[To += Mathieu]
"Nysal Jan K.A." <ny...@linux.ibm.com> writes:
> From: "Nysal Jan K.A" <ny...@linux.ibm.com>
>
> 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 the load of mm->membarrier_state
> for this case as the atomic_read() cannot be optimized away.
I was wondering if this was caused by powerpc's arch_atomic_read() which
uses asm volatile.
But replacing arch_atomic_read() with READ_ONCE() makes no difference,
presumably because the compiler still can't see that the READ_ONCE() is
unnecessary (which is kind of by design).
> Here's a snippet of the code generated for finish_task_switch() on powerpc:
>
> 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.
So it was causing a noticable performance blip? But isn't anymore?
> There is a minor improvement in the size of finish_task_switch().
> The following are results from bloat-o-meter:
>
> 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
GCC 12 is a couple of years old, I assume GCC 14 behaves similarly?
> 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
>
> Signed-off-by: Nysal Jan K.A <ny...@linux.ibm.com>
> ---
> include/linux/sched/mm.h | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 07bb8d4181d7..042e60ab853a 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -540,6 +540,8 @@ enum {
>
> static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct
> *mm)
> {
> + if (!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE))
> + return;
> if (current->mm != mm)
> return;
> if (likely(!(atomic_read(&mm->membarrier_state) &
The other option would be to have a completely separate stub, eg:
#ifdef CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct
*mm)
{
if (current->mm != mm)
return;
if (likely(!(atomic_read(&mm->membarrier_state) &
MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE)))
return;
sync_core_before_usermode();
}
#else
static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct
*mm) { }
#endif
Not sure what folks prefer.
In either case I think it's probably worth a short comment explaining
why it's worth the trouble (ie. that the atomic_read() prevents the
compiler from doing DCE).
cheers
