Re: [PATCH] sched/membarrier: Fix redundant load of membarrier_state

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-10-24 20:29, Michael Ellerman wrote:
> [To += Mathieu]
> 
> "Nysal Jan K.A." <nysal@linux.ibm.com> writes:
>> From: "Nysal Jan K.A" <nysal@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?

I indeed moved mm_count into its own cacheline in response to
performance regressions reports, which were caused by simply
loading the pcpu_cid pointer frequently enough. So if membarrier_state
was also sharing that cache line, it makes sense that moving
mm_count away helped there as well.

[...]

>> ---
>>   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) &

I prefer the approach above, because it requires fewer kernel
configurations to reach the same compilation code coverage.

Thanks,

Mathieu


> 
> 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

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com