[PATCH v12 12/13] arm64: factor work_pending state machine to C

[PATCH v12 12/13] arm64: factor work_pending state machine to C

[Chris Metcalf]

Currently ret_fast_syscall, work_pending, and ret_to_user form an ad-hoc
state machine that can be difficult to reason about due to duplicated
code and a large number of branch targets.

This patch factors the common logic out into the existing
do_notify_resume function, converting the code to C in the process,
making the code more legible.

This patch tries to closely mirror the existing behaviour while using
the usual C control flow primitives. As local_irq_{disable,enable} may
be instrumented, we balance exception entry (where we will almost most
likely enable IRQs) with a call to trace_hardirqs_on just before the
return to userspace.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
---
 arch/arm64/kernel/entry.S  | 12 ++++--------
 arch/arm64/kernel/signal.c | 36 ++++++++++++++++++++++++++----------
 2 files changed, 30 insertions(+), 18 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 12e8d2bcb3f9..d70a9e44b7d6 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -674,18 +674,13 @@ ret_fast_syscall_trace:
  * Ok, we need to do extra processing, enter the slow path.
  */
 work_pending:
-	tbnz	x1, #TIF_NEED_RESCHED, work_resched
-	/* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
 	mov	x0, sp				// 'regs'
-	enable_irq				// enable interrupts for do_notify_resume()
 	bl	do_notify_resume
-	b	ret_to_user
-work_resched:
 #ifdef CONFIG_TRACE_IRQFLAGS
-	bl	trace_hardirqs_off		// the IRQs are off here, inform the tracing code
+	bl	trace_hardirqs_on		// enabled while in userspace
 #endif
-	bl	schedule
-
+	ldr	x1, [tsk, #TI_FLAGS]		// re-check for single-step
+	b	finish_ret_to_user
 /*
  * "slow" syscall return path.
  */
@@ -694,6 +689,7 @@ ret_to_user:
 	ldr	x1, [tsk, #TI_FLAGS]
 	and	x2, x1, #_TIF_WORK_MASK
 	cbnz	x2, work_pending
+finish_ret_to_user:
 	enable_step_tsk x1, x2
 	kernel_exit 0
 ENDPROC(ret_to_user)
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index a8eafdbc7cb8..404dd67080b9 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -402,15 +402,31 @@ static void do_signal(struct pt_regs *regs)
 asmlinkage void do_notify_resume(struct pt_regs *regs,
 				 unsigned int thread_flags)
 {
-	if (thread_flags & _TIF_SIGPENDING)
-		do_signal(regs);
-
-	if (thread_flags & _TIF_NOTIFY_RESUME) {
-		clear_thread_flag(TIF_NOTIFY_RESUME);
-		tracehook_notify_resume(regs);
-	}
-
-	if (thread_flags & _TIF_FOREIGN_FPSTATE)
-		fpsimd_restore_current_state();
+	/*
+	 * The assembly code enters us with IRQs off, but it hasn't
+	 * informed the tracing code of that for efficiency reasons.
+	 * Update the trace code with the current status.
+	 */
+	trace_hardirqs_off();
+	do {
+		if (thread_flags & _TIF_NEED_RESCHED) {
+			schedule();
+		} else {
+			local_irq_enable();
+
+			if (thread_flags & _TIF_SIGPENDING)
+				do_signal(regs);
+
+			if (thread_flags & _TIF_NOTIFY_RESUME) {
+				clear_thread_flag(TIF_NOTIFY_RESUME);
+				tracehook_notify_resume(regs);
+			}
+
+			if (thread_flags & _TIF_FOREIGN_FPSTATE)
+				fpsimd_restore_current_state();
+		}
 
+		local_irq_disable();
+		thread_flags = READ_ONCE(current_thread_info()->flags);
+	} while (thread_flags & _TIF_WORK_MASK);
 }
-- 
2.7.2

[PATCH v12 13/13] arch/arm64: enable task isolation functionality

[Chris Metcalf]

In do_notify_resume(), call task_isolation_ready() for
TIF_TASK_ISOLATION tasks when we are checking the thread-info flags;
and after we've handled the other work, call task_isolation_enter()
for such tasks.  To ensure we always call task_isolation_enter() when
returning to userspace, add _TIF_TASK_ISOLATION to _TIF_WORK_MASK,
while leaving the old bitmask value as _TIF_WORK_LOOP_MASK to
check while looping.

We tweak syscall_trace_enter() slightly to carry the "flags"
value from current_thread_info()->flags for each of the tests,
rather than doing a volatile read from memory for each one.  This
avoids a small overhead for each test, and in particular avoids
that overhead for TIF_NOHZ when TASK_ISOLATION is not enabled.

We instrument the smp_cross_call() routine so that it checks for
isolated tasks and generates a suitable warning if we are about
to disturb one of them in strict or debug mode.

Finally, add an explicit check for STRICT mode in do_mem_abort()
to handle the case of page faults.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
---
 arch/arm64/Kconfig                   |  1 +
 arch/arm64/include/asm/thread_info.h |  5 ++++-
 arch/arm64/kernel/ptrace.c           | 15 ++++++++++++---
 arch/arm64/kernel/signal.c           | 10 ++++++++++
 arch/arm64/kernel/smp.c              |  2 ++
 arch/arm64/mm/fault.c                |  4 ++++
 6 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 4f436220384f..ec033abee9d5 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -57,6 +57,7 @@ config ARM64
 	select HAVE_ARCH_MMAP_RND_BITS
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
 	select HAVE_ARCH_SECCOMP_FILTER
+	select HAVE_ARCH_TASK_ISOLATION
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_BPF_JIT
 	select HAVE_C_RECORDMCOUNT
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index abd64bd1f6d9..bdc6426b9968 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -109,6 +109,7 @@ static inline struct thread_info *current_thread_info(void)
 #define TIF_NEED_RESCHED	1
 #define TIF_NOTIFY_RESUME	2	/* callback before returning to user */
 #define TIF_FOREIGN_FPSTATE	3	/* CPU's FP state is not current's */
+#define TIF_TASK_ISOLATION	4
 #define TIF_NOHZ		7
 #define TIF_SYSCALL_TRACE	8
 #define TIF_SYSCALL_AUDIT	9
@@ -124,6 +125,7 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
 #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
 #define _TIF_FOREIGN_FPSTATE	(1 << TIF_FOREIGN_FPSTATE)
+#define _TIF_TASK_ISOLATION	(1 << TIF_TASK_ISOLATION)
 #define _TIF_NOHZ		(1 << TIF_NOHZ)
 #define _TIF_SYSCALL_TRACE	(1 << TIF_SYSCALL_TRACE)
 #define _TIF_SYSCALL_AUDIT	(1 << TIF_SYSCALL_AUDIT)
@@ -132,7 +134,8 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_32BIT		(1 << TIF_32BIT)
 
 #define _TIF_WORK_MASK		(_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
-				 _TIF_NOTIFY_RESUME | _TIF_FOREIGN_FPSTATE)
+				 _TIF_NOTIFY_RESUME | _TIF_FOREIGN_FPSTATE | \
+				 _TIF_TASK_ISOLATION)
 
 #define _TIF_SYSCALL_WORK	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
 				 _TIF_SYSCALL_TRACEPOINT | _TIF_SECCOMP | \
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 3f6cd5c5234f..ae336065733d 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -37,6 +37,7 @@
 #include <linux/regset.h>
 #include <linux/tracehook.h>
 #include <linux/elf.h>
+#include <linux/isolation.h>
 
 #include <asm/compat.h>
 #include <asm/debug-monitors.h>
@@ -1246,14 +1247,22 @@ static void tracehook_report_syscall(struct pt_regs *regs,
 
 asmlinkage int syscall_trace_enter(struct pt_regs *regs)
 {
-	/* Do the secure computing check first; failures should be fast. */
+	unsigned long work = ACCESS_ONCE(current_thread_info()->flags);
+
+	/* In isolation mode, we may prevent the syscall from running. */
+	if (work & _TIF_TASK_ISOLATION) {
+		if (task_isolation_syscall(regs->syscallno) == -1)
+			return -1;
+	}
+
+	/* Do the secure computing check early; failures should be fast. */
 	if (secure_computing() == -1)
 		return -1;
 
-	if (test_thread_flag(TIF_SYSCALL_TRACE))
+	if (work & _TIF_SYSCALL_TRACE)
 		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
 
-	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
+	if (work & _TIF_SYSCALL_TRACEPOINT)
 		trace_sys_enter(regs, regs->syscallno);
 
 	audit_syscall_entry(regs->syscallno, regs->orig_x0, regs->regs[1],
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 404dd67080b9..f9b9b25636ca 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -25,6 +25,7 @@
 #include <linux/uaccess.h>
 #include <linux/tracehook.h>
 #include <linux/ratelimit.h>
+#include <linux/isolation.h>
 
 #include <asm/debug-monitors.h>
 #include <asm/elf.h>
@@ -424,9 +425,18 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
 
 			if (thread_flags & _TIF_FOREIGN_FPSTATE)
 				fpsimd_restore_current_state();
+
+			if (thread_flags & _TIF_TASK_ISOLATION)
+				task_isolation_enter();
 		}
 
 		local_irq_disable();
 		thread_flags = READ_ONCE(current_thread_info()->flags);
+
+		/* Clear task isolation from cached_flags manually. */
+		if ((thread_flags & _TIF_TASK_ISOLATION) &&
+		    task_isolation_ready())
+			thread_flags &= ~_TIF_TASK_ISOLATION;
+
 	} while (thread_flags & _TIF_WORK_MASK);
 }
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index b2d5f4ee9a1c..83ed6b5baa4d 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -37,6 +37,7 @@
 #include <linux/completion.h>
 #include <linux/of.h>
 #include <linux/irq_work.h>
+#include <linux/isolation.h>
 
 #include <asm/alternative.h>
 #include <asm/atomic.h>
@@ -710,6 +711,7 @@ static const char *ipi_types[NR_IPI] __tracepoint_string = {
 static void smp_cross_call(const struct cpumask *target, unsigned int ipinr)
 {
 	trace_ipi_raise(target, ipi_types[ipinr]);
+	task_isolation_debug_cpumask(target);
 	__smp_cross_call(target, ipinr);
 }
 
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 95df28bc875f..77f827b02c6d 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -29,6 +29,7 @@
 #include <linux/sched.h>
 #include <linux/highmem.h>
 #include <linux/perf_event.h>
+#include <linux/isolation.h>
 
 #include <asm/cpufeature.h>
 #include <asm/exception.h>
@@ -482,6 +483,9 @@ asmlinkage void __exception do_mem_abort(unsigned long addr, unsigned int esr,
 	const struct fault_info *inf = fault_info + (esr & 63);
 	struct siginfo info;
 
+	if (user_mode(regs))
+		task_isolation_exception("%s at %#lx", inf->name, addr);
+
 	if (!inf->fn(addr, esr, regs))
 		return;
 
-- 
2.7.2

[PATCH]: arm64: factor work_pending state machine to C

[Chris Metcalf]

Ping!

I am hopeful that this patch [1] can be picked up for the 4.8 merge window
in the arm64 tree.  As I mentioned in my last patch series that included
this patch [2], I'm hopeful that this version addresses the performance
issues that were seen with Mark's original patch.  This version tests the
TIF flags prior to calling out to the loop in C code.

It makes more sense for it go via the arm64 tree than to wait and go
through the nohz_full tree, I suspect.

Thanks!

[1] https://lkml.kernel.org/g/1459877922-15512-13-git-send-email-cmetcalf at mellanox.com
[2] https://lkml.kernel.org/g/56E9A3CE.2040209 at mellanox.com

On 4/5/2016 1:38 PM, Chris Metcalf wrote:
> Currently ret_fast_syscall, work_pending, and ret_to_user form an ad-hoc
> state machine that can be difficult to reason about due to duplicated
> code and a large number of branch targets.
>
> This patch factors the common logic out into the existing
> do_notify_resume function, converting the code to C in the process,
> making the code more legible.
>
> This patch tries to closely mirror the existing behaviour while using
> the usual C control flow primitives. As local_irq_{disable,enable} may
> be instrumented, we balance exception entry (where we will almost most
> likely enable IRQs) with a call to trace_hardirqs_on just before the
> return to userspace.
>
> Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
> ---
>   arch/arm64/kernel/entry.S  | 12 ++++--------
>   arch/arm64/kernel/signal.c | 36 ++++++++++++++++++++++++++----------
>   2 files changed, 30 insertions(+), 18 deletions(-)
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 12e8d2bcb3f9..d70a9e44b7d6 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -674,18 +674,13 @@ ret_fast_syscall_trace:
>    * Ok, we need to do extra processing, enter the slow path.
>    */
>   work_pending:
> -	tbnz	x1, #TIF_NEED_RESCHED, work_resched
> -	/* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
>   	mov	x0, sp				// 'regs'
> -	enable_irq				// enable interrupts for do_notify_resume()
>   	bl	do_notify_resume
> -	b	ret_to_user
> -work_resched:
>   #ifdef CONFIG_TRACE_IRQFLAGS
> -	bl	trace_hardirqs_off		// the IRQs are off here, inform the tracing code
> +	bl	trace_hardirqs_on		// enabled while in userspace
>   #endif
> -	bl	schedule
> -
> +	ldr	x1, [tsk, #TI_FLAGS]		// re-check for single-step
> +	b	finish_ret_to_user
>   /*
>    * "slow" syscall return path.
>    */
> @@ -694,6 +689,7 @@ ret_to_user:
>   	ldr	x1, [tsk, #TI_FLAGS]
>   	and	x2, x1, #_TIF_WORK_MASK
>   	cbnz	x2, work_pending
> +finish_ret_to_user:
>   	enable_step_tsk x1, x2
>   	kernel_exit 0
>   ENDPROC(ret_to_user)
> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index a8eafdbc7cb8..404dd67080b9 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -402,15 +402,31 @@ static void do_signal(struct pt_regs *regs)
>   asmlinkage void do_notify_resume(struct pt_regs *regs,
>   				 unsigned int thread_flags)
>   {
> -	if (thread_flags & _TIF_SIGPENDING)
> -		do_signal(regs);
> -
> -	if (thread_flags & _TIF_NOTIFY_RESUME) {
> -		clear_thread_flag(TIF_NOTIFY_RESUME);
> -		tracehook_notify_resume(regs);
> -	}
> -
> -	if (thread_flags & _TIF_FOREIGN_FPSTATE)
> -		fpsimd_restore_current_state();
> +	/*
> +	 * The assembly code enters us with IRQs off, but it hasn't
> +	 * informed the tracing code of that for efficiency reasons.
> +	 * Update the trace code with the current status.
> +	 */
> +	trace_hardirqs_off();
> +	do {
> +		if (thread_flags & _TIF_NEED_RESCHED) {
> +			schedule();
> +		} else {
> +			local_irq_enable();
> +
> +			if (thread_flags & _TIF_SIGPENDING)
> +				do_signal(regs);
> +
> +			if (thread_flags & _TIF_NOTIFY_RESUME) {
> +				clear_thread_flag(TIF_NOTIFY_RESUME);
> +				tracehook_notify_resume(regs);
> +			}
> +
> +			if (thread_flags & _TIF_FOREIGN_FPSTATE)
> +				fpsimd_restore_current_state();
> +		}
>   
> +		local_irq_disable();
> +		thread_flags = READ_ONCE(current_thread_info()->flags);
> +	} while (thread_flags & _TIF_WORK_MASK);
>   }

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com

[PATCH]: arm64: factor work_pending state machine to C

[Will Deacon]

Hi Chris,

On Fri, Jul 08, 2016 at 11:43:50AM -0400, Chris Metcalf wrote:
> Ping!
> 
> I am hopeful that this patch [1] can be picked up for the 4.8 merge window
> in the arm64 tree.  As I mentioned in my last patch series that included
> this patch [2], I'm hopeful that this version addresses the performance
> issues that were seen with Mark's original patch.  This version tests the
> TIF flags prior to calling out to the loop in C code.

I still need to get round to measuring that again. This might also
conflict with a late fix I just sent for 4.7.

Is your task isolation series all ready apart from this? It seems like
there's still discussion over there.

Will

[PATCH]: arm64: factor work_pending state machine to C

[Chris Metcalf]

On 7/8/2016 11:49 AM, Will Deacon wrote:
> Hi Chris,
>
> On Fri, Jul 08, 2016 at 11:43:50AM -0400, Chris Metcalf wrote:
>> Ping!
>>
>> I am hopeful that this patch [1] can be picked up for the 4.8 merge window
>> in the arm64 tree.  As I mentioned in my last patch series that included
>> this patch [2], I'm hopeful that this version addresses the performance
>> issues that were seen with Mark's original patch.  This version tests the
>> TIF flags prior to calling out to the loop in C code.
> I still need to get round to measuring that again. This might also
> conflict with a late fix I just sent for 4.7.
>
> Is your task isolation series all ready apart from this? It seems like
> there's still discussion over there.

No, discussion of task isolation is still ongoing.  Alas :-)

I'm trying to pick off low-hanging fruit where possible - I think this refactoring
is independently good for arm64 regardless of whether task isolation makes it in for
the 4.8 merge window or not, so figured I'd break it out.

If you get a chance to remeasure, that will be great.  Thanks!

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com

[PATCH]: arm64: factor work_pending state machine to C

[Will Deacon]

On Fri, Jul 08, 2016 at 04:49:01PM +0100, Will Deacon wrote:
> On Fri, Jul 08, 2016 at 11:43:50AM -0400, Chris Metcalf wrote:
> > I am hopeful that this patch [1] can be picked up for the 4.8 merge window
> > in the arm64 tree.  As I mentioned in my last patch series that included
> > this patch [2], I'm hopeful that this version addresses the performance
> > issues that were seen with Mark's original patch.  This version tests the
> > TIF flags prior to calling out to the loop in C code.
> 
> I still need to get round to measuring that again. This might also
> conflict with a late fix I just sent for 4.7.

FWIW, I've failed to measure any performance overhead from this change,
so it certainly looks better thsn Mark's original patch from that angle.

Will

[PATCH]: arm64: factor work_pending state machine to C

[Mark Rutland]

On Mon, Jul 11, 2016 at 12:42:37PM +0100, Will Deacon wrote:
> On Fri, Jul 08, 2016 at 04:49:01PM +0100, Will Deacon wrote:
> > On Fri, Jul 08, 2016 at 11:43:50AM -0400, Chris Metcalf wrote:
> > > I am hopeful that this patch [1] can be picked up for the 4.8 merge window
> > > in the arm64 tree.  As I mentioned in my last patch series that included
> > > this patch [2], I'm hopeful that this version addresses the performance
> > > issues that were seen with Mark's original patch.  This version tests the
> > > TIF flags prior to calling out to the loop in C code.
> > 
> > I still need to get round to measuring that again. This might also
> > conflict with a late fix I just sent for 4.7.
> 
> FWIW, I've failed to measure any performance overhead from this change,
> so it certainly looks better thsn Mark's original patch from that angle.

FWIW, likewise, with perf bench sched atop of v4.7-rc6.

Mark.

[PATCH]: arm64: factor work_pending state machine to C

[Chris Metcalf]

On 7/11/2016 8:19 AM, Mark Rutland wrote:
> On Mon, Jul 11, 2016 at 12:42:37PM +0100, Will Deacon wrote:
>> On Fri, Jul 08, 2016 at 04:49:01PM +0100, Will Deacon wrote:
>>> On Fri, Jul 08, 2016 at 11:43:50AM -0400, Chris Metcalf wrote:
>>>> I am hopeful that this patch [1] can be picked up for the 4.8 merge window
>>>> in the arm64 tree.  As I mentioned in my last patch series that included
>>>> this patch [2], I'm hopeful that this version addresses the performance
>>>> issues that were seen with Mark's original patch.  This version tests the
>>>> TIF flags prior to calling out to the loop in C code.
>>> I still need to get round to measuring that again. This might also
>>> conflict with a late fix I just sent for 4.7.
>> FWIW, I've failed to measure any performance overhead from this change,
>> so it certainly looks better thsn Mark's original patch from that angle.
> FWIW, likewise, with perf bench sched atop of v4.7-rc6.

I don't see this commit pushed up as part of what's in 4.8 so far. Any chance
it's going to go in later in the merge window?  I'm just hoping to trim down the
task-isolation series by being able to drop this patch from its next iteration... :-)

Thanks!

[1] https://lkml.kernel.org/r/1459877922-15512-13-git-send-email-cmetcalf at mellanox.com

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com

[PATCH]: arm64: factor work_pending state machine to C

[Will Deacon]

On Fri, Jul 29, 2016 at 02:16:07PM -0400, Chris Metcalf wrote:
> On 7/11/2016 8:19 AM, Mark Rutland wrote:
> >On Mon, Jul 11, 2016 at 12:42:37PM +0100, Will Deacon wrote:
> >>On Fri, Jul 08, 2016 at 04:49:01PM +0100, Will Deacon wrote:
> >>>On Fri, Jul 08, 2016 at 11:43:50AM -0400, Chris Metcalf wrote:
> >>>>I am hopeful that this patch [1] can be picked up for the 4.8 merge window
> >>>>in the arm64 tree.  As I mentioned in my last patch series that included
> >>>>this patch [2], I'm hopeful that this version addresses the performance
> >>>>issues that were seen with Mark's original patch.  This version tests the
> >>>>TIF flags prior to calling out to the loop in C code.
> >>>I still need to get round to measuring that again. This might also
> >>>conflict with a late fix I just sent for 4.7.
> >>FWIW, I've failed to measure any performance overhead from this change,
> >>so it certainly looks better thsn Mark's original patch from that angle.
> >FWIW, likewise, with perf bench sched atop of v4.7-rc6.
> 
> I don't see this commit pushed up as part of what's in 4.8 so far. Any chance
> it's going to go in later in the merge window?  I'm just hoping to trim down the
> task-isolation series by being able to drop this patch from its next iteration... :-)

I'll queue it up for 4.9, once I've had a closer look.

Will