[PATCH 0/3] entry: inline syscall enter/exit functions

[PATCH 0/3] entry: inline syscall enter/exit functions

[Sven Schnelle]

Hi List,

looking into the performance of syscall entry/exit after s390 switched
to generic entry showed that there's quite some overhead calling some
of the entry/exit work functions even when there's nothing to do.
This patchset moves the entry and exit function to entry-common.h, so
non inlined code gets only called when there is some work pending.

I wrote a small program that just issues invalid syscalls in a loop.
On an s390 machine, this results in the following numbers:

without this series:

# ./syscall 1000000000
runtime: 94.886581s / per-syscall 9.488658e-08s

with this series:

./syscall 1000000000
runtime: 84.732391s / per-syscall 8.473239e-08s

so the time required for one syscall dropped from 94.8ns to
84.7ns, which is a drop of about 11%.

Sven Schnelle (3):
  entry: move exit to usermode functions to header file
  move enter_from_user_mode() to header file
  entry: move syscall_enter_from_user_mode() to header file

 include/linux/entry-common.h | 137 ++++++++++++++++++++++++++++++++-
 kernel/entry/common.c        | 145 ++---------------------------------
 2 files changed, 138 insertions(+), 144 deletions(-)

-- 
2.40.1

[PATCH 1/3] entry: move exit to usermode functions to header file

[Sven Schnelle]

To allow inlining, move exit_to_user_mode() and
exit_to_user_mode_loop() to common.h.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
---
 include/linux/entry-common.h | 95 +++++++++++++++++++++++++++++++++++-
 kernel/entry/common.c        | 89 +--------------------------------
 2 files changed, 96 insertions(+), 88 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index d95ab85f96ba..f0f1a26dc638 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -7,6 +7,10 @@
 #include <linux/syscalls.h>
 #include <linux/seccomp.h>
 #include <linux/sched.h>
+#include <linux/context_tracking.h>
+#include <linux/livepatch.h>
+#include <linux/resume_user_mode.h>
+#include <linux/tick.h>
 
 #include <asm/entry-common.h>
 
@@ -258,6 +262,85 @@ static __always_inline void arch_exit_to_user_mode(void) { }
  */
 void arch_do_signal_or_restart(struct pt_regs *regs);
 
+/**
+ * exit_to_user_mode_loop - do any pending work before leaving to user space
+ */
+static __always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
+							    unsigned long ti_work)
+{
+	/*
+	 * Before returning to user space ensure that all pending work
+	 * items have been completed.
+	 */
+	while (ti_work & EXIT_TO_USER_MODE_WORK) {
+
+		local_irq_enable_exit_to_user(ti_work);
+
+		if (ti_work & _TIF_NEED_RESCHED)
+			schedule();
+
+		if (ti_work & _TIF_UPROBE)
+			uprobe_notify_resume(regs);
+
+		if (ti_work & _TIF_PATCH_PENDING)
+			klp_update_patch_state(current);
+
+		if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
+			arch_do_signal_or_restart(regs);
+
+		if (ti_work & _TIF_NOTIFY_RESUME)
+			resume_user_mode_work(regs);
+
+		/* Architecture specific TIF work */
+		arch_exit_to_user_mode_work(regs, ti_work);
+
+		/*
+		 * Disable interrupts and reevaluate the work flags as they
+		 * might have changed while interrupts and preemption was
+		 * enabled above.
+		 */
+		local_irq_disable_exit_to_user();
+
+		/* Check if any of the above work has queued a deferred wakeup */
+		tick_nohz_user_enter_prepare();
+
+		ti_work = read_thread_flags();
+	}
+
+	/* Return the latest work state for arch_exit_to_user_mode() */
+	return ti_work;
+}
+
+/**
+ * exit_to_user_mode_prepare - call exit_to_user_mode_loop() if required
+ *
+ * 1) check that interrupts are disabled
+ * 2) call tick_nohz_user_enter_prepare()
+ * 3) call exit_to_user_mode_loop() if any flags from
+ *    EXIT_TO_USER_MODE_WORK are set
+ * 4) check that interrupts are still disabled
+ */
+static __always_inline void exit_to_user_mode_prepare(struct pt_regs *regs)
+{
+	unsigned long ti_work;
+
+	lockdep_assert_irqs_disabled();
+
+	/* Flush pending rcuog wakeup before the last need_resched() check */
+	tick_nohz_user_enter_prepare();
+
+	ti_work = read_thread_flags();
+	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
+		ti_work = exit_to_user_mode_loop(regs, ti_work);
+
+	arch_exit_to_user_mode_prepare(regs, ti_work);
+
+	/* Ensure that kernel state is sane for a return to userspace */
+	kmap_assert_nomap();
+	lockdep_assert_irqs_disabled();
+	lockdep_sys_exit();
+}
+
 /**
  * exit_to_user_mode - Fixup state when exiting to user mode
  *
@@ -276,7 +359,17 @@ void arch_do_signal_or_restart(struct pt_regs *regs);
  * non-instrumentable.
  * The caller has to invoke syscall_exit_to_user_mode_work() before this.
  */
-void exit_to_user_mode(void);
+static __always_inline void exit_to_user_mode(void)
+{
+	instrumentation_begin();
+	trace_hardirqs_on_prepare();
+	lockdep_hardirqs_on_prepare();
+	instrumentation_end();
+
+	user_enter_irqoff();
+	arch_exit_to_user_mode();
+	lockdep_hardirqs_on(CALLER_ADDR0);
+}
 
 /**
  * syscall_exit_to_user_mode_work - Handle work before returning to user mode
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index d7ee4bc3f2ba..6ba2bcfbe32c 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -123,94 +123,9 @@ noinstr void syscall_enter_from_user_mode_prepare(struct pt_regs *regs)
 	instrumentation_end();
 }
 
-/* See comment for exit_to_user_mode() in entry-common.h */
-static __always_inline void __exit_to_user_mode(void)
-{
-	instrumentation_begin();
-	trace_hardirqs_on_prepare();
-	lockdep_hardirqs_on_prepare();
-	instrumentation_end();
-
-	user_enter_irqoff();
-	arch_exit_to_user_mode();
-	lockdep_hardirqs_on(CALLER_ADDR0);
-}
-
-void noinstr exit_to_user_mode(void)
-{
-	__exit_to_user_mode();
-}
-
 /* Workaround to allow gradual conversion of architecture code */
 void __weak arch_do_signal_or_restart(struct pt_regs *regs) { }
 
-static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
-					    unsigned long ti_work)
-{
-	/*
-	 * Before returning to user space ensure that all pending work
-	 * items have been completed.
-	 */
-	while (ti_work & EXIT_TO_USER_MODE_WORK) {
-
-		local_irq_enable_exit_to_user(ti_work);
-
-		if (ti_work & _TIF_NEED_RESCHED)
-			schedule();
-
-		if (ti_work & _TIF_UPROBE)
-			uprobe_notify_resume(regs);
-
-		if (ti_work & _TIF_PATCH_PENDING)
-			klp_update_patch_state(current);
-
-		if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
-			arch_do_signal_or_restart(regs);
-
-		if (ti_work & _TIF_NOTIFY_RESUME)
-			resume_user_mode_work(regs);
-
-		/* Architecture specific TIF work */
-		arch_exit_to_user_mode_work(regs, ti_work);
-
-		/*
-		 * Disable interrupts and reevaluate the work flags as they
-		 * might have changed while interrupts and preemption was
-		 * enabled above.
-		 */
-		local_irq_disable_exit_to_user();
-
-		/* Check if any of the above work has queued a deferred wakeup */
-		tick_nohz_user_enter_prepare();
-
-		ti_work = read_thread_flags();
-	}
-
-	/* Return the latest work state for arch_exit_to_user_mode() */
-	return ti_work;
-}
-
-static void exit_to_user_mode_prepare(struct pt_regs *regs)
-{
-	unsigned long ti_work;
-
-	lockdep_assert_irqs_disabled();
-
-	/* Flush pending rcuog wakeup before the last need_resched() check */
-	tick_nohz_user_enter_prepare();
-
-	ti_work = read_thread_flags();
-	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
-		ti_work = exit_to_user_mode_loop(regs, ti_work);
-
-	arch_exit_to_user_mode_prepare(regs, ti_work);
-
-	/* Ensure that kernel state is sane for a return to userspace */
-	kmap_assert_nomap();
-	lockdep_assert_irqs_disabled();
-	lockdep_sys_exit();
-}
-
 /*
  * If SYSCALL_EMU is set, then the only reason to report is when
  * SINGLESTEP is set (i.e. PTRACE_SYSEMU_SINGLESTEP).  This syscall
@@ -295,7 +210,7 @@ __visible noinstr void syscall_exit_to_user_mode(struct pt_regs *regs)
 	instrumentation_begin();
 	__syscall_exit_to_user_mode_work(regs);
 	instrumentation_end();
-	__exit_to_user_mode();
+	exit_to_user_mode();
 }
 
 noinstr void irqentry_enter_from_user_mode(struct pt_regs *regs)
@@ -308,7 +223,7 @@ noinstr void irqentry_exit_to_user_mode(struct pt_regs *regs)
 	instrumentation_begin();
 	exit_to_user_mode_prepare(regs);
 	instrumentation_end();
-	__exit_to_user_mode();
+	exit_to_user_mode();
 }
 
 noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
-- 
2.40.1

[PATCH 2/3] move enter_from_user_mode() to header file

[Sven Schnelle]

To allow inlining of enter_from_user_mode(), move it to common.h.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
---
 include/linux/entry-common.h | 15 ++++++++++++++-
 kernel/entry/common.c        | 26 +++-----------------------
 2 files changed, 17 insertions(+), 24 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index f0f1a26dc638..e2c62d111318 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -11,6 +11,7 @@
 #include <linux/livepatch.h>
 #include <linux/resume_user_mode.h>
 #include <linux/tick.h>
+#include <linux/kmsan.h>
 
 #include <asm/entry-common.h>
 
@@ -102,7 +103,19 @@ static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs) {}
  * done between establishing state and enabling interrupts. The caller must
  * enable interrupts before invoking syscall_enter_from_user_mode_work().
  */
-void enter_from_user_mode(struct pt_regs *regs);
+static __always_inline void enter_from_user_mode(struct pt_regs *regs)
+{
+	arch_enter_from_user_mode(regs);
+	lockdep_hardirqs_off(CALLER_ADDR0);
+
+	CT_WARN_ON(__ct_state() != CONTEXT_USER);
+	user_exit_irqoff();
+
+	instrumentation_begin();
+	kmsan_unpoison_entry_regs(regs);
+	trace_hardirqs_off_finish();
+	instrumentation_end();
+}
 
 /**
  * syscall_enter_from_user_mode_prepare - Establish state and enable interrupts
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 6ba2bcfbe32c..90b995facd7a 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -15,26 +15,6 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/syscalls.h>
 
-/* See comment for enter_from_user_mode() in entry-common.h */
-static __always_inline void __enter_from_user_mode(struct pt_regs *regs)
-{
-	arch_enter_from_user_mode(regs);
-	lockdep_hardirqs_off(CALLER_ADDR0);
-
-	CT_WARN_ON(__ct_state() != CONTEXT_USER);
-	user_exit_irqoff();
-
-	instrumentation_begin();
-	kmsan_unpoison_entry_regs(regs);
-	trace_hardirqs_off_finish();
-	instrumentation_end();
-}
-
-void noinstr enter_from_user_mode(struct pt_regs *regs)
-{
-	__enter_from_user_mode(regs);
-}
-
 static inline void syscall_enter_audit(struct pt_regs *regs, long syscall)
 {
 	if (unlikely(audit_context())) {
@@ -105,7 +85,7 @@ noinstr long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)
 {
 	long ret;
 
-	__enter_from_user_mode(regs);
+	enter_from_user_mode(regs);
 
 	instrumentation_begin();
 	local_irq_enable();
@@ -117,7 +97,7 @@ noinstr long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)
 
 noinstr void syscall_enter_from_user_mode_prepare(struct pt_regs *regs)
 {
-	__enter_from_user_mode(regs);
+	enter_from_user_mode(regs);
 	instrumentation_begin();
 	local_irq_enable();
 	instrumentation_end();
@@ -215,7 +195,7 @@ __visible noinstr void syscall_exit_to_user_mode(struct pt_regs *regs)
 
 noinstr void irqentry_enter_from_user_mode(struct pt_regs *regs)
 {
-	__enter_from_user_mode(regs);
+	enter_from_user_mode(regs);
 }
 
 noinstr void irqentry_exit_to_user_mode(struct pt_regs *regs)
-- 
2.40.1

[PATCH 3/3] entry: move syscall_enter_from_user_mode() to header file

[Sven Schnelle]

To allow inlining of syscall_enter_from_user_mode(), move it
to the entry-common.h header file.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
---
 include/linux/entry-common.h | 27 +++++++++++++++++++++++++--
 kernel/entry/common.c        | 32 +-------------------------------
 2 files changed, 26 insertions(+), 33 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index e2c62d111318..ed26d626d587 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -134,6 +134,9 @@ static __always_inline void enter_from_user_mode(struct pt_regs *regs)
  */
 void syscall_enter_from_user_mode_prepare(struct pt_regs *regs);
 
+long syscall_trace_enter(struct pt_regs *regs, long syscall,
+			 unsigned long work);
+
 /**
  * syscall_enter_from_user_mode_work - Check and handle work before invoking
  *				       a syscall
@@ -157,7 +160,15 @@ void syscall_enter_from_user_mode_prepare(struct pt_regs *regs);
  *     ptrace_report_syscall_entry(), __secure_computing(), trace_sys_enter()
  *  2) Invocation of audit_syscall_entry()
  */
-long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall);
+static __always_inline long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall)
+{
+	unsigned long work = READ_ONCE(current_thread_info()->syscall_work);
+
+	if (work & SYSCALL_WORK_ENTER)
+		syscall = syscall_trace_enter(regs, syscall, work);
+
+	return syscall;
+}
 
 /**
  * syscall_enter_from_user_mode - Establish state and check and handle work
@@ -176,7 +187,19 @@ long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall);
  * Returns: The original or a modified syscall number. See
  * syscall_enter_from_user_mode_work() for further explanation.
  */
-long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall);
+static __always_inline long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)
+{
+	long ret;
+
+	enter_from_user_mode(regs);
+
+	instrumentation_begin();
+	local_irq_enable();
+	ret = syscall_enter_from_user_mode_work(regs, syscall);
+	instrumentation_end();
+
+	return ret;
+}
 
 /**
  * local_irq_enable_exit_to_user - Exit to user variant of local_irq_enable()
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 90b995facd7a..dde4a9c1f9f3 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -25,7 +25,7 @@ static inline void syscall_enter_audit(struct pt_regs *regs, long syscall)
 	}
 }
 
-static long syscall_trace_enter(struct pt_regs *regs, long syscall,
+long syscall_trace_enter(struct pt_regs *regs, long syscall,
 				unsigned long work)
 {
 	long ret = 0;
@@ -65,36 +65,6 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall,
 	return ret ? : syscall;
 }
 
-static __always_inline long
-__syscall_enter_from_user_work(struct pt_regs *regs, long syscall)
-{
-	unsigned long work = READ_ONCE(current_thread_info()->syscall_work);
-
-	if (work & SYSCALL_WORK_ENTER)
-		syscall = syscall_trace_enter(regs, syscall, work);
-
-	return syscall;
-}
-
-long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall)
-{
-	return __syscall_enter_from_user_work(regs, syscall);
-}
-
-noinstr long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)
-{
-	long ret;
-
-	enter_from_user_mode(regs);
-
-	instrumentation_begin();
-	local_irq_enable();
-	ret = __syscall_enter_from_user_work(regs, syscall);
-	instrumentation_end();
-
-	return ret;
-}
-
 noinstr void syscall_enter_from_user_mode_prepare(struct pt_regs *regs)
 {
 	enter_from_user_mode(regs);
-- 
2.40.1

Re: [PATCH 0/3] entry: inline syscall enter/exit functions

[Peter Zijlstra]

On Tue, Dec 05, 2023 at 02:30:12PM +0100, Sven Schnelle wrote:
> Hi List,
> 
> looking into the performance of syscall entry/exit after s390 switched
> to generic entry showed that there's quite some overhead calling some
> of the entry/exit work functions even when there's nothing to do.
> This patchset moves the entry and exit function to entry-common.h, so
> non inlined code gets only called when there is some work pending.

So per that logic you wouldn't need to inline exit_to_user_mode_loop()
for example, that's only called when there is a EXIT_TO_USER_MODE_WORK
bit set.

That is, I'm just being pedantic here and pointing out that your
justification doesn't cover the extent of the changes.

> I wrote a small program that just issues invalid syscalls in a loop.
> On an s390 machine, this results in the following numbers:
> 
> without this series:
> 
> # ./syscall 1000000000
> runtime: 94.886581s / per-syscall 9.488658e-08s
> 
> with this series:
> 
> ./syscall 1000000000
> runtime: 84.732391s / per-syscall 8.473239e-08s
> 
> so the time required for one syscall dropped from 94.8ns to
> 84.7ns, which is a drop of about 11%.

That is obviously very nice, and I don't immediately see anything wrong
with moving the lot to header based inlines.

Thomas?

Re: [PATCH 0/3] entry: inline syscall enter/exit functions

[Sven Schnelle]

Peter Zijlstra <peterz@infradead.org> writes:

> On Tue, Dec 05, 2023 at 02:30:12PM +0100, Sven Schnelle wrote:
>> Hi List,
>> 
>> looking into the performance of syscall entry/exit after s390 switched
>> to generic entry showed that there's quite some overhead calling some
>> of the entry/exit work functions even when there's nothing to do.
>> This patchset moves the entry and exit function to entry-common.h, so
>> non inlined code gets only called when there is some work pending.
>
> So per that logic you wouldn't need to inline exit_to_user_mode_loop()
> for example, that's only called when there is a EXIT_TO_USER_MODE_WORK
> bit set.
>
> That is, I'm just being pedantic here and pointing out that your
> justification doesn't cover the extent of the changes.
>
>> I wrote a small program that just issues invalid syscalls in a loop.
>> On an s390 machine, this results in the following numbers:
>> 
>> without this series:
>> 
>> # ./syscall 1000000000
>> runtime: 94.886581s / per-syscall 9.488658e-08s
>> 
>> with this series:
>> 
>> ./syscall 1000000000
>> runtime: 84.732391s / per-syscall 8.473239e-08s
>> 
>> so the time required for one syscall dropped from 94.8ns to
>> 84.7ns, which is a drop of about 11%.
>
> That is obviously very nice, and I don't immediately see anything wrong
> with moving the lot to header based inlines.
>
> Thomas?

Thomas, any opinion on this change?

Re: [PATCH 0/3] entry: inline syscall enter/exit functions

[Thomas Gleixner]

On Thu, Dec 14 2023 at 09:24, Sven Schnelle wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
>>> so the time required for one syscall dropped from 94.8ns to
>>> 84.7ns, which is a drop of about 11%.
>>
>> That is obviously very nice, and I don't immediately see anything wrong
>> with moving the lot to header based inlines.
>>
>> Thomas?

No objections in principle. Let me look at the lot

Re: [PATCH 1/3] entry: move exit to usermode functions to header file

[Thomas Gleixner]

On Tue, Dec 05 2023 at 14:30, Sven Schnelle wrote:
> +/**
> + * exit_to_user_mode_loop - do any pending work before leaving to user space
> + */
> +static __always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> +							    unsigned long ti_work)
> +{
> +	/*
> +	 * Before returning to user space ensure that all pending work
> +	 * items have been completed.
> +	 */
> +	while (ti_work & EXIT_TO_USER_MODE_WORK) {
> +
> +		local_irq_enable_exit_to_user(ti_work);
> +
> +		if (ti_work & _TIF_NEED_RESCHED)
> +			schedule();
> +
> +		if (ti_work & _TIF_UPROBE)
> +			uprobe_notify_resume(regs);
> +
> +		if (ti_work & _TIF_PATCH_PENDING)
> +			klp_update_patch_state(current);
> +
> +		if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
> +			arch_do_signal_or_restart(regs);
> +
> +		if (ti_work & _TIF_NOTIFY_RESUME)
> +			resume_user_mode_work(regs);
> +
> +		/* Architecture specific TIF work */
> +		arch_exit_to_user_mode_work(regs, ti_work);
> +
> +		/*
> +		 * Disable interrupts and reevaluate the work flags as they
> +		 * might have changed while interrupts and preemption was
> +		 * enabled above.
> +		 */
> +		local_irq_disable_exit_to_user();
> +
> +		/* Check if any of the above work has queued a deferred wakeup */
> +		tick_nohz_user_enter_prepare();
> +
> +		ti_work = read_thread_flags();
> +	}
> +
> +	/* Return the latest work state for arch_exit_to_user_mode() */
> +	return ti_work;
> +}

I'm not really sure about this part. exit_to_user_mode_loop() is the
slowpath when a TIF work flag is set. I can see the benefit on the
fastpath functions which are way smaller.

Thanks,

        tglx

Re: [PATCH 1/3] entry: move exit to usermode functions to header file

[Sven Schnelle]

Hi Thomas,

Thomas Gleixner <tglx@linutronix.de> writes:

> On Tue, Dec 05 2023 at 14:30, Sven Schnelle wrote:
>> +/**
>> + * exit_to_user_mode_loop - do any pending work before leaving to user space
>> + */
>> +static __always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
>> +							    unsigned long ti_work)
>> +{
>> +	/*
>> +	 * Before returning to user space ensure that all pending work
>> +	 * items have been completed.
>> +	 */
>> +	while (ti_work & EXIT_TO_USER_MODE_WORK) {
>> +
>> +		local_irq_enable_exit_to_user(ti_work);
>> +
>> +		if (ti_work & _TIF_NEED_RESCHED)
>> +			schedule();
>> +
>> +		if (ti_work & _TIF_UPROBE)
>> +			uprobe_notify_resume(regs);
>> +
>> +		if (ti_work & _TIF_PATCH_PENDING)
>> +			klp_update_patch_state(current);
>> +
>> +		if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
>> +			arch_do_signal_or_restart(regs);
>> +
>> +		if (ti_work & _TIF_NOTIFY_RESUME)
>> +			resume_user_mode_work(regs);
>> +
>> +		/* Architecture specific TIF work */
>> +		arch_exit_to_user_mode_work(regs, ti_work);
>> +
>> +		/*
>> +		 * Disable interrupts and reevaluate the work flags as they
>> +		 * might have changed while interrupts and preemption was
>> +		 * enabled above.
>> +		 */
>> +		local_irq_disable_exit_to_user();
>> +
>> +		/* Check if any of the above work has queued a deferred wakeup */
>> +		tick_nohz_user_enter_prepare();
>> +
>> +		ti_work = read_thread_flags();
>> +	}
>> +
>> +	/* Return the latest work state for arch_exit_to_user_mode() */
>> +	return ti_work;
>> +}
>
> I'm not really sure about this part. exit_to_user_mode_loop() is the
> slowpath when a TIF work flag is set. I can see the benefit on the
> fastpath functions which are way smaller.

Indeed, the main performance improvement comes from inlining the small
functions. As Peter mentioned the same, i sent out a v2 which doesn't
move exit_to_user_mode_loop().

Thanks!
Sven