diff mbox series

[RFC,v3,11/15] context-tracking: Introduce work deferral infrastructure

Message ID 20241119153502.41361-12-vschneid@redhat.com (mailing list archive)
State New
Headers show
Series context_tracking,x86: Defer some IPIs until a user->kernel transition | expand

Commit Message

Valentin Schneider Nov. 19, 2024, 3:34 p.m. UTC 
smp_call_function() & friends have the unfortunate habit of sending IPIs to
isolated, NOHZ_FULL, in-userspace CPUs, as they blindly target all online
CPUs.

Some callsites can be bent into doing the right, such as done by commit:

  cc9e303c91f5 ("x86/cpu: Disable frequency requests via aperfmperf IPI for nohz_full CPUs")

Unfortunately, not all SMP callbacks can be omitted in this
fashion. However, some of them only affect execution in kernelspace, which
means they don't have to be executed *immediately* if the target CPU is in
userspace: stashing the callback and executing it upon the next kernel entry
would suffice. x86 kernel instruction patching or kernel TLB invalidation
are prime examples of it.

Reduce the RCU dynticks counter width to free up some bits to be used as a
deferred callback bitmask. Add some build-time checks to validate that
setup.

Presence of CT_STATE_KERNEL in the ct_state prevents queuing deferred work.

Later commits introduce the bit:callback mappings.

Link: https://lore.kernel.org/all/20210929151723.162004989@infradead.org/
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 arch/Kconfig                                 |  9 ++++
 arch/x86/Kconfig                             |  1 +
 arch/x86/include/asm/context_tracking_work.h | 14 ++++++
 include/linux/context_tracking.h             | 21 +++++++++
 include/linux/context_tracking_state.h       | 30 ++++++++-----
 include/linux/context_tracking_work.h        | 26 +++++++++++
 kernel/context_tracking.c                    | 46 +++++++++++++++++++-
 kernel/time/Kconfig                          |  5 +++
 8 files changed, 140 insertions(+), 12 deletions(-)
 create mode 100644 arch/x86/include/asm/context_tracking_work.h
 create mode 100644 include/linux/context_tracking_work.h

Comments

Frederic Weisbecker Nov. 20, 2024, 10:54 a.m. UTC | #1 
Le Tue, Nov 19, 2024 at 04:34:58PM +0100, Valentin Schneider a écrit :
> +bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
> +{
> +	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> +	unsigned int old;
> +	bool ret = false;
> +
> +	preempt_disable();
> +
> +	old = atomic_read(&ct->state);
> +	/*
> +	 * Try setting the work until either
> +	 * - the target CPU has entered kernelspace
> +	 * - the work has been set
> +	 */
> +	do {
> +		ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START));
> +	} while (!ret && ((old & CT_STATE_MASK) != CT_STATE_KERNEL));
> +
> +	preempt_enable();
> +	return ret;

Does it ignore the IPI even if:

     (ret && (old & CT_STATE_MASK) == CT_STATE_KERNEL))

?

And what about CT_STATE_IDLE?

Is the work ignored in those two cases?

But would it be cleaner to never set the work if the target is elsewhere
than CT_STATE_USER. So you don't need to clear the work on kernel exit
but rather on kernel entry.

That is:

bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
{
	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
	unsigned int old;
	bool ret = false;

	preempt_disable();

	old = atomic_read(&ct->state);

	/* Start with our best wishes */
	old &= ~CT_STATE_MASK;
	old |= CT_STATE_USER

	/*
	 * Try setting the work until either
	 * - the target CPU has exited userspace
	 * - the work has been set
	 */
	do {
		ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START));
	} while (!ret && ((old & CT_STATE_MASK) == CT_STATE_USER));

	preempt_enable();

	return ret;
}

Thanks.
Frederic Weisbecker Nov. 20, 2024, 2:23 p.m. UTC | #2 
Le Wed, Nov 20, 2024 at 11:54:36AM +0100, Frederic Weisbecker a écrit :
> Le Tue, Nov 19, 2024 at 04:34:58PM +0100, Valentin Schneider a écrit :
> > +bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
> > +{
> > +	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> > +	unsigned int old;
> > +	bool ret = false;
> > +
> > +	preempt_disable();
> > +
> > +	old = atomic_read(&ct->state);
> > +	/*
> > +	 * Try setting the work until either
> > +	 * - the target CPU has entered kernelspace
> > +	 * - the work has been set
> > +	 */
> > +	do {
> > +		ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START));
> > +	} while (!ret && ((old & CT_STATE_MASK) != CT_STATE_KERNEL));
> > +
> > +	preempt_enable();
> > +	return ret;
> 
> Does it ignore the IPI even if:
> 
>      (ret && (old & CT_STATE_MASK) == CT_STATE_KERNEL))
> 
> ?
> 
> And what about CT_STATE_IDLE?
> 
> Is the work ignored in those two cases?
> 
> But would it be cleaner to never set the work if the target is elsewhere
> than CT_STATE_USER. So you don't need to clear the work on kernel exit
> but rather on kernel entry.
> 
> That is:
> 
> bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
> {
> 	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> 	unsigned int old;
> 	bool ret = false;
> 
> 	preempt_disable();
> 
> 	old = atomic_read(&ct->state);
> 
> 	/* Start with our best wishes */
> 	old &= ~CT_STATE_MASK;
> 	old |= CT_STATE_USER
> 
> 	/*
> 	 * Try setting the work until either
> 	 * - the target CPU has exited userspace
> 	 * - the work has been set
> 	 */
> 	do {
> 		ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START));
> 	} while (!ret && ((old & CT_STATE_MASK) == CT_STATE_USER));
> 
> 	preempt_enable();
> 
> 	return ret;
> }

Ah but there is CT_STATE_GUEST and I see the last patch also applies that to
CT_STATE_IDLE.

So that could be:

bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
{
	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
	unsigned int old;
	bool ret = false;

	preempt_disable();

	old = atomic_read(&ct->state);

	/* CT_STATE_IDLE can be added to last patch here */
	if (!(old & (CT_STATE_USER | CT_STATE_GUEST))) {
		old &= ~CT_STATE_MASK;
		old |= CT_STATE_USER;
	}

	/*
	 * Try setting the work until either
	 * - the target CPU has exited userspace / guest
	 * - the work has been set
	 */
	do {
		ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START));
	} while (!ret && old & (CT_STATE_USER | CT_STATE_GUEST));

	preempt_enable();

	return ret;
}

Thanks.
Valentin Schneider Nov. 20, 2024, 5:10 p.m. UTC | #3 
On 20/11/24 15:23, Frederic Weisbecker wrote:

> Ah but there is CT_STATE_GUEST and I see the last patch also applies that to
> CT_STATE_IDLE.
>
> So that could be:
>
> bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
> {
> 	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> 	unsigned int old;
> 	bool ret = false;
>
> 	preempt_disable();
>
> 	old = atomic_read(&ct->state);
>
> 	/* CT_STATE_IDLE can be added to last patch here */
> 	if (!(old & (CT_STATE_USER | CT_STATE_GUEST))) {
> 		old &= ~CT_STATE_MASK;
> 		old |= CT_STATE_USER;
> 	}

Hmph, so that lets us leverage the cmpxchg for a !CT_STATE_KERNEL check,
but we get an extra loop if the target CPU exits kernelspace not to
userspace (e.g. vcpu or idle) in the meantime - not great, not terrible.

At the cost of one extra bit for the CT_STATE area, with CT_STATE_KERNEL=1
we could do: 

  old = atomic_read(&ct->state);
  old &= ~CT_STATE_KERNEL;
Frederic Weisbecker Nov. 20, 2024, 5:30 p.m. UTC | #4 
Le Wed, Nov 20, 2024 at 06:10:43PM +0100, Valentin Schneider a écrit :
> On 20/11/24 15:23, Frederic Weisbecker wrote:
> 
> > Ah but there is CT_STATE_GUEST and I see the last patch also applies that to
> > CT_STATE_IDLE.
> >
> > So that could be:
> >
> > bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
> > {
> > 	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> > 	unsigned int old;
> > 	bool ret = false;
> >
> > 	preempt_disable();
> >
> > 	old = atomic_read(&ct->state);
> >
> > 	/* CT_STATE_IDLE can be added to last patch here */
> > 	if (!(old & (CT_STATE_USER | CT_STATE_GUEST))) {
> > 		old &= ~CT_STATE_MASK;
> > 		old |= CT_STATE_USER;
> > 	}
> 
> Hmph, so that lets us leverage the cmpxchg for a !CT_STATE_KERNEL check,
> but we get an extra loop if the target CPU exits kernelspace not to
> userspace (e.g. vcpu or idle) in the meantime - not great, not terrible.

The thing is, what you read with atomic_read() should be close to reality.
If it already is != CT_STATE_KERNEL then you're good (minus racy changes).
If it is CT_STATE_KERNEL then you still must do a failing cmpxchg() in any case,
at least to make sure you didn't miss a context tracking change. So the best
you can do is a bet.

> 
> At the cost of one extra bit for the CT_STATE area, with CT_STATE_KERNEL=1
> we could do: 
> 
>   old = atomic_read(&ct->state);
>   old &= ~CT_STATE_KERNEL;

And perhaps also old |= CT_STATE_IDLE (I'm seeing the last patch now),
so you at least get a chance of making it right (only ~CT_STATE_KERNEL
will always fail) and CPUs usually spend most of their time idle.

Thanks.
diff mbox series

Patch

diff --git a/arch/Kconfig b/arch/Kconfig
index bd9f095d69fa0..e7f3f797a34a4 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -912,6 +912,15 @@  config HAVE_CONTEXT_TRACKING_USER_OFFSTACK
 	  - No use of instrumentation, unless instrumentation_begin() got
 	    called.
 
+config HAVE_CONTEXT_TRACKING_WORK
+	bool
+	help
+	  Architecture supports deferring work while not in kernel context.
+	  This is especially useful on setups with isolated CPUs that might
+	  want to avoid being interrupted to perform housekeeping tasks (for
+	  ex. TLB invalidation or icache invalidation). The housekeeping
+	  operations are performed upon re-entering the kernel.
+
 config HAVE_TIF_NOHZ
 	bool
 	help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 16354dfa6d965..c703376dd326b 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -213,6 +213,7 @@  config X86
 	select HAVE_CMPXCHG_LOCAL
 	select HAVE_CONTEXT_TRACKING_USER		if X86_64
 	select HAVE_CONTEXT_TRACKING_USER_OFFSTACK	if HAVE_CONTEXT_TRACKING_USER
+	select HAVE_CONTEXT_TRACKING_WORK		if X86_64
 	select HAVE_C_RECORDMCOUNT
 	select HAVE_OBJTOOL_MCOUNT		if HAVE_OBJTOOL
 	select HAVE_OBJTOOL_NOP_MCOUNT		if HAVE_OBJTOOL_MCOUNT
diff --git a/arch/x86/include/asm/context_tracking_work.h b/arch/x86/include/asm/context_tracking_work.h
new file mode 100644
index 0000000000000..5bc29e6b2ed38
--- /dev/null
+++ b/arch/x86/include/asm/context_tracking_work.h
@@ -0,0 +1,14 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_CONTEXT_TRACKING_WORK_H
+#define _ASM_X86_CONTEXT_TRACKING_WORK_H
+
+static __always_inline void arch_context_tracking_work(int work)
+{
+	switch (work) {
+	case CONTEXT_WORK_n:
+		// Do work...
+		break;
+	}
+}
+
+#endif
diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index af9fe87a09225..16a2eb7525f1f 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -5,6 +5,7 @@ 
 #include <linux/sched.h>
 #include <linux/vtime.h>
 #include <linux/context_tracking_state.h>
+#include <linux/context_tracking_work.h>
 #include <linux/instrumentation.h>
 
 #include <asm/ptrace.h>
@@ -137,6 +138,26 @@  static __always_inline unsigned long ct_state_inc(int incby)
 	return raw_atomic_add_return(incby, this_cpu_ptr(&context_tracking.state));
 }
 
+#ifdef CONTEXT_TRACKING_WORK
+static __always_inline unsigned long ct_state_inc_clear_work(int incby)
+{
+	struct context_tracking *ct = this_cpu_ptr(&context_tracking);
+	unsigned long new, old, state;
+
+	state = arch_atomic_read(&ct->state);
+	do {
+		old = state;
+		new = old & ~CONTEXT_WORK_MASK;
+		new += incby;
+		state = arch_atomic_cmpxchg(&ct->state, old, new);
+	} while (old != state);
+
+	return new;
+}
+#else
+#define ct_state_inc_clear_work(x) ct_state_inc(x)
+#endif
+
 static __always_inline bool warn_rcu_enter(void)
 {
 	bool ret = false;
diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
index 0b81248aa03e2..d1d37dbdf7195 100644
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -5,6 +5,7 @@ 
 #include <linux/percpu.h>
 #include <linux/static_key.h>
 #include <linux/context_tracking_irq.h>
+#include <linux/context_tracking_work.h>
 
 /* Offset to allow distinguishing irq vs. task-based idle entry/exit. */
 #define CT_NESTING_IRQ_NONIDLE	((LONG_MAX / 2) + 1)
@@ -39,16 +40,19 @@  struct context_tracking {
 };
 
 /*
- * We cram two different things within the same atomic variable:
+ * We cram up to three different things within the same atomic variable:
  *
- *                     CT_RCU_WATCHING_START  CT_STATE_START
- *                                |                |
- *                                v                v
- *     MSB [ RCU watching counter ][ context_state ] LSB
- *         ^                       ^
- *         |                       |
- * CT_RCU_WATCHING_END        CT_STATE_END
+ *                     CT_RCU_WATCHING_START                  CT_STATE_START
+ *                                |         CT_WORK_START          |
+ *                                |               |                |
+ *                                v               v                v
+ *     MSB [ RCU watching counter ][ context work ][ context_state ] LSB
+ *         ^                       ^               ^
+ *         |                       |               |
+ *         |                  CT_WORK_END          |
+ * CT_RCU_WATCHING_END                        CT_STATE_END
  *
+ * The [ context work ] region spans 0 bits if CONFIG_CONTEXT_WORK=n
  * Bits are used from the LSB upwards, so unused bits (if any) will always be in
  * upper bits of the variable.
  */
@@ -59,18 +63,24 @@  struct context_tracking {
 #define CT_STATE_START 0
 #define CT_STATE_END   (CT_STATE_START + CT_STATE_WIDTH - 1)
 
-#define CT_RCU_WATCHING_MAX_WIDTH (CT_SIZE - CT_STATE_WIDTH)
+#define CT_WORK_WIDTH (IS_ENABLED(CONFIG_CONTEXT_TRACKING_WORK) ? CONTEXT_WORK_MAX_OFFSET : 0)
+#define	CT_WORK_START (CT_STATE_END + 1)
+#define CT_WORK_END   (CT_WORK_START + CT_WORK_WIDTH - 1)
+
+#define CT_RCU_WATCHING_MAX_WIDTH (CT_SIZE - CT_WORK_WIDTH - CT_STATE_WIDTH)
 #define CT_RCU_WATCHING_WIDTH     (IS_ENABLED(CONFIG_RCU_DYNTICKS_TORTURE) ? 2 : CT_RCU_WATCHING_MAX_WIDTH)
-#define CT_RCU_WATCHING_START     (CT_STATE_END + 1)
+#define CT_RCU_WATCHING_START     (CT_WORK_END + 1)
 #define CT_RCU_WATCHING_END       (CT_RCU_WATCHING_START + CT_RCU_WATCHING_WIDTH - 1)
 #define CT_RCU_WATCHING           BIT(CT_RCU_WATCHING_START)
 
 #define CT_STATE_MASK        GENMASK(CT_STATE_END,        CT_STATE_START)
+#define CT_WORK_MASK         GENMASK(CT_WORK_END,         CT_WORK_START)
 #define CT_RCU_WATCHING_MASK GENMASK(CT_RCU_WATCHING_END, CT_RCU_WATCHING_START)
 
 #define CT_UNUSED_WIDTH (CT_RCU_WATCHING_MAX_WIDTH - CT_RCU_WATCHING_WIDTH)
 
 static_assert(CT_STATE_WIDTH        +
+	      CT_WORK_WIDTH         +
 	      CT_RCU_WATCHING_WIDTH +
 	      CT_UNUSED_WIDTH       ==
 	      CT_SIZE);
diff --git a/include/linux/context_tracking_work.h b/include/linux/context_tracking_work.h
new file mode 100644
index 0000000000000..fb74db8876dd2
--- /dev/null
+++ b/include/linux/context_tracking_work.h
@@ -0,0 +1,26 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_CONTEXT_TRACKING_WORK_H
+#define _LINUX_CONTEXT_TRACKING_WORK_H
+
+#include <linux/bitops.h>
+
+enum {
+	CONTEXT_WORK_n_OFFSET,
+	CONTEXT_WORK_MAX_OFFSET
+};
+
+enum ct_work {
+	CONTEXT_WORK_n        = BIT(CONTEXT_WORK_n_OFFSET),
+	CONTEXT_WORK_MAX      = BIT(CONTEXT_WORK_MAX_OFFSET)
+};
+
+#include <asm/context_tracking_work.h>
+
+#ifdef CONFIG_CONTEXT_TRACKING_WORK
+extern bool ct_set_cpu_work(unsigned int cpu, unsigned int work);
+#else
+static inline bool
+ct_set_cpu_work(unsigned int cpu, unsigned int work) { return false; }
+#endif
+
+#endif
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 938c48952d265..37b094ea56fb6 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -72,6 +72,47 @@  static __always_inline void rcu_task_trace_heavyweight_exit(void)
 #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
 }
 
+#ifdef CONFIG_CONTEXT_TRACKING_WORK
+static noinstr void ct_work_flush(unsigned long seq)
+{
+	int bit;
+
+	seq = (seq & CT_WORK_MASK) >> CT_WORK_START;
+
+	/*
+	 * arch_context_tracking_work() must be noinstr, non-blocking,
+	 * and NMI safe.
+	 */
+	for_each_set_bit(bit, &seq, CONTEXT_WORK_MAX)
+		arch_context_tracking_work(BIT(bit));
+}
+
+bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
+{
+	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
+	unsigned int old;
+	bool ret = false;
+
+	preempt_disable();
+
+	old = atomic_read(&ct->state);
+	/*
+	 * Try setting the work until either
+	 * - the target CPU has entered kernelspace
+	 * - the work has been set
+	 */
+	do {
+		ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START));
+	} while (!ret && ((old & CT_STATE_MASK) != CT_STATE_KERNEL));
+
+	preempt_enable();
+	return ret;
+}
+#else
+static __always_inline void ct_work_flush(unsigned long work) { }
+static __always_inline void ct_work_clear(struct context_tracking *ct) { }
+#endif
+
 /*
  * Record entry into an extended quiescent state.  This is only to be
  * called when not already in an extended quiescent state, that is,
@@ -88,7 +129,7 @@  static noinstr void ct_kernel_exit_state(int offset)
 	 * next idle sojourn.
 	 */
 	rcu_task_trace_heavyweight_enter();  // Before CT state update!
-	seq = ct_state_inc(offset);
+	seq = ct_state_inc_clear_work(offset);
 	// RCU is no longer watching.  Better be in extended quiescent state!
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & CT_RCU_WATCHING));
 }
@@ -100,7 +141,7 @@  static noinstr void ct_kernel_exit_state(int offset)
  */
 static noinstr void ct_kernel_enter_state(int offset)
 {
-	int seq;
+	unsigned long seq;
 
 	/*
 	 * CPUs seeing atomic_add_return() must see prior idle sojourns,
@@ -108,6 +149,7 @@  static noinstr void ct_kernel_enter_state(int offset)
 	 * critical section.
 	 */
 	seq = ct_state_inc(offset);
+	ct_work_flush(seq);
 	// RCU is now watching.  Better not be in an extended quiescent state!
 	rcu_task_trace_heavyweight_exit();  // After CT state update!
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & CT_RCU_WATCHING));
diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 8ebb6d5a106be..04efc2b605823 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -186,6 +186,11 @@  config CONTEXT_TRACKING_USER_FORCE
 	  Say N otherwise, this option brings an overhead that you
 	  don't want in production.
 
+config CONTEXT_TRACKING_WORK
+	bool
+	depends on HAVE_CONTEXT_TRACKING_WORK && CONTEXT_TRACKING_USER
+	default y
+
 config NO_HZ
 	bool "Old Idle dynticks config"
 	help