include/linux/sched/mm.h | 7 +++++++ 1 file changed, 7 insertions(+)
On architectures where ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
is not selected, sync_core_before_usermode() is a no-op.
In membarrier_mm_sync_core_before_usermode() the compiler does not
eliminate redundant branches and load of mm->membarrier_state
for this case as the atomic_read() cannot be optimized away.
Here's a snippet of the code generated for finish_task_switch() on powerpc
prior to this change:
1b786c: ld r26,2624(r30) # mm = rq->prev_mm;
.......
1b78c8: cmpdi cr7,r26,0
1b78cc: beq cr7,1b78e4 <finish_task_switch+0xd0>
1b78d0: ld r9,2312(r13) # current
1b78d4: ld r9,1888(r9) # current->mm
1b78d8: cmpd cr7,r26,r9
1b78dc: beq cr7,1b7a70 <finish_task_switch+0x25c>
1b78e0: hwsync
1b78e4: cmplwi cr7,r27,128
.......
1b7a70: lwz r9,176(r26) # atomic_read(&mm->membarrier_state)
1b7a74: b 1b78e0 <finish_task_switch+0xcc>
This was found while analyzing "perf c2c" reports on kernels prior
to commit c1753fd02a00 ("mm: move mm_count into its own cache line")
where mm_count was false sharing with membarrier_state.
There is a minor improvement in the size of finish_task_switch().
The following are results from bloat-o-meter for ppc64le:
GCC 7.5.0
---------
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
Function old new delta
finish_task_switch 884 852 -32
GCC 12.2.1
----------
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
Function old new delta
finish_task_switch.isra 852 820 -32
LLVM 17.0.6
-----------
add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-36 (-36)
Function old new delta
rt_mutex_schedule 120 104 -16
finish_task_switch 792 772 -20
Results on aarch64:
GCC 14.1.1
----------
add/remove: 0/2 grow/shrink: 1/1 up/down: 4/-60 (-56)
Function old new delta
get_nohz_timer_target 352 356 +4
e843419@0b02_0000d7e7_408 8 - -8
e843419@01bb_000021d2_868 8 - -8
finish_task_switch.isra 592 548 -44
Signed-off-by: Nysal Jan K.A. <nysal@linux.ibm.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
---
V1 -> V2:
- Add results for aarch64
- Add a comment describing the changes
---
include/linux/sched/mm.h | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 928a626725e6..b13474825130 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -531,6 +531,13 @@ enum {
static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
{
+ /*
+ * The atomic_read() below prevents CSE. The following should
+ * help the compiler generate more efficient code on architectures
+ * where sync_core_before_usermode() is a no-op.
+ */
+ if (!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE))
+ return;
if (current->mm != mm)
return;
if (likely(!(atomic_read(&mm->membarrier_state) &
--
2.48.1The following commit has been merged into the sched/core branch of tip:
Commit-ID: 7ab02bd36eb444654183ad6c5b15211ddfa32a8f
Gitweb: https://git.kernel.org/tip/7ab02bd36eb444654183ad6c5b15211ddfa32a8f
Author: Nysal Jan K.A <nysal@linux.ibm.com>
AuthorDate: Mon, 03 Mar 2025 11:34:50 +05:30
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 03 Mar 2025 11:25:40 +01:00
sched/membarrier: Fix redundant load of membarrier_state
On architectures where ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
is not selected, sync_core_before_usermode() is a no-op.
In membarrier_mm_sync_core_before_usermode() the compiler does not
eliminate redundant branches and load of mm->membarrier_state
for this case as the atomic_read() cannot be optimized away.
Here's a snippet of the code generated for finish_task_switch() on powerpc
prior to this change:
1b786c: ld r26,2624(r30) # mm = rq->prev_mm;
.......
1b78c8: cmpdi cr7,r26,0
1b78cc: beq cr7,1b78e4 <finish_task_switch+0xd0>
1b78d0: ld r9,2312(r13) # current
1b78d4: ld r9,1888(r9) # current->mm
1b78d8: cmpd cr7,r26,r9
1b78dc: beq cr7,1b7a70 <finish_task_switch+0x25c>
1b78e0: hwsync
1b78e4: cmplwi cr7,r27,128
.......
1b7a70: lwz r9,176(r26) # atomic_read(&mm->membarrier_state)
1b7a74: b 1b78e0 <finish_task_switch+0xcc>
This was found while analyzing "perf c2c" reports on kernels prior
to commit c1753fd02a00 ("mm: move mm_count into its own cache line")
where mm_count was false sharing with membarrier_state.
There is a minor improvement in the size of finish_task_switch().
The following are results from bloat-o-meter for ppc64le:
GCC 7.5.0
---------
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
Function old new delta
finish_task_switch 884 852 -32
GCC 12.2.1
----------
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
Function old new delta
finish_task_switch.isra 852 820 -32
LLVM 17.0.6
-----------
add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-36 (-36)
Function old new delta
rt_mutex_schedule 120 104 -16
finish_task_switch 792 772 -20
Results on aarch64:
GCC 14.1.1
----------
add/remove: 0/2 grow/shrink: 1/1 up/down: 4/-60 (-56)
Function old new delta
get_nohz_timer_target 352 356 +4
e843419@0b02_0000d7e7_408 8 - -8
e843419@01bb_000021d2_868 8 - -8
finish_task_switch.isra 592 548 -44
Signed-off-by: Nysal Jan K.A. <nysal@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Link: https://lore.kernel.org/r/20250303060457.531293-1-nysal@linux.ibm.com
---
include/linux/sched/mm.h | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 928a626..b134748 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -531,6 +531,13 @@ enum {
static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
{
+ /*
+ * The atomic_read() below prevents CSE. The following should
+ * help the compiler generate more efficient code on architectures
+ * where sync_core_before_usermode() is a no-op.
+ */
+ if (!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE))
+ return;
if (current->mm != mm)
return;
if (likely(!(atomic_read(&mm->membarrier_state) &© 2016 - 2025 Red Hat, Inc.