Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush

[Nadav Amit]

> On May 13, 2019, at 9:37 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> 
> On Mon, May 13, 2019 at 09:11:38AM +0000, Nadav Amit wrote:
>>> On May 13, 2019, at 1:36 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>> 
>>> On Thu, May 09, 2019 at 09:21:35PM +0000, Nadav Amit wrote:
>>> 
>>>>>>> And we can fix that by having tlb_finish_mmu() sync up. Never let a
>>>>>>> concurrent tlb_finish_mmu() complete until all concurrenct mmu_gathers
>>>>>>> have completed.
>>>>>>> 
>>>>>>> This should not be too hard to make happen.
>>>>>> 
>>>>>> This synchronization sounds much more expensive than what I proposed. But I
>>>>>> agree that cache-lines that move from one CPU to another might become an
>>>>>> issue. But I think that the scheme I suggested would minimize this overhead.
>>>>> 
>>>>> Well, it would have a lot more unconditional atomic ops. My scheme only
>>>>> waits when there is actual concurrency.
>>>> 
>>>> Well, something has to give. I didnât think that if the same core does the
>>>> atomic op it would be too expensive.
>>> 
>>> They're still at least 20 cycles a pop, uncontended.
>>> 
>>>>> I _think_ something like the below ought to work, but its not even been
>>>>> near a compiler. The only problem is the unconditional wakeup; we can
>>>>> play games to avoid that if we want to continue with this.
>>>>> 
>>>>> Ideally we'd only do this when there's been actual overlap, but I've not
>>>>> found a sensible way to detect that.
>>>>> 
>>>>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>>>>> index 4ef4bbe78a1d..b70e35792d29 100644
>>>>> --- a/include/linux/mm_types.h
>>>>> +++ b/include/linux/mm_types.h
>>>>> @@ -590,7 +590,12 @@ static inline void dec_tlb_flush_pending(struct mm_struct *mm)
>>>>> 	 *
>>>>> 	 * Therefore we must rely on tlb_flush_*() to guarantee order.
>>>>> 	 */
>>>>> -	atomic_dec(&mm->tlb_flush_pending);
>>>>> +	if (atomic_dec_and_test(&mm->tlb_flush_pending)) {
>>>>> +		wake_up_var(&mm->tlb_flush_pending);
>>>>> +	} else {
>>>>> +		wait_event_var(&mm->tlb_flush_pending,
>>>>> +			       !atomic_read_acquire(&mm->tlb_flush_pending));
>>>>> +	}
>>>>> }
>>>> 
>>>> It still seems very expensive to me, at least for certain workloads (e.g.,
>>>> Apache with multithreaded MPM).
>>> 
>>> Is that Apache-MPM workload triggering this lots? Having a known
>>> benchmark for this stuff is good for when someone has time to play with
>>> things.
>> 
>> Setting Apache2 with mpm_worker causes every request to go through
>> mmap-writev-munmap flow on every thread. I didnât run this workload after
>> the patches that downgrade the mmap_sem to read before the page-table
>> zapping were introduced. I presume these patches would allow the page-table
>> zapping to be done concurrently, and therefore would hit this flow.
> 
> Hmm, I don't think so: munmap() still has to take the semaphore for write
> initially, so it will be serialised against other munmap() threads even
> after they've downgraded afaict.
> 
> The initial bug report was about concurrent madvise() vs munmap().

I guess you are right (and Iâm wrong).

Short search suggests that ebizzy might be affected (a thread by Mel
Gorman): https://lkml.org/lkml/2015/2/2/493