[PATCH 0/2 v2] x86,tlb: context switch optimizations

[PATCH 0/2 v2] x86,tlb: context switch optimizations

[Rik van Riel]

While profiling switch_mm_irqs_off with several workloads,
it appears there are two hot spots that probably don't need
to be there.

The patch placing the mm_cpumask test inside the prev == next
branch behind CONFIG_DEBUG_VM got merged into x86/mm already,
so here are the other two.

The approach used in v2 to ensure the call to flush_mm_tlb_range()
from __text_poke() remains a noop is to clear the CPU from the
mm_cpumask of poke_mm. Fix suggested by Peter Zijlstra.

That way the only thing flush_mm_tlb_range() really ends up
doing is increment the tlb_gen, resulting in future users of
poke_mm flushing the TLB.

[PATCH 1/2] x86,tlb: update mm_cpumask lazily

[Rik van Riel]

On busy multi-threaded workloads, there can be significant contention
on the mm_cpumask at context switch time.

Reduce that contention by updating mm_cpumask lazily, setting the CPU bit
at context switch time (if not already set), and clearing the CPU bit at
the first TLB flush sent to a CPU where the process isn't running.

When a flurry of TLB flushes for a process happen, only the first one
will be sent to CPUs where the process isn't running. The others will
be sent to CPUs where the process is currently running.

On an AMD Milan system with 36 cores, there is a noticeable difference:
$ hackbench --groups 20 --loops 10000

Before: ~4.5s +/- 0.1s
After:  ~4.2s +/- 0.1s

Signed-off-by: Rik van Riel <riel@surriel.com>
---
 arch/x86/kernel/alternative.c | 10 +++++++---
 arch/x86/mm/tlb.c             | 19 +++++++++----------
 2 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d17518ca19b8..8b66a555d2f0 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1825,11 +1825,18 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
 	return temp_state;
 }
 
+__ro_after_init struct mm_struct *poking_mm;
+__ro_after_init unsigned long poking_addr;
+
 static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
 {
 	lockdep_assert_irqs_disabled();
+
 	switch_mm_irqs_off(NULL, prev_state.mm, current);
 
+	/* Clear the cpumask, to indicate no TLB flushing is needed anywhere */
+	cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(poking_mm));
+
 	/*
 	 * Restore the breakpoints if they were disabled before the temporary mm
 	 * was loaded.
@@ -1838,9 +1845,6 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
 		hw_breakpoint_restore();
 }
 
-__ro_after_init struct mm_struct *poking_mm;
-__ro_after_init unsigned long poking_addr;
-
 static void text_poke_memcpy(void *dst, const void *src, size_t len)
 {
 	memcpy(dst, src, len);
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index b0d5a644fc84..cc4e57ae690f 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -606,18 +606,15 @@ void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next,
 		cond_mitigation(tsk);
 
 		/*
-		 * Stop remote flushes for the previous mm.
-		 * Skip kernel threads; we never send init_mm TLB flushing IPIs,
-		 * but the bitmap manipulation can cause cache line contention.
+		 * Leave this CPU in prev's mm_cpumask. Atomic writes to
+		 * mm_cpumask can be expensive under contention. The CPU
+		 * will be removed lazily at TLB flush time.
 		 */
-		if (prev != &init_mm) {
-			VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu,
-						mm_cpumask(prev)));
-			cpumask_clear_cpu(cpu, mm_cpumask(prev));
-		}
+		VM_WARN_ON_ONCE(prev != &init_mm && !cpumask_test_cpu(cpu,
+				mm_cpumask(prev)));
 
 		/* Start receiving IPIs and then read tlb_gen (and LAM below) */
-		if (next != &init_mm)
+		if (next != &init_mm && !cpumask_test_cpu(cpu, mm_cpumask(next)))
 			cpumask_set_cpu(cpu, mm_cpumask(next));
 		next_tlb_gen = atomic64_read(&next->context.tlb_gen);
 
@@ -761,8 +758,10 @@ static void flush_tlb_func(void *info)
 		count_vm_tlb_event(NR_TLB_REMOTE_FLUSH_RECEIVED);
 
 		/* Can only happen on remote CPUs */
-		if (f->mm && f->mm != loaded_mm)
+		if (f->mm && f->mm != loaded_mm) {
+			cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(f->mm));
 			return;
+		}
 	}
 
 	if (unlikely(loaded_mm == &init_mm))
-- 
2.45.2

[tip: x86/mm] x86/mm/tlb: Update mm_cpumask lazily

[tip-bot2 for Rik van Riel]

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     209954cbc7d0ce1a190fc725d20ce303d74d2680
Gitweb:        https://git.kernel.org/tip/209954cbc7d0ce1a190fc725d20ce303d74d2680
Author:        Rik van Riel <riel@surriel.com>
AuthorDate:    Thu, 14 Nov 2024 10:26:16 -05:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 19 Nov 2024 12:02:46 +01:00

x86/mm/tlb: Update mm_cpumask lazily

On busy multi-threaded workloads, there can be significant contention
on the mm_cpumask at context switch time.

Reduce that contention by updating mm_cpumask lazily, setting the CPU bit
at context switch time (if not already set), and clearing the CPU bit at
the first TLB flush sent to a CPU where the process isn't running.

When a flurry of TLB flushes for a process happen, only the first one
will be sent to CPUs where the process isn't running. The others will
be sent to CPUs where the process is currently running.

On an AMD Milan system with 36 cores, there is a noticeable difference:
$ hackbench --groups 20 --loops 10000

  Before: ~4.5s +/- 0.1s
  After:  ~4.2s +/- 0.1s

Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Link: https://lore.kernel.org/r/20241114152723.1294686-2-riel@surriel.com
---
 arch/x86/kernel/alternative.c | 10 +++++++---
 arch/x86/mm/tlb.c             | 19 +++++++++----------
 2 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d17518c..8b66a55 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1825,11 +1825,18 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
 	return temp_state;
 }
 
+__ro_after_init struct mm_struct *poking_mm;
+__ro_after_init unsigned long poking_addr;
+
 static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
 {
 	lockdep_assert_irqs_disabled();
+
 	switch_mm_irqs_off(NULL, prev_state.mm, current);
 
+	/* Clear the cpumask, to indicate no TLB flushing is needed anywhere */
+	cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(poking_mm));
+
 	/*
 	 * Restore the breakpoints if they were disabled before the temporary mm
 	 * was loaded.
@@ -1838,9 +1845,6 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
 		hw_breakpoint_restore();
 }
 
-__ro_after_init struct mm_struct *poking_mm;
-__ro_after_init unsigned long poking_addr;
-
 static void text_poke_memcpy(void *dst, const void *src, size_t len)
 {
 	memcpy(dst, src, len);
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index b0d5a64..cc4e57a 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -606,18 +606,15 @@ void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next,
 		cond_mitigation(tsk);
 
 		/*
-		 * Stop remote flushes for the previous mm.
-		 * Skip kernel threads; we never send init_mm TLB flushing IPIs,
-		 * but the bitmap manipulation can cause cache line contention.
+		 * Leave this CPU in prev's mm_cpumask. Atomic writes to
+		 * mm_cpumask can be expensive under contention. The CPU
+		 * will be removed lazily at TLB flush time.
 		 */
-		if (prev != &init_mm) {
-			VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu,
-						mm_cpumask(prev)));
-			cpumask_clear_cpu(cpu, mm_cpumask(prev));
-		}
+		VM_WARN_ON_ONCE(prev != &init_mm && !cpumask_test_cpu(cpu,
+				mm_cpumask(prev)));
 
 		/* Start receiving IPIs and then read tlb_gen (and LAM below) */
-		if (next != &init_mm)
+		if (next != &init_mm && !cpumask_test_cpu(cpu, mm_cpumask(next)))
 			cpumask_set_cpu(cpu, mm_cpumask(next));
 		next_tlb_gen = atomic64_read(&next->context.tlb_gen);
 
@@ -761,8 +758,10 @@ static void flush_tlb_func(void *info)
 		count_vm_tlb_event(NR_TLB_REMOTE_FLUSH_RECEIVED);
 
 		/* Can only happen on remote CPUs */
-		if (f->mm && f->mm != loaded_mm)
+		if (f->mm && f->mm != loaded_mm) {
+			cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(f->mm));
 			return;
+		}
 	}
 
 	if (unlikely(loaded_mm == &init_mm))

Re: [tip: x86/mm] x86/mm/tlb: Update mm_cpumask lazily

[Peter Zijlstra]

On Tue, Nov 19, 2024 at 11:27:35AM -0000, tip-bot2 for Rik van Riel wrote:
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index d17518c..8b66a55 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1825,11 +1825,18 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
>  	return temp_state;
>  }
>  
> +__ro_after_init struct mm_struct *poking_mm;
> +__ro_after_init unsigned long poking_addr;
> +
>  static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
>  {
>  	lockdep_assert_irqs_disabled();
> +
>  	switch_mm_irqs_off(NULL, prev_state.mm, current);
>  
> +	/* Clear the cpumask, to indicate no TLB flushing is needed anywhere */
> +	cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(poking_mm));

Oh bugger, this is really unfortunate.

Let me try and fix this.

[PATCH 2/2] x86,tlb: add tracepoint for TLB flush IPI to stale CPU

[Rik van Riel]

Add a tracepoint when we send a TLB flush IPI to a CPU that used
to be in the mm_cpumask, but isn't any more.

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
---
 arch/x86/mm/tlb.c        | 1 +
 include/linux/mm_types.h | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index cc4e57ae690f..1aac4fa90d3d 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -760,6 +760,7 @@ static void flush_tlb_func(void *info)
 		/* Can only happen on remote CPUs */
 		if (f->mm && f->mm != loaded_mm) {
 			cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(f->mm));
+			trace_tlb_flush(TLB_REMOTE_WRONG_CPU, 0);
 			return;
 		}
 	}
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6e3bdf8e38bc..6b6f05404304 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1335,6 +1335,7 @@ enum tlb_flush_reason {
 	TLB_LOCAL_SHOOTDOWN,
 	TLB_LOCAL_MM_SHOOTDOWN,
 	TLB_REMOTE_SEND_IPI,
+	TLB_REMOTE_WRONG_CPU,
 	NR_TLB_FLUSH_REASONS,
 };
 
-- 
2.45.2

[tip: x86/mm] x86/mm/tlb: Add tracepoint for TLB flush IPI to stale CPU

[tip-bot2 for Rik van Riel]

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     2815a56e4b7252a836969f5674ee356ea1ce482c
Gitweb:        https://git.kernel.org/tip/2815a56e4b7252a836969f5674ee356ea1ce482c
Author:        Rik van Riel <riel@surriel.com>
AuthorDate:    Thu, 14 Nov 2024 10:26:17 -05:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 19 Nov 2024 12:02:46 +01:00

x86/mm/tlb: Add tracepoint for TLB flush IPI to stale CPU

Add a tracepoint when we send a TLB flush IPI to a CPU that used
to be in the mm_cpumask, but isn't any more.

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20241114152723.1294686-3-riel@surriel.com
---
 arch/x86/mm/tlb.c        | 1 +
 include/linux/mm_types.h | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index cc4e57a..1aac4fa 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -760,6 +760,7 @@ static void flush_tlb_func(void *info)
 		/* Can only happen on remote CPUs */
 		if (f->mm && f->mm != loaded_mm) {
 			cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(f->mm));
+			trace_tlb_flush(TLB_REMOTE_WRONG_CPU, 0);
 			return;
 		}
 	}
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6e3bdf8..6b6f054 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1335,6 +1335,7 @@ enum tlb_flush_reason {
 	TLB_LOCAL_SHOOTDOWN,
 	TLB_LOCAL_MM_SHOOTDOWN,
 	TLB_REMOTE_SEND_IPI,
+	TLB_REMOTE_WRONG_CPU,
 	NR_TLB_FLUSH_REASONS,
 };
 

[PATCH 2/3] x86,tlb: add tracepoint for TLB flush IPI to stale CPU

[Rik van Riel]

Add a tracepoint when we send a TLB flush IPI to a CPU that used
to be in the mm_cpumask, but isn't any more.

This can be used to evaluate whether there any workloads where
we end up in this path problematically often. Hopefully they
don't exist.

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
---
 arch/x86/mm/tlb.c        | 1 +
 include/linux/mm_types.h | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index f19f6378cabf..9d0d34576928 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -760,6 +760,7 @@ static void flush_tlb_func(void *info)
 		/* Can only happen on remote CPUs */
 		if (f->mm && f->mm != loaded_mm) {
 			cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(f->mm));
+			trace_tlb_flush(TLB_REMOTE_WRONG_CPU, 0);
 			return;
 		}
 	}
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6e3bdf8e38bc..6b6f05404304 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1335,6 +1335,7 @@ enum tlb_flush_reason {
 	TLB_LOCAL_SHOOTDOWN,
 	TLB_LOCAL_MM_SHOOTDOWN,
 	TLB_REMOTE_SEND_IPI,
+	TLB_REMOTE_WRONG_CPU,
 	NR_TLB_FLUSH_REASONS,
 };
 
-- 
2.45.2

[tip: x86/mm] x86/mm/tlb: Add tracepoint for TLB flush IPI to stale CPU

[tip-bot2 for Rik van Riel]

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     c058cc5fedf0cc2d05d44b166e155690e101aa6e
Gitweb:        https://git.kernel.org/tip/c058cc5fedf0cc2d05d44b166e155690e101aa6e
Author:        Rik van Riel <riel@surriel.com>
AuthorDate:    Fri, 08 Nov 2024 19:27:49 -05:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 13 Nov 2024 03:42:41 +01:00

x86/mm/tlb: Add tracepoint for TLB flush IPI to stale CPU

Add a tracepoint when we send a TLB flush IPI to a CPU that used
to be in the mm_cpumask, but isn't any more.

This can be used to evaluate whether there any workloads where
we end up in this path problematically often. Hopefully they
don't exist.

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20241109003727.3958374-3-riel@surriel.com
---
 arch/x86/mm/tlb.c        | 1 +
 include/linux/mm_types.h | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index cc4e57a..1aac4fa 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -760,6 +760,7 @@ static void flush_tlb_func(void *info)
 		/* Can only happen on remote CPUs */
 		if (f->mm && f->mm != loaded_mm) {
 			cpumask_clear_cpu(raw_smp_processor_id(), mm_cpumask(f->mm));
+			trace_tlb_flush(TLB_REMOTE_WRONG_CPU, 0);
 			return;
 		}
 	}
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6e3bdf8..6b6f054 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1335,6 +1335,7 @@ enum tlb_flush_reason {
 	TLB_LOCAL_SHOOTDOWN,
 	TLB_LOCAL_MM_SHOOTDOWN,
 	TLB_REMOTE_SEND_IPI,
+	TLB_REMOTE_WRONG_CPU,
 	NR_TLB_FLUSH_REASONS,
 };