EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);
> @@ -622,18 +564,15 @@ static noinstr void rcu_eqs_enter(bool u
>
> lockdep_assert_irqs_disabled();
> instrumentation_begin();
> - trace_rcu_dyntick(TPS("Start"), rdp->dynticks_nesting, 0, atomic_read(&rdp->dynticks));
> + trace_rcu_dyntick(TPS("Start"), rdp->dynticks_nesting, 0, 0);
Why not get the value from __context_tracking_cpu_seq()?
Ditto for similar changes later on.
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> rcu_prepare_for_idle();
> rcu_preempt_deferred_qs(current);
>
> - // instrumentation for the noinstr rcu_dynticks_eqs_enter()
> - instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
>
> instrumentation_end();
> WRITE_ONCE(rdp->dynticks_nesting, 0); /* Avoid irq-access tearing. */
> // RCU is watching here ...
> - rcu_dynticks_eqs_enter();
> // ... but is no longer watching here.
OK, at the very least, these two comments are now nonsense. ;-)
Is the intent to drive this from context tracking, so that these functions
only push the various required RCU state transitions?
Same for the similar changes below.
Anyway, unless I am missing something subtle (quite possible given
that I haven't looked closely at context tracking), you did succeed in
breaking RCU! ;-)
> rcu_dynticks_task_enter();
> }
> @@ -756,8 +695,7 @@ noinstr void rcu_nmi_exit(void)
> * leave it in non-RCU-idle state.
> */
> if (rdp->dynticks_nmi_nesting != 1) {
> - trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2,
> - atomic_read(&rdp->dynticks));
> + trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, 0);
> WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
> rdp->dynticks_nmi_nesting - 2);
> instrumentation_end();
> @@ -765,18 +703,15 @@ noinstr void rcu_nmi_exit(void)
> }
>
> /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
> - trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
> + trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, 0);
> WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
>
> if (!in_nmi())
> rcu_prepare_for_idle();
>
> - // instrumentation for the noinstr rcu_dynticks_eqs_enter()
> - instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
> instrumentation_end();
>
> // RCU is watching here ...
> - rcu_dynticks_eqs_enter();
> // ... but is no longer watching here.
>
> if (!in_nmi())
> @@ -865,15 +800,11 @@ static void noinstr rcu_eqs_exit(bool us
> }
> rcu_dynticks_task_exit();
> // RCU is not watching here ...
> - rcu_dynticks_eqs_exit();
> // ... but is watching here.
> instrumentation_begin();
>
> - // instrumentation for the noinstr rcu_dynticks_eqs_exit()
> - instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
> -
> rcu_cleanup_after_idle();
> - trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, atomic_read(&rdp->dynticks));
> + trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, 0);
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> WRITE_ONCE(rdp->dynticks_nesting, 1);
> WARN_ON_ONCE(rdp->dynticks_nmi_nesting);
> @@ -1011,7 +942,6 @@ noinstr void rcu_nmi_enter(void)
> rcu_dynticks_task_exit();
>
> // RCU is not watching here ...
> - rcu_dynticks_eqs_exit();
> // ... but is watching here.
>
> if (!in_nmi()) {
> @@ -1021,11 +951,6 @@ noinstr void rcu_nmi_enter(void)
> }
>
> instrumentation_begin();
> - // instrumentation for the noinstr rcu_dynticks_curr_cpu_in_eqs()
> - instrument_atomic_read(&rdp->dynticks, sizeof(rdp->dynticks));
> - // instrumentation for the noinstr rcu_dynticks_eqs_exit()
> - instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
> -
> incby = 1;
> } else if (!in_nmi()) {
> instrumentation_begin();
> @@ -1036,7 +961,7 @@ noinstr void rcu_nmi_enter(void)
>
> trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
> rdp->dynticks_nmi_nesting,
> - rdp->dynticks_nmi_nesting + incby, atomic_read(&rdp->dynticks));
> + rdp->dynticks_nmi_nesting + incby, 0);
> instrumentation_end();
> WRITE_ONCE(rdp->dynticks_nmi_nesting, /* Prevent store tearing. */
> rdp->dynticks_nmi_nesting + incby);
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -184,7 +184,6 @@ struct rcu_data {
> int dynticks_snap; /* Per-GP tracking for dynticks. */
> long dynticks_nesting; /* Track process nesting level. */
> long dynticks_nmi_nesting; /* Track irq/NMI nesting level. */
> - atomic_t dynticks; /* Even value for idle, else odd. */
> bool rcu_need_heavy_qs; /* GP old, so heavy quiescent state! */
> bool rcu_urgent_qs; /* GP old need light quiescent state. */
> bool rcu_forced_tick; /* Forced tick to provide QS. */
>
>
